Chapter 2: Q 25. (page 108)
Density curves Sketch a density curve that might describe a distribution that is symmetric but has two peaks.
Short Answer
Density curve is

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Chapter 2: Q 25. (page 108)
Density curves Sketch a density curve that might describe a distribution that is symmetric but has two peaks.
Density curve is

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Comparing bone density Refer to the previous exercise. One of Judy’s friends, Mary, has the bone density in her hip measured using DEXA. Mary is 35 years old. Her bone density is also reported as , but her standardized score is. The mean bone density in the hip for the reference population of -year-old women is .
(a) Whose bones are healthier—Judy’s or Mary’s? Justify your answer.
(b) Calculate the standard deviation of the bone density in Mary’s reference population. How does
this compare with your answer to Exercise 13(b)? Are you surprised?
Teacher raises A school system employs teachers at salaries between and . The teachers’ union and the school board are negotiating the form of next year’s increase in the salary schedule.
(a) If every teacher is given a flat raise, what will this do to the mean salary? To the median salary? Explain your answers.
(b) What would a flat raise do to the extremes and quartiles of the salary distribution? To the standard deviation of teachers’ salaries? Explain your answers.
Comparing batting averages Three landmarks of baseball achievement are Ty Cobb’s batting average of in , Ted Williams’s in , and George Brett’s in . These batting averages cannot be compared directly because the distribution of major league batting averages has changed over the years. The distributions are quite symmetric, except for outliers such as Cobb, Williams, and Brett. While the mean batting average has been held roughly constant
by rule changes and the balance between hitting and pitching, the standard deviation has dropped over time. Here are the facts: Compute the standardized batting averages for Cobb, Williams, and Brett to compare how far each stood above his peers.

Weights aren’t Normal The heights of people of the same gender and similar ages follow Normal distributions reasonably closely. Weights, on the other hand, are not Normally distributed. The weights of women aged to have mean pounds and median pounds. The first and third quartiles are pounds and pounds. What can you say about the shape of the weight distribution? Why?
Questions T2.9 and T2.10 refer to the following setting. Until the scale was changed in , SAT scores were based on a scale set many years ago. For Math scores, the mean under the old scale in the was and the standard deviation was . In , the mean was and the standard deviation was .
T2.10. Jane took the SAT in and scored . Her sister Colleen took the SAT in and scored . Who did better on the exam, and how can you tell?
(a) Colleen-she scored points higher than Jane.
(b) Colleen-her standardized score is higher than Jane's.
(c) Jane-her standardized score is higher than Colleen's.
(d) Jane-the standard deviation was bigger in
(e) The two sisters did equally well-their -scores are the same.
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