EBook Problems

Probability and Statistics EBook Practice Problems

The problems provided below may be useful for practicing the concepts, methods and analysis protocols, and for self-evaluation of learning of the materials presented in the EBook.

II. Describing, Exploring, and Comparing Data

Pictures of Data

1. Two random samples were taken to determine backpack load difference between seniors and freshmen, in pounds. The following are the summaries:

Year	Mean	SD	Median	Min	Max	Range	Count
Freshmen	20.43	4.21	17.20	5.78	31.68	25.9	115
Senior	18.67	3.56	18.67	5.31	27.66	22.35	157

Which of the following plots would be the most useful in comparing the two sets of backpack weights?

Choose One Answer:

A. Histograms

B. Dot Plots

C. Scatter Plots

D. Box Plots

Answer

 D.

Measures of Central Tendency

1. Suppose that in a certain country, the average yearly income for 75% of the population is below average, what would you use as the measure of center and spread?

Choose one answer.

A. Mean and interquartile range

B. Mean and standard deviation

C. Median and interquartile range

D. Mean and standard deviation

Measures of Variation

1. The number of flaws of an electroplated automobile grill is known to have the following probability distribution:

X	0	1	2	3
P(X)	0.8	0.1	0.05	0.05

What would be the standard deviation of the sample means if we took 100 samples, each sample with 200 grills, and computed their sample means?

Choose One Answer.

A. 0.6275

B. 0.0560

C. None of the Above

D. 0.89269

2. Suppose that in a certain country, the average yearly income for 75% of the population is below average, what would you use as the measure of center and spread?

Choose one answer.

A. Mean and interquartile range

B. Mean and standard deviation

C. Median and interquartile range

D. Mean and standard deviation

III. Probability

Fundamentals

1. In a large midwestern university with 30 different departments, the university is considering eliminating standardized scores from their admission requirements. The university wants to find out whether the students agree with this plan. They decide to randomly select 100 students from each department, send them a survey, and follow up with a phone call if they do not return the survey within a week. What kind of sampling plan did they use?

Choose one answer.

A. Stratified random sampling

B. Simple random sampling

C. Multi-stage sampling

D. Cluster sampling

Rules for Computing Probabilities

1. A professor who teaches 500 students in an introductory psychology course reports that 250 of the students have taken at least one introductory statistics course, and the other 250 have not taken any statistics courses. 200 of the students were freshmen, and the other 300 students were not freshmen. Exactly 50 of the students were freshmen who had taken at least one introductory statistics course.

If you select one of these psychology students at random, what is the probability that the student is not a freshman and has never taken a statistics course?

A. 30%

B. 40%

C. 50%

D. 60%

E. 20%

2. A box contains 30 pens, where 5 are red, 14 are black, and 11 are blue. If you pick three pens from the box at random without replacement, what is the probability that these three pens will all be black?

Choose one answer.

A. 14/30 + 14/30 + 14/30

B. 14/30 + 13/29 + 12/28

C. 14/30 x 13/29 x 12/28

D. 1 - (14/30 x 13/29 x 12/28)

3. When three fair dice are simultaneously thrown, which of these three results is least likely to be obtained?

Choose one answer.

A. All three results are equally unlikely.

B. Two fives and a 3 in any order.

C. A 5, a 3 and a 6 in any order.

D. Three 5's.

4. Suppose that you take a three question "true/false" quiz for which you are completely unprepared. You have to guess the correct answer for each question. What is the probability of answering at least one question correctly?

Choose one answer.

A. 4/8

B. 5/8

C. 7/8

D. 1/8

E. 3/8

IV. Probability Distributions

Expectation(Mean) and Variance)

1. Ming’s Seafood Shop stocks live lobsters. Ming pays $6.00 for each lobster and sells each one for $12.00. The demand X for these lobsters in a given day has the following probability mass function.

X	0	1	2	3	4	5
P(x)	0.05	0.15	0.30	0.20	0.20	0.1

What is the Expected Demand?

