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AP Statistics 5.1 Graphical Representations Between Two Quantitative Variables- Exam Style Questions - FRQs - New Syllabus

Question

Wildlife biologists are interested in the health of tule elk, a species of deer found in California. An important measurement of tule elk health is their weight. The weight of a tule elk is difficult to measure in the wild. However, chest circumference, which is believed to be related to the weight of a tule elk, can easily be measured from a safe distance using a harmless laser. A study was done to investigate whether chest circumference, in centimeters (cm), could be used to accurately estimate the weight, in kilograms (kg), of male tule elk. For the study, wildlife biologists captured 30 male tule elk, measured their chest circumference and weight, and then released the elk. The data for the 30 male tule elk are shown in the scatterplot.
(a) Describe the relationship between chest circumference and weight of male tule elk in context.
Following is the equation of the least-squares regression line relating chest circumference and weight for male tule elk.
$\text{predicted weight} = -350.3 + 3.7455(\text{chest circumference})$
(b) The weight of one male tule elk with a chest circumference of $145.9$ cm is $204.3$ kg.
(i) Using the equation of the least-squares regression line, calculate the predicted weight for this male tule elk. Show your work.
(ii) Calculate the residual for this male tule elk. Show your work.
(c) Interpret the slope of the least-squares regression line in context.
(d) The sambar, another species of deer, is similar in size to the tule elk. The slope of the population regression line relating chest circumference and weight for all male sambars is $4.5$ kilograms per centimeter. A wildlife biologist wants to determine whether the slope of the population regression line for male tule elk is different than that for male sambars. Let $\beta$ represent the slope of the population regression line for male tule elk. The wildlife biologist conducted a test of the following hypotheses using the sample of 30 tule elk.
$H_0: \beta = 4.5$
$H_a: \beta \ne 4.5$
The test statistic was calculated to be $3.408$. Assume all conditions for inference were met.
(i) Determine the p-value of the test.
(ii) At a significance level of $\alpha = 0.05$, what conclusion should the wildlife biologist make regarding the slope of the population regression line for male tule elk? Justify your response.
 

Most-appropriate topic codes (AP Statistics):

• Topic \(5.1\) — Graphical Representations Between Two Quantitative Variables (Part \( \mathrm{a} \))
• Topic \(5.3\) — Linear Regression Models (Parts \( \mathrm{b} \), \( \mathrm{c} \))
• Topic \(5.4\) — Residuals (Part \( \mathrm{d} \))
• Topic \(5.5\) — Least-Squares Regression (Part \( \mathrm{d} \))
▶️ Answer/Explanation

(a)
The relationship between chest circumference and weight of male tule elk is strong, positive, and linear. As the chest circumference of male tule elk increases, their weight tends to increase.

When describing a scatterplot, always cover the Direction, Form, and Strength (DFS), and make sure to include context by explicitly naming the variables (chest circumference and weight).

(b) (i)
$\text{predicted weight} = -350.3 + 3.7455(145.9)$
$\text{predicted weight} = 196.168 \text{ kg}$

(b) (ii)
$\text{residual} = \text{actual weight} – \text{predicted weight}$
$\text{residual} = 204.3 – 196.168$
$\text{residual} = 8.132 \text{ kg}$

(c)
For each additional centimeter increase in chest circumference, the predicted weight of a male tule elk increases by $3.7455$ kilograms.

(d) (i)
The degrees of freedom for regression slope inference is $df = n – 2$. With a sample size of $n = 30$, $df = 30 – 2 = 28$.
Using the t-distribution table with $df = 28$ and a test statistic of $t = 3.408$, the one-tail area is exactly $0.001$.
Since the alternative hypothesis ($H_a: \beta \ne 4.5$) is two-sided, the p-value is $2 \times 0.001 = 0.002$.

(d) (ii)
Because the p-value of $0.002$ is less than the significance level of $\alpha = 0.05$, we reject the null hypothesis.
There is convincing statistical evidence to conclude that the slope of the population regression line relating chest circumference to weight for male tule elk is different than $4.5$ kg/cm.

Question

A biologist gathered data on the length, in millimeters (mm), and the mass, in grams (g), for 11 bullfrogs. The data are shown in Plot 1.
(a) Based on the scatterplot, describe the relationship between mass and length, in context.
From the data, the biologist calculated the least-squares regression line for predicting mass from length. The least-squares regression line is shown in Plot 2.
(b) Identify and interpret the slope of the least-squares regression line in context.
(c) Interpret the coefficient of determination of the least-squares regression line, $r^2 \approx 0.819$, in context.
(d) From Plot 2, consider the residuals of the 11 bullfrogs.
(i) Based on the plot, approximately what is the length and mass of the bullfrog with the largest absolute value residual?
(ii) Does the least-squares regression line overestimate or underestimate the mass of the bullfrog identified in part (d-i)? Explain your answer.

