Name __________________________________________
Date________________________
Worksheet 2.1A – Percentiles, Cumulative Relative Freq. Graphs, z-scores
1. How many pairs of shoes do students have? Do girls have more shoes than boys? Here are data from a
random sample of 20 female and 20 male students at a large high school:
(a) Find and interpret the percentile in the female distribution for the girl with 22 pairs of shoes.
(b) Find and interpret the percentile in the male distribution for the boy with 22 pairs of shoes.
(c) Who is more unusual: the girl with 22 pairs of shoes or the boy with 22 pairs of shoes? Explain.
2. Larry came home very excited after a visit to his doctor. He announced proudly to his wife, “My doctor says
my blood pressure is at the 90th percentile among men like me. That means I’m better off than about 90% of
similar men.” How should his wife, who is a statistician, respond to Larry’s statement?
3. The graph below is a cumulative relative frequency graph showing the lifetimes (in hours) of 200 lamps.
(a) Estimate the 60th percentile of this
distribution. Show your method.
(b) What is the percentile for a lamp that lasted 900
hours?
4. Eleanor scores 680 on the SAT Mathematics test. The distribution of SAT scores is symmetric and singlepeaked, with mean 500 and standard deviation 100. Gerald takes the American College Testing (ACT)
Mathematics test and scores 27. ACT scores also follow a symmetric, single-peaked distribution—but with
mean 18 and standard deviation 6. Find the standardized scores for both students. Assuming that both tests
measure the same kind of ability, who has the higher score?
5. Three landmarks of baseball achievement are Ty Cobb’s batting average of .420 in 1911, Ted Williams’s .406
in 1941, and George Brett’s .390 in 1980. 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.
6. Mrs. Munson is concerned about how her daughter’s height and weight compare with those of other girls of
the same age. She uses an online calculator to determine that her daughter is at the 87th percentile for weight
and the 67th percentile for height. Explain to Mrs. Munson what this means.
7. The graph displays the cumulative relative frequency of the lengths of phone calls made from the
mathematics department office at White Plains High last month.
(a) About what percent of calls lasted less than 30
minutes?
(b) 30 minutes or more?
7. Mrs. Navard's statistics class has just completed the first three steps of the “Where Do I Stand?” Activity. The
figure below shows a dotplot of the class’s height distribution, along with summary statistics from computer
output.
(a) Lynette, a student in the class, is 65 inches tall. Find and interpret her z-score.
(b) Another student in the class, Brent, is 74 inches tall. How tall is Brent compared with the rest of the class?
Give appropriate numerical evidence to support your answer.
(c) Brent is a member of the school’s basketball team. The mean height of the players on the team is 76 inches.
Brent’s height translates to a z-score of –0.85 in the team’s height distribution. What is the standard deviation
of the team members’ heights?
8. Brad Lidge played a crucial role as the Phillies’ “closer,” pitching the end of many games throughout the
season. Lidge’s salary for the 2008 season was $6,350,000.
(a) Find the percentile corresponding to Lidge’s salary. Explain what this value means.
(b) Find the z-score corresponding to Lidge’s salary. Explain what this value means.
(c) Did Ryan Madson, who was paid $1,400,000, have a high salary or a low salary compared with the rest of the
team? Justify your answer by calculating and interpreting Madson’s percentile and z-score