Choose one answer.

A. 13.5

B. 3.1

C. 2.65

D. 5.2

V. Normal Probability Distribution

The Standard Normal Distribution

1. Weight is a measure that tends to be normally distributed. Suppose the mean weight of all women at a large university is 135 pounds, with a standard deviation of 12 pounds. If you were to randomly sample 9 women at the university, there would be a 68% chance that the sample mean weight would be between:

Choose one answer.

A. 131 and 139 pounds.

B. 133 and 137 pounds.

C. 119 and 151 pounds

D. 125 and 145 pounds.

E. 123 and 147 pounds.

Nonstandard Normal Distribution: Finding Probabilities

1. A researcher converts 100 lung capacity measurements to z-scores. The lung capacity measurements do not follow a normal distribution. What can we say about the standard deviation of the 100 z-scores?

Choose one answer.

A. It depends on the standard deviation of the raw scores

B. It equals 1

C. It equals 100

D. It must always be less than the standard deviation of the raw scores

E. It depends on the shape of the raw score distribution

VI. Relations Between Distributions

==The Central Limit Theorem==A. A smaller population standard deviation 1. Which of the following would make the sampling distribution of the sample mean narrower? Check all answers that apply.

Choose at least one answer.

A. A smaller population standard deviation

B. A smaller sample size

C. A larger standard error

D. A larger sample size

E. A larger population standard deviation

VII. Point and Interval Estimates

Estimating a Population Proportion

1. A 1996 poll of 1,200 African American adults found that 708 think that the American dream has become impossible to achieve. The New Yorker magazine editors want to estimate the proportion of all African American adults who feel this way. Which of the following is an approximate 90% confidence interval for the proportion of all African American adults who feel this way?

Choose one answer.

A. (.56, .62)

B. (.57, .61)

C. Can't be calculated because the population size is too small.

D. Can't be calculated because the sample size is too small.

VIII. Hypothesis Testing

Fundamentals of Hypothesis Testing

1. Suppose you were hired to conduct a study to find out which of two brands of soda college students think taste better. In your study, students are given a blind taste test. They rate one brand and then rated the other, in random order. The ratings are given on a scale of 1 (awful) to 5 (delicious). Which type of test would be the best to compare these ratings?

A. One-Sample t

B. Chi-Square

C. Paired Difference t

D. Two-Sample t

2. USA Today's AD Track examined the effectiveness of the new ads involving the Pets.com Sock Puppet (which is now extinct). In particular, they conducted a nationwide poll of 428 adults who had seen the Pets.com ads and asked for their opinions. They found that 36% of the respondents said they liked the ads. Suppose you increased the sample size for this poll to 1000, but you had the same sample percentage who like the ads (36%). How would this change the p-value of the hypothesis test you want to conduct?

Choose One Answer.

A. No way to tell

B. The new p-value would be the same as before

C. The new p-value would be smaller than before

D. The new p-value would be larger than before

3. A marketing director for a radio station collects a random sample of three hundred 18 to 25 year-olds and two hundred and fifty 25 to 40 year-olds. She records the percent of each group that had purchased music online in the last 30 days. She performs a hypothesis test, and the p-value of her test turns out to be 0.15. From this she should conclude:

Choose one answer.

A. that about 15% more people purchased on-line music in the younger group than in the older group.

B. there is insufficient evidence to conclude that there is a difference in the proportion of on-line music purchases in the younger and older group.

C. the proportion of on-line music purchasers is the same in the under-25 year-old group as in the older group.

D. the probability of getting the same results again is 0.15.

4. If we want to estimate the mean difference in scores on a pre-test and post-test for a sample of students, how should we proceed?

Choose one answer.

A. We should construct a confidence interval or conduct a hypothesis test

B. We should collect one sample, two samples, or conduct a paired data procedure

C. We should calculate a z or a t statistic

5. The paint used to make lines on roads must reflect enough light to be clearly visible at night. Let mu denote the true average reflectometer reading for a new type of paint under consideration. A test of the null hypothesis that mu = 20 versus the alternative hypothesis that mu > 20 will be based on a random sample of size n from a normal population distribution. In which of the following scenarios is there significant evidence that mu is larger than 20?