Most-appropriate topic codes (AP Statistics):

• Topic \(5.1\) — Graphical Representations Between Two Quantitative Variables (Part \( \mathrm{a} \))
• Topic \(5.3\) — Linear Regression Models (Parts \( \mathrm{b} \), \( \mathrm{c} \), \( \mathrm{d} \))
▶️ Answer/Explanation

(a)
The scatterplot reveals a strong, positive, roughly linear association between the mass and length of bullfrogs. There are no points that seriously deviate from the straight-line pattern of the points in the plot.

(b)
The value of the slope of the least-squares regression line is 6.086. This value indicates that the predicted mass of a bullfrog increases by 6.086 grams for each additional millimeter of length.

(c)
The coefficient of determination is $r^2 \approx 0.819$. This value indicates that 81.9% of the variation in bullfrog mass can be explained by variation in bullfrog length as described by the least-squares line.

(d)(i)
The largest residual in absolute value belongs to the bullfrog with length 162 mm and mass 356 grams.

(d)(ii)
The least-squares regression line overestimates the mass of the bullfrog with length 162 mm. Plot 2 shows that the point for the bullfrog with length 162 mm is below the least-squares regression line.

Question

Attendance at games for a certain baseball team is being investigated by the team owner. The following boxplots summarize the attendance, measured as average number of attendees per game, for \(47\) years of the team’s existence. The boxplots include the \(30\) years of games played in the old stadium and the \(17\) years played in the new stadium.
(a) Compare the distributions of average attendance between the old and new stadiums.
The following scatterplot shows average attendance versus year.
(b) Compare the trends in average attendance over time between the old and new stadium.
(c) Consider the following scatterplots.
i. Graph I shows the average attendance versus number of games won for each year. Describe the relationship between the variables.
ii. Graph II shows the same information as Graph I, but also indicates the old and new stadiums. Does Graph II suggest that the rate at which attendance changes as number of games won increases is different in the new stadium compared to the old stadium? Explain your reasoning.
(d) Consider the three variables: number of games won, year, and stadium. Based on the graphs, explain how one of those variables could be a confounding variable in the relationship between average attendance and the other variables.

Most-appropriate topic codes (AP Statistics):

• Topic \(1.9\) — Comparing the Distributions of One Quantitative Variable (Part \( \mathrm{a} \))
• Topic \(5.1\) — Graphical Representations Between Two Quantitative Variables (Parts \( \mathrm{b} \), \( \mathrm{c} \))
• Topic \(5.3\) — Linear Regression Models (Parts \( \mathrm{c} \), \( \mathrm{d} \))
▶️ Answer/Explanation

(a)
The median average attendance is noticeably higher in the new stadium (around \(25,000\)) compared to the old stadium (around \(16,000\)).
While the interquartile ranges are fairly similar, indicating comparable variability in the middle \(50\%\) of attendance figures, the overall range is slightly wider for the new stadium.
Neither stadium’s distribution shows any clear outliers.

(b)
Average attendance in the old stadium remained relatively flat over time with no clear upward or downward trend.
In contrast, the new stadium exhibits a strong, positive, upward trend in average attendance over the years it was actively used.

(c)(i)
There is a strong, positive, linear relationship between the number of games won and the average attendance for each year.

(c)(ii)
No, the rate of change appears to be roughly the same for both stadiums.
If you drew separate lines of best fit through the points representing the old stadium and the new stadium, the slopes of both lines would be nearly identical, meaning attendance increases at a similar rate per win regardless of the venue.

(d)
The number of games won could serve as a confounding variable when assessing the relationship between the stadium type and average attendance.
Since the team won significantly more games playing in the new stadium, and winning is strongly associated with higher attendance, it’s impossible to tell if the attendance spike was caused by the appeal of the new stadium or simply because the team was performing better on the field.

Question

The manager of a grocery store selected a random sample of $11$ customers to investigate the relationship between the number of customers in a checkout line and the time to finish checkout. As soon as the selected customer entered the end of a checkout line, data were collected on the number of customers in line who were in front of the selected customer and the time, in seconds, until the selected customer was finished with the checkout. The data are shown in the following scatterplot along with the corresponding least-squares regression line and computer output.
(a) Identify and interpret in context the estimate of the intercept for the least-squares regression line.
(b) Identify and interpret in context the coefficient of determination, $r^{2}$.
(c) One of the data points was determined to be an outlier. Circle the point on the scatterplot and explain why the point is considered an outlier.

Most-appropriate topic codes (AP Statistics):

• Topic \(5.1\) — Tabular and Graphical Representations for Bivariate Quantitative Data (Part \( \mathrm{c} \))
• Topic \(5.3\) — Linear Regression Models (Parts \( \mathrm{a} \), \( \mathrm{b} \))
▶️ Answer/Explanation

(a)
The estimate of the intercept is $72.95$.
Detailed Solution:
To find the intercept, we look at the computer output table under the “Coef” column right next to “Constant”, which is $72.95$.
In the context of the problem, the $y$-intercept represents the predicted $y$-value when $x=0$.
Therefore, if a customer gets into a line with $0$ people ahead of them, we predict their checkout process will take an average of $72.95$ seconds.

