# Finding Proportions

**Chapter 5, numbers 5.11, 5.13, 5.15, and 5.18**

**Finding Proportions**

5.11 Scores on the Wechsler Adult Intelligence Scale (WAIS) approximate a normal curve with a mean of 100 and a standard deviation of 15. What proportion of IQ scores are

(a) above Kristen’s 125?

(b) below 82?

(c) within 9 points of the mean?

(d) more than 40 points from the mean?

**Finding Scores**

5.13 IQ scores on the WAIS test approximate a normal curve with a mean of 100 and a standard deviation of 15. What IQ score is identified with

(a) the upper 2 percent, that is, 2 percent to the right (and 98 percent to the left)?

(b) the lower 10 percent?

(c) the upper 60 percent?

(d) the middle 95 percent? [Remember, the middle 95 percent straddles the line perpendicular to the mean (or the 50th percentile), with half of 95 percent, or 47.5 percent, above this line and the remaining 47.5 percent below this line.]

(e) the middle 99 percent**?**

**Finding Proportions and Scores IMPORTANT NOTE: When doing Questions 5.15 and 5.16, remember to decide first whether a proportion or a score is to be found.**

*5. 15 An investigator polls common cold sufferer, asking them to estimate the number of hours of physical discomfort caused by their most recent colds. Assume that their estimates approximate a normal curve with a mean of 83 hours and a standard deviation of 20 hours.

(a) What is the estimated number of hours for the shortest-suffering 5 percent?

(b) What proportion of sufferers estimate that their colds lasted longer than 48 hours?

(c) What proportion suffered for fewer than 61 hours?

(d) What is the estimated number of hours suffered by the extreme 1 percent either above or below the mean?

(e) What proportion suffered for between 1 and 3 days, that is, between 24 and 72 hours?

(f) What is the estimated number of hours suffered by the middle 95 percent? [See the comment about “middle 95 percent” in Question 5.13(d).]

(g) What proportion suffered for between 2 and 4 days?

(h) A medical researcher wishes to concentrate on the 20 percent who suffered the most. She will work only with those who estimate that they suffered for more than ——— hours.

(I) Another researcher wishes to compare those who suffered least with those who suffered most. If each group is to consist of only the extreme 3 percent, the mild group will consist of those who suffered for fewer than _____ hours, and the severe group will consist of those who suffered for more than _____ hours.

(j) Another survey found that people with colds who took daily doses of vitamin C suf-fered, on the average, for 61 hours. What proportion of the original survey (with a mean of 83 hours and a standard deviation of 20 hours) suffered for more than 61 hours?

(k) What proportion of the original survey suffered for exactly 61 hours?

(Be careful!) Answers on page 427.

**NORMAL DISTRIBUTIONS AND STANDARD (z) SCORES**

*5.18 The body mass index (BMI) measures body size in people by dividing weight (in pounds) by the square of height (in inches) and then multiplying by a factor of 703. A BMI less than 18.5 is defined as underweight; between 18.5 to 24.9 is normal; between 25 and 29.9 is overweight; and 30 or more is obese. It is well established that Americans have become heavier during the last half century. Assume that the positively skewed distribution of BMIs for adult American males has a mean of 28 with a standard deviation of 4.

(a) Would the median BMI score exceed, equal, or be exceeded by the mean BMI score of 28?

**Answer:**

Mean exceeds median

(b) What z score defines overweight

**Answer:**

-0.75

(c) What z score defines obese. Answers on page 427.

0.50

**Chapter 8, numbers 8.10, 8.14, 8.16, 8.19, and 8.21**

8.10 Television stations sometimes solicit feedback volunteered by viewers about a tele-vised event. Following a televised debate between Barack Obama and Mitt Romney in the 2012 presidential election campaign, a TV station conducted a telephone poll to determine the “winner.” Callers were given two phone numbers, one for Obama and the other for Romney, to register their opinions automatically.

(a) Comment on whether this was a random sample.

(b) How might this poll have been improved?

*8.14 The probability of a boy being born equals .50, or 1/2, as does the probability of a girl being born. For a randomly selected family with two children, what’s the probability of

(a) two boys, that is, a boy and a boy? (Reminder: Before using either the addition or multiplication rule, satisfy yourself that the various events are either mutually exclusive or independent, respectively.)

**Answer:**

(b) two girls?

**Answer:**

(c) either two boys or two girls?

**Answer:**

**Note:** Answers on page 431.

8.16 A traditional test for extrasensory perception (ESP) involves a set of playing cards, each of which shows a different symbol (circle, square, cross, star, or wavy lines). If C represents a correct guess and I an incorrect guess, what is the probability of

(a) C?

(b) CI (in that order) for two guesses?

(c) CCC for three guesses?

(d) III for three guesses?

8.19 A sensor is used to monitor the performance of a nuclear reactor. The sensor accurately reflects the state of the reactor with a probability of .97. But with a probability of .02, it gives a false alarm (by reporting excessive radiation even though the reactor is performing normally), and with a probability of .01, it misses excessive radiation (by failing to report excessive radiation even though the reactor is performing abnormally).

(a) What is the probability that a sensor will give an incorrect report, that is, either a false alarm or a miss?

(b) To reduce costly shutdowns caused by false alarms, management introduces a second completely independent sensor, and the reactor is shut down only when both sensors report excessive radiation. (According to this perspective, solitary reports of excessive radiation should be viewed as false alarms and ignored, since both sensors provide accurate information much of the time.) What is the new probability that the reactor will be shut down because of simultaneous false alarms by both the first and second sensors?

(c) Being more concerned about failures to detect excessive radiation, someone who lives near the nuclear reactor proposes an entirely different strategy: Shut down the reactor whenever either sensor reports excessive radiation. (According to this point of view, even a solitary report of excessive radiation should trigger a shutdown, since a failure to detect excessive radiation is potentially catastrophic.) If this policy were adopted, what is the new probability that excessive radiation will be missed simultaneously by both the first and second sensors?

8.21 Assume that the probability of breast cancer equals .01 for women in the 50-59 age group. Furthermore, if a woman does have breast cancer, the probability of a true positive mammogram (correct detection of breast cancer) equals .80 and the probability of a false negative mammogram (a miss) equals .20. On the other hand, if a woman does not have breast cancer, the probability of a true negative mammogram (correct non-detection) equals .90 and the probability of a false positive mammogram (a false alarm) equals .10.

(a) What is the probability that a randomly selected woman will have a positive mammogram?

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(b) What is the probability of having breast cancer, given a positive mammogram?

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c) What is the probability of not having breast cancer, given a negative mammogram? **Note:** Answers on page 431.

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**(Hint: **Use a frequency analysis to answer questions. To facilitate checking your answers with those in the book, begin with a total of 1,000 women, then branch into the number of women who do or do not have breast cancer, and finally, under each of these numbers, branch into the number of women with positive and negative mammograms.)