(b)
The coefficient of determination is $r^{2}=73.33\%$.
Detailed Solution:
The coefficient of determination is clearly labeled as “$R\text{-}Sq$” in the given computer output, which reads $73.33\%$.
This metric tells us the proportion of variance in the response variable that is predictable from the explanatory variable.
So, we can comfortably say that about $73.33\%$ of the changes in the total checkout time are directly accounted for by the linear relationship with the number of customers waiting in line.

(c)
The outlier is the point with $x=3$ and $y$ close to $100$ (this point should be circled on the scatterplot).
This point is considered an outlier because the combination of $x$ and $y$ values differs from the pattern of the rest of the data. Specifically, the value of $y$ (time to finish checkout) is much lower than would be expected when there are $x=3$ customers in line in front of the selected customer, given the remaining data.
Detailed Solution:
When observing the scatterplot, most data points cluster relatively close to the least-squares regression line, showing a clear positive trend.
However, there is one distinct point at $(3, \sim 100)$ that sits far below the regression line.
This point is considered an outlier because the checkout time is unusually fast for having $3$ customers in front, severely breaking the linear pattern established by the rest of the sample.

Question

Researchers studying a pack of gray wolves in North America collected data on the length \(x\), in meters, from nose to tip of tail, and the weight \(y\), in kilograms, of the wolves. A scatterplot of weight versus length revealed a relationship between the two variables described as positive, linear, and strong.
(a) For the situation described above, explain what is meant by each of the following words.
(i) Positive:
(ii) Linear:
(iii) Strong:
The data collected from the wolves were used to create the least-squares equation \(\hat{y} = -16.46 + 35.02x\).
(b) Interpret the meaning of the slope of the least-squares regression line in context.
(c) One wolf in the pack with a length of \(1.4\) meters had a residual of \(-9.67\) kilograms. What was the weight of the wolf?

Most-appropriate topic codes (AP Statistics):

• Topic \(5.1\) — Tabular and Graphical Representations for Bivariate Quantitative Data (Part \( \mathrm{a} \))
• Topic \(5.3\) — Linear Regression Models (Part \( \mathrm{b} \))
• Topic \(5.4\) — Residuals (Part \( \mathrm{c} \))
▶️ Answer/Explanation

(a)(i) Positive:
A positive relationship means that wolves with greater length also tend to have greater weight — so as one variable goes up, so does the other. Think of it visually: the data points on the scatterplot trend upward from left to right when length \(x\) is plotted on the horizontal axis and weight \(y\) on the vertical axis.

(a)(ii) Linear:
A linear relationship means that as the length of a wolf increases by one meter, the weight tends to change by a roughly constant amount on average. In other words, the pattern of the data follows a straight-line shape rather than a curve.

(a)(iii) Strong:
A strong relationship means that the data points fall close to the regression line, so there is little scatter around it. Equivalently, the observed weights are close to the predicted weights — the residuals are generally small.

(b)
The least-squares regression equation is
\(\hat{y} = -16.46 + 35.02x\)
The slope \(35.02\) means that for every one-meter increase in the length of a wolf, the predicted weight increases by \(35.02\) kilograms, on average. In other words, two wolves that differ by one meter in length are predicted to differ by \(35.02\) kg in weight, with the longer wolf expected to be heavier.
\(\boxed{\text{For each 1-meter increase in length, predicted weight increases by } 35.02 \text{ kg}}\)

(c)
Recall the residual formula:
\(\text{Residual} = \text{Actual weight} – \text{Predicted weight}\)
First, find the predicted weight for \(x = 1.4\) meters:
\(\hat{y} = -16.46 + 35.02(1.4)\)
\(\hat{y} = -16.46 + 49.028 = 32.568 \text{ kg}\)
Now use the residual to find the actual weight:
\(\text{Actual weight} = \hat{y} + \text{Residual}\)
\(\text{Actual weight} = 32.568 + (-9.67)\)
\(\boxed{\text{Actual weight} = 22.898 \approx 22.9 \text{ kg}}\)

Question

A newspaper in Germany reported that the more semesters needed to complete an academic program at the university, the greater the starting salary in the first year of a job. The report was based on a study that used a random sample of 24 people who had recently completed an academic program. Information was collected on the number of semesters each person in the sample needed to complete the program and the starting salary, in thousands of euros, for the first year of a job. The data are shown in the scatterplot below.
(a) Does the scatterplot support the newspaper report about number of semesters and starting salary? Justify your answer.
The table below shows computer output from a linear regression analysis on the data.
(b) Identify the slope of the least-squares regression line, and interpret the slope in context.
An independent researcher received the data from the newspaper and conducted a new analysis by separating the data into three groups based on the major of each person. A revised scatterplot identifying the major of each person is shown below.
(c) Based on the people in the sample, describe the association between starting salary and number of semesters for the business majors.
(d) Based on the people in the sample, compare the median starting salaries for the three majors.
(e) Based on the analysis conducted by the independent researcher, how could the newspaper report be modified to give a better description of the relationship between the number of semesters and the starting salary for the people in the sample?

Most-appropriate topic codes (AP Statistics):

• Topic \(5.1\) — Graphical Representations Between Two Quantitative Variables (Parts \( \mathrm{a} \), \( \mathrm{c} \))
• Topic \(5.2\) — Correlation (Part \( \mathrm{e} \))
• Topic \(5.3\) — Linear Regression Models (Part \( \mathrm{b} \))
• Topic \(1.9\) — Comparisons of the Distributions for One Quantitative Variable (Part \( \mathrm{d} \))
▶️ Answer/Explanation

(a)

Yes, the scatterplot supports the newspaper report. The scatterplot shows a positive association between the number of semesters needed to complete an academic program and starting salary — as the number of semesters increases, starting salary tends to increase as well. This is consistent with the newspaper’s claim that more semesters are associated with a greater starting salary.

(b)

The slope of the least-squares regression line is \(b_1 = 1.1594\).
The least-squares regression equation is:
\( \hat{y} = 34.018 + 1.1594x \)
Interpretation: For each additional semester needed to complete an academic program, the predicted starting salary in the first year of a job increases by approximately €1,159.40 (i.e., 1.1594 thousand euros).

(c)

For the business majors alone, there is a strong, negative, linear association between the number of semesters and starting salary. Business majors who need more semesters to complete their academic program tend to have lower starting salaries — which is the opposite direction from the overall trend seen in the combined scatterplot.

(d)

Business majors have the lowest median starting salary, at approximately €38,000. Physics majors have the next highest median starting salary, at approximately €48,000. Chemistry majors have the highest median starting salary, at approximately €55,000.
So in order from lowest to highest median starting salary: Business \(\approx\) €38,000 < Physics \(\approx\) €48,000 < Chemistry \(\approx\) €55,000.

(e)

The newspaper report should be modified to account for the major of each person. The overall positive association in the original report is largely explained by the fact that different majors — chemistry, physics, and business — tend to require different numbers of semesters and also have very different starting salary levels. Chemistry majors take more semesters on average and also earn the highest salaries; business majors take fewer semesters and earn the lowest salaries. This creates an apparent positive association when all three majors are pooled together.
However, within each individual major, students who take a greater number of semesters to complete their program tend to have lower starting salaries, not higher. The newspaper report should therefore be revised to state that, while majors requiring more semesters overall tend to have higher starting salaries (chemistry highest, physics next, business lowest), within any given major, taking more semesters to complete the program is associated with a lower starting salary.

Question

A student measured the heights and the arm spans, rounded to the nearest inch, of each person in a random sample of \(12\) seniors at a high school. A scatterplot of arm span versus height for the \(12\) seniors is shown.
(a) Based on the scatterplot, describe the relationship between arm span and height for the sample of \(12\) seniors.
Let \(x\) represent height, in inches, and let \(y\) represent arm span, in inches. Two scatterplots of the same data are shown below. Graph \(1\) shows the data with the least squares regression line \(\hat{y}=11.74+0.8247x\), and graph \(2\) shows the data with the line \(y=x\).
(b) The criteria described in the table below can be used to classify people into one of three body shape categories: square, tall rectangle, or short rectangle.
i. For which graph, \(1\) or \(2\), is the line helpful in classifying a student’s body shape as square, tall rectangle, or short rectangle? Explain.
ii. Complete the table of classifications for the \(12\) seniors.
(c) Using the best model for prediction, calculate the predicted arm span for a senior with height \(61\) inches.

Most-appropriate topic codes (AP Statistics):

• Topic \(5.1\) — Graphical Representations Between Two Quantitative Variables (Parts \( \mathrm{a} \), \( \mathrm{b} \))
• Topic \(5.5\) — Least-Squares Regression (Part \( \mathrm{c} \))
▶️ Answer/Explanation

(a)
There is a moderately strong, positive, linear relationship between height and arm span so that taller students tend to have longer arm spans.

(b)(i)
The line in Graph \(2\) is the one that is helpful. For each student, the graph illustrates whether arm span is equal to height (points on the line), arm span is greater than height (points above the line), or arm span is less than height (points below the line).

(b)(ii)
The frequencies of the \(12\) seniors classified by body shape are:
Square: \(3\)
Tall Rectangle: \(4\)
Short Rectangle: \(5\)

(c)
The predicted arm span is calculated using the given least squares regression line equation:
\( \hat{y} = 11.74 + 0.8247x \)
\( \hat{y} = 11.74 + 0.8247(61) \)
\( \hat{y} = 62.05 \text{ inches} \)

Question

The scatterplot below displays the price in dollars and quality rating for 14 different sewing machines.
(a) Describe the nature of the association between price and quality rating for the sewing machines.
(b) One of the 14 sewing machines substantially affects the appropriateness of using a linear regression model to predict quality rating based on price. Report the approximate price and quality rating of that machine and explain your choice.
(c) Chris is interested in buying one of the 14 sewing machines. He will consider buying only those machines for which there is no other machine that has both higher quality and lower price. On the scatterplot reproduced below, circle all data points corresponding to machines that Chris will consider buying.

Most-appropriate topic codes (AP Statistics):

• Topic 5.1 — Graphical Representations Between Two Quantitative Variables (Part a)
• Topic 5.3 — Linear Regression Models (Part b)
• Topic 5.2 — Correlation (Part c)
▶️ Answer/Explanation

(a)
The data show a weak to moderate, positive association between price and quality rating for these sewing machines.
The overall form of the relationship is curved or nonlinear rather than a straight line.
Among the machines costing under \$500, there appears to be little to no visible association between price and quality rating.
However, machines costing above \$500 generally tend to achieve much higher quality ratings than the cheaper group, which creates the overall positive direction.

(b)
The machine that most heavily influences and reduces the appropriateness of a linear regression model is the one located at an approximate price of \(\$2,200\) with a quality rating of approximately \(65\).
The general trend among the other four machines priced over \$500 suggests that quality starts to level off or approach a maximum potential limit rather than increasing continuously with price.
This particular \(\$2,200\) sewing machine is the absolute most expensive model in the entire dataset, yet its quality rating drops significantly compared to the models around \$1,500.
Including this point in a linear regression model would heavily drag the least-squares line down toward it, creating a poor overall fit for the rest of the data points.

(c)
According to Chris’s rule, he wants a machine only if there isn’t another choice available that is both cheaper and higher in quality. Following this strategy, only two models fit his standard and should be circled on the scatterplot:
1. The model positioned at a price slightly above \(\$100\) with a quality rating of \(65\).
2. The model positioned at a price slightly below \(\$500\) with a quality rating of \(81\) (or \(82\)).
The data points corresponding to these two machines have been circled on the scatterplot below.

Question

Agricultural experts are trying to develop a bird deterrent to reduce costly damage to crops in the United States. An experiment is to be conducted using garlic oil to study its effectiveness as a nontoxic, environmentally safe bird repellant. The experiment will use European starlings, a bird species that causes considerable damage annually to the corn crop in the United States. Food granules made from corn are to be infused with garlic oil in each of five concentrations of garlic—0 percent, 2 percent, 10 percent, 25 percent, and 50 percent. The researchers will determine the adverse reaction of the birds to the repellant by measuring the number of food granules consumed during a two-hour period following overnight food deprivation. There are forty birds available for the experiment, and the researchers will use eight birds for each concentration of garlic. Each bird will be kept in a separate cage and provided with the same number of food granules.
(a) For the experiment, identify
i. the treatments
ii. the experimental units
iii. the response that will be measured
(b) After performing the experiment, the researchers recorded the data shown in the table below.
i. Construct a graph of the data that could be used to investigate the appropriateness of a linear regression model for analyzing the results of the experiment.

ii. Based on your graph, do you think a linear regression model is appropriate? Explain.

Most-appropriate topic codes (AP Statistics):

• Topic 1.13 — Experimental Design (Part a)
• Topic 5.1 — Graphical Representations Between Two Quantitative Variables (Part b.i)
• Topic 5.3 — Linear Regression Models (Part b.ii)
▶️ Answer/Explanation

(a)

i. The treatments: The five different concentrations of garlic oil infused into the food granules, which are 0%, 2%, 10%, 25%, and 50%.
ii. The experimental units: The 40 individual European starlings (or the individual cages housing each bird).
iii. The response that will be measured: The number of food granules consumed by an individual bird during the two-hour period.

(b)
i. Graph construction:
To investigate linearity, we construct a scatterplot with Garlic Oil Concentration (%) on the horizontal $x$-axis and Mean Number of Food Granules Consumed on the vertical $y$-axis.

The points to plot are $(0, 58)$, $(2, 48)$, $(10, 29)$, $(25, 24)$, and $(50, 20)$.
ii. Linearity Assessment:
No, a linear regression model is not appropriate.
The scatterplot reveals a clear, distinct curved pattern rather than a straight line trend.
As the garlic oil concentration increases, the mean number of food granules consumed drops sharply at first and then begins to level off, indicating a non-linear relationship.

Question

The nerves that supply sensation to the front portion of a person’s foot run between the long bones of the foot. Tight-fitting shoes can squeeze these nerves between the bones, causing pain when the nerves swell. This condition is called Morton’s neuroma. Because most people have a dominant foot, muscular development is not the same in both feet. People who have Morton’s neuroma may have the condition in only one foot or they may have it in both feet.
Investigators selected a random sample of 12 adult female patients with Morton’s neuroma to study this disease further. The data below are measurements of nerve swelling as recorded by a physician. A value of 1.0 is considered “normal,” and 2.0 is considered extreme swelling. The population distribution of the swelling measurements is approximately normal for adult females who have Morton’s neuroma.
(a) A scatterplot of the ordered pairs (swelling in left foot, swelling in right foot), is shown below.
The scatterplot suggests there are two distinct groups of patients. Patients within each group share a common trait. Use the scatterplot above and the table to determine the common trait and explain how this trait differs for the two groups.
(b) A scatterplot of the ordered pairs (swelling in dominant foot, swelling in nondominant foot), is shown below.
What conclusion can be drawn from this scatterplot that is not apparent from the scatterplot in part (a)?
(c) Can you conclude that there is a difference between the mean swelling in the dominant foot and the mean swelling in the nondominant foot for adult females who have Morton’s neuroma in at least one foot? Give a statistical justification to support your answer.
(For easy reference, the table of data from above also appears at the bottom of this question.)
(d) The nerve swelling measurement is used to indicate whether a foot has Morton’s neuroma. Use the 24 measurements of nerve swelling to suggest a criterion for diagnosing Morton’s neuroma. Justify your suggestion graphically.
(For easy reference, the table of data from above also appears below.)

Most-appropriate topic codes (AP Statistics):

• Topic 5.1 — Graphical Representations Between Two Quantitative Variables (Parts \(\mathrm{a}\), \(\mathrm{b}\))
• Topic 4.4 — Setting Up a Test for a Population Mean or Population Mean Difference (Part \(\mathrm{c}\))
• Topic 4.5 — Carrying Out a Test for a Population Mean or Population Mean Difference (Part \(\mathrm{c}\))
• Topic 1.5 — Graphical Representations for One Quantitative Variable (Part \(\mathrm{d}\))
• Topic 1.9 — Comparisons of the Distributions for One Quantitative Variable (Part \(\mathrm{d}\))
▶️ Answer/Explanation

(a)

The trait that distinguishes the two groups in the scatterplot is the dominant foot (left or right). All the points in the upper-left cluster represent patients whose dominant foot is the right foot, while all the points in the lower-right cluster represent patients whose dominant foot is the left foot. The dominant foot type is the common trait, and it differs between the two groups.

(b)

Two conclusions become clear from this scatterplot that were not visible before:
First, there is a positive linear relationship between swelling in the dominant foot and swelling in the nondominant foot — as swelling in the dominant foot increases, swelling in the nondominant foot tends to increase as well.
Second, and importantly, every single point lies below the line \(y = x\), which means swelling in the dominant foot is consistently greater than swelling in the nondominant foot for all patients in the sample. This pattern across both groups combined is something you simply could not see in the left-foot vs. right-foot scatterplot from part (a).

(c)

We perform a matched-pairs \(t\)-test on the differences \(d_i = \text{(dominant swelling)} – \text{(nondominant swelling)}\).
The 12 differences are:
\(0.30,\ 0.30,\ 0.45,\ 0.15,\ 0.30,\ 0.35,\ 0.25,\ 0.35,\ 0.20,\ 0.25,\ 0.40,\ 0.15\)
State hypotheses (where \(\mu_d\) is the mean difference, dominant minus nondominant):
\(H_0: \mu_d = 0\)
\(H_a: \mu_d \neq 0\)
Check conditions:
1. We are told a random sample was selected from the population of adult females with Morton’s neuroma.
2. A dotplot of the differences shows a roughly symmetric, unimodal distribution with no outliers — it is reasonable to treat the population of differences as approximately normal.
Compute the test statistic:
\(\bar{x}_d = 0.2875, \quad s_d = 0.0932, \quad n = 12, \quad df = 11\)
\(t = \dfrac{\bar{x}_d – 0}{\dfrac{s_d}{\sqrt{n}}} = \dfrac{0.2875 – 0}{\dfrac{0.0932}{\sqrt{12}}} = 10.68\)
\(p\text{-value} \approx 0.0000004 \approx 0\)
Since the \(p\)-value is essentially \(0\), which is far less than any reasonable significance level \(\alpha\), we reject \(H_0\). There is very convincing statistical evidence that the mean swelling in the dominant foot is different from (and specifically greater than) the mean swelling in the nondominant foot for adult females who have Morton’s neuroma in at least one foot.

(d)

To suggest a diagnostic criterion, we separate all 24 swelling measurements into two groups: the 17 foot measurements from feet that have Morton’s neuroma and the 7 foot measurements from feet that do not have Morton’s neuroma. A stacked dotplot of the two groups is shown below:

The dotplot makes it visually clear that all 7 feet without Morton’s neuroma have swelling measurements of \(1.40\) or below, while the feet with Morton’s neuroma have swelling values of \(1.40\) and above (with the measurements extending up to \(1.85\)). Based on this graphical display, a reasonable diagnostic criterion is:
\(\boxed{\text{Swelling measurement} \geq 1.4 \Rightarrow \text{diagnose Morton’s neuroma}}\)
A cutoff of approximately \(1.4\) or higher serves as a sensible threshold for diagnosing Morton’s neuroma, since it cleanly separates the feet with and without the condition in this dataset.

Question

Lead, found in some paints, is a neurotoxin that can be especially harmful to the developing brain and nervous system of children. Children frequently put their hands in their mouth after touching painted surfaces, and this is the most common type of exposure to lead.
A study was conducted to investigate whether there were differences in children’s exposure to lead between suburban day-care centers and urban day-care centers in one large city. For this study, researchers used a random sample of 20 children in suburban day-care centers. Ten of these 20 children were randomly selected to play outside; the remaining 10 children played inside. All children had their hands wiped clean before beginning their assigned one-hour play period either outside or inside. After the play period ended, the amount of lead in micrograms (mcg) on each child’s dominant hand was recorded.
The mean amount of lead on the dominant hand for the children playing inside was \(3.75\) mcg, and the mean amount of lead for the children playing outside was \(5.65\) mcg. A \(95\) percent confidence interval for the difference in the mean amount of lead after one hour inside versus one hour outside was calculated to be \((-2.46, -1.34)\).
A random sample of 18 children in urban day-care centers in the same large city was selected. For this sample, the same process was used, including randomly assigning children to play inside or outside. The data for the amount (in mcg) of lead on each child’s dominant hand are shown in the table below.
(a) Use a 95 percent confidence interval to estimate the difference in the mean amount of lead on a child’s dominant hand after an hour of play inside versus an hour of play outside at urban day-care centers in this city. Be sure to interpret your interval.
(b) On the figure below,
• Using the vertical axis for the mean amount of lead, plot the mean for the amounts of lead on the dominant hand of children who played inside at the suburban day-care center and then plot the mean for the amounts of lead on the dominant hand of children who played inside at the urban day-care center.
• Connect these two points with a line segment.
• Plot the two means (suburban and urban) for the children who played outside at the two types of day-care centers.
• Connect these two points with a second line segment.
(c) From the study, what conclusions can be drawn about the impact of setting (inside, outside), environment (suburban, urban), and the relationship between the two on the amount of lead on the dominant hand of children after play in this city? Justify your answer.

Most-appropriate topic codes (AP Statistics):

• Topic 4.7 — Constructing a Confidence Interval for the Difference Between Two Population Means (Part a)
• Topic 4.8 — Justifying a Claim Based on a Confidence Interval for the Difference Between Two Population Means (Part a, Part c)
• Topic 5.1 — Graphical Representations Between Two Quantitative Variables (Part b)
▶️ Answer/Explanation

(a)
First check conditions for a two-sample \(t\)-interval: the two groups of urban children were assigned at random and independently to play inside or outside, and dotplots of each group’s data show no strong skew or outliers, so it’s reasonable to treat the underlying populations as approximately normal.

Summary statistics for the urban sample:
\( n_{\text{in}}=9,\quad \bar{x}_{\text{in}}=4.56,\quad s_{\text{in}}=0.846 \)
\( n_{\text{out}}=9,\quad \bar{x}_{\text{out}}=17.56,\quad s_{\text{out}}=4.61 \)
The two-sample \(t\)-confidence interval formula is
\( (\bar{x}_{\text{in}}-\bar{x}_{\text{out}})\pm t^*\sqrt{\dfrac{s_{\text{in}}^2}{n_{\text{in}}}+\dfrac{s_{\text{out}}^2}{n_{\text{out}}}} \)
Using the conservative degrees of freedom \(df=\min(n_{\text{in}}-1,\,n_{\text{out}}-1)=8\), so \(t^*=2.306\):
\( (4.56-17.56)\pm 2.306\sqrt{\dfrac{(0.846)^2}{9}+\dfrac{(4.61)^2}{9}} \)
\( -13.00\pm 2.306(1.564) \)
\( -13.00\pm 3.61 \)
\( \boxed{(-16.60,\ -9.40)\text{ mcg}} \)
Interpretation: we are 95% confident that, for the population of urban day-care children, the mean amount of lead on the dominant hand after an hour of play inside is between 9.40 and 16.60 mcg lower than after an hour of play outside. Since this interval doesn’t contain zero, the difference is meaningful — urban children who play outside pick up noticeably more lead on their hands.

(b)

Plot the points \((\text{Suburban},3.75)\) and \((\text{Urban},4.56)\), connect them with a line labeled “inside.” Then plot \((\text{Suburban},5.65)\) and \((\text{Urban},17.56)\), connect them with a line labeled “outside.” The “inside” line should be nearly flat and low on the graph, while the “outside” line should rise sharply from suburban to urban.
\( \boxed{\text{Inside line: nearly flat, low values; Outside line: steep increase from suburban to urban}} \)

(c)
Setting (inside vs. outside): In both suburban and urban environments, children who played outside ended up with more lead on their hands than children who played inside. This is supported by the fact that all four endpoints of the two confidence intervals (inside minus outside) are negative, and the graph shows the “outside” line sitting above the “inside” line everywhere.
Environment (suburban vs. urban): For both inside and outside play, urban children had more lead on their hands on average than suburban children. The graph shows both lines sloping upward from suburban to urban.
Relationship between the two: The effect of going inside versus outside depends heavily on the environment. In the suburban setting, the inside and outside means are fairly close together (3.75 vs. 5.65), but in the urban setting the gap is much larger (4.56 vs. 17.56). In other words, playing outside makes a much bigger difference in lead exposure in the urban environment than in the suburban environment.
\( \boxed{\text{Outside > Inside in both settings; Urban > Suburban in both settings; the inside/outside gap is much larger for urban than suburban}} \)

Question

The Earth’s Moon has many impact craters that were created when the inner solar system was subjected to heavy bombardment of small celestial bodies. Scientists studied 11 impact craters on the Moon to determine whether there was any relationship between the age of the craters (based on radioactive dating of lunar rocks) and the impact rate (as deduced from the density of the craters). The data are displayed in the scatterplot below.
 
(a) Describe the nature of the relationship between impact rate and age.
Prior to fitting a linear regression model, the researchers transformed both impact rate and age by using logarithms. The following computer output and residual plot were produced.
(b) Interpret the value of \(r^2\).
(c) Comment on the appropriateness of this linear regression for modeling the relationship between the transformed variables.

Most-appropriate topic codes (AP Statistics):

• Topic 5.1 — Graphical Representations Between Two Quantitative Variables (Part a)
• Topic 5.2 — Correlation (Part b)
• Topic 5.3 — Linear Regression Models (Parts b, c)
• Topic 5.4 — Residuals (Part c)
▶️ Answer/Explanation

(a)

The relationship between impact rate and age is negative and nonlinear (curved). As age increases, impact rate decreases, but not at a constant rate — the decrease is very steep for craters with ages less than about 0.7 billion years, and then the impact rate levels off and remains close to zero for older craters. The spread of the data also decreases as age increases, suggesting a fan-shaped, non-constant variance pattern.

(b)

The value of \(r^2 = 0.894\), or \(89.4\%\).
This means that approximately \(89.4\%\) of the variability in \(\ln(\text{rate})\) is explained by the linear relationship with \(\ln(\text{age})\). In other words, after taking logarithms of both variables, the linear model accounts for nearly \(89.4\%\) of the variation seen in the log-transformed impact rate values among the 11 craters studied.

(c)

The linear regression model is appropriate for the log-transformed variables. Here’s the reasoning:
First, the residual plot shows no obvious curved pattern — the residuals appear to be scattered roughly randomly around zero, with no systematic curvature, which supports linearity.
Second, the residuals do not show a clear fan shape (no dramatic increase in spread), so the equal variance condition appears reasonably satisfied.
Third, the \(R^2\) value of \(89.4\%\) is quite high, indicating the linear model fits the transformed data well.
However, one concern is that the residual plot shows a slight tendency for residuals to be negative in the middle range of fitted values and positive at the extremes, hinting at a mild pattern. Given the small sample size of only 11 observations, this could simply be due to natural sampling variability. Overall, the linear model on the log-transformed data is a reasonable fit.

Question

A simple random sample of 9 students was selected from a large university. Each of these students reported the number of hours he or she had allocated to studying and the number of hours allocated to work each week. A least squares linear regression was performed and part of the resulting computer output is shown below.
The scatterplot below displays the data that were collected from the 9 students.
(a) After point \(P\), labeled on the graph on the previous page, was removed from the data, a second linear regression was performed and the computer output is shown below.
Does point \(P\) exercise a large influence on the regression line? Explain.
(b) The researcher who conducted the study discovered that the number of hours spent studying reported by the student represented by \(P\) was recorded incorrectly. The corrected data point for this student is represented by the letter \(Q\) in the scatterplot below.
Study Work 5 10 15 0 10 20 30 Q•
Explain how the least squares regression line for the corrected data (in this part) would differ from the least squares regression line for the original data.

Most-appropriate topic codes (AP Statistics):

• Topic 5.3 — Linear Regression Models (Part a)
• Topic 5.4 — Residuals (Part a)
• Topic 5.5 — Least-Squares Regression (Parts a, b)
• Topic 5.1 — Graphical Representations Between Two Quantitative Variables (Part b)
▶️ Answer/Explanation

(a)

Yes, point \(P\) does exercise a large influence on the regression line.
When point \(P\) is included, the slope of the regression line is \(b_1 = 0.4919\), and it is statistically significant (\(p = 0.040\)).
When point \(P\) is removed, the slope drops dramatically to \(b_1 = 0.1500\), which is no longer statistically significant (\(p = 0.709\)).
The intercept also changes from \(8.107\) to \(11.123\), and \(R^2\) falls sharply from \(47.6\%\) to only \(2.5\%\).
Because removing a single point caused such substantial changes in the slope, intercept, statistical significance, and \(R^2\), point \(P\) is clearly an influential point — it is extreme in the \(x\)-direction (a work value of about 30, far beyond the rest of the data), which gives it high leverage and strong pull on the regression line.

(b)

In the original data, point \(P\) is at approximately \((\text{Work} = 30,\ \text{Study} = 25)\), which is high on both variables and pulls the regression line upward to the right, producing a positive slope.
The corrected data point \(Q\) is at approximately \((\text{Work} = 30,\ \text{Study} = 6)\), which is far below the trend of the other data points at that work value.
With \(Q\) replacing \(P\), the regression line for the corrected data will have a negative slope rather than a positive slope — the corrected point now pulls the line downward on the right side.
The intercept would also be considerably larger for the corrected data, since the line must start higher on the \(y\)-axis to accommodate the negative slope while passing near the rest of the data.

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