(i) n=15, t=3.2, alpha=0.05

(ii) n=9, t=1.8, alpha=0.01

(iii) n=24, t=-0.2, alpha=0.01

Choose one answer.

A. (ii) and (iii)

B. (i)

C. (iii)

D. (ii)

Testing a Claim About a Mean: Large Samples

1. Hong is a pharmacist studying the effect of an anti-depressant drug. She organizes a simple random sample of 100 patients, and then collect their anxiety test scores before and after administering the anti-depressant drug. Hong wants to estimate the mean difference between the pre-drug and post-drug test scores. How should she proceed?

Choose one answer.

A. She should compute a confidence interval or conduct a hypothesis test

B. She should calculate the z or the t statistics

C. She should compute the correlation between the two samples

D. Not enough information to tell

Testing a Claim About a Mean: Small Samples

1. To test the claim that the average home in a certain town is within 5.5 miles of the nearest fire station, and insurance company measured the distances from 25 randomly selected homes to the nearest fire station and found x-bar = 5.8 miles and sd = 2.4 miles. Determine what the insurance company found out with a test of significance. Check all that apply.

Choose at least one answer.

A. There is no evidence in the data to conclude that the distance is different from 5.5.

B. The average of 5.8 miles observed is by chance.

C. We cannot reject the null.

D. There is evidence in the data to conclude that the distance is 5.5.

X. Correlation and regression

Correlation

1. A positive correlation between two variables X and Y means that if X increases, this will cause the value of Y to increase.

A. This is always true.

B. This is sometimes true.

C. This is never true.

Answer

 C.

2. The correlation between high school algebra and geometry scores was found to be + 0.8. Which of the following statements is not true?

A. Most of the students who have above average scores in algebra also have above average scores in geometry.

B. Most people who have above average scores in algebra will have below average scores in geometry

C. If we increase a student's score in algebra (ie. with extra tutoring in algebra), then the student's geometry scores will always increase accordingly.

D. Most students who have below average scores in algebra also have below average scores in geometry.

Answer

 C.

3. Researchers discover that the correlation between miles ran per week and cardiovascular endurance is +0.75. They also discover that the correlation between hours spent watching television per week and cardiovascular endurance is -0.75. What is the conclusion that best characterizes the result of this study?

Choose one answer.

A. Most people who spend a lot of hours watching television have low cardiovascular endurance.

B. Most people who have good cardiovascular endurance spend a lot of time running and little time watching television.

C. Based on the correlation, if you increase your running hours per week, your cardiovascular endurance will decrease.

D. Based on the correlation, if you increases your television watching time, your cardiovascular endurance will decrease.

E. Most people with a lot of miles ran per week have high cardiovascular endurance.

Regression

1. Use the information from the Heights of Fathers and Sons to write the linear model that best predicts the height of the son from the height of the father.

Choose one answer.

A. Son's height = 35 + 0.5*Father's height'

B. Son's height = 1.00 + 1.00* Father's height

C. The model cannot be determined without the actual data

D. Son's height = 0.5 + 35*Father's height

Variation and Prediction Intervals

1. Two researchers are going to take a sample of data from the same population of physics students. Researcher A will select a random sample of students from among all students taking physics. Researcher B's sample will consist only of the students in her class. Both researchers will construct a 95% confidence interval for the mean score on the physics final exam using their own sample data. Which researcher's method has a 95% chance of capturing the true mean of the population of all students taking physics?

Choose one answer.

A. Research B

B. Researcher A

C. Both methods have a 95% chance of capturing the true mean

D. Neither

2. A random sample of 150 UCLA students found that 35% of the respondants wanted a elevator to replace Bruin Walk. A 95% confidence interval for the percentage of all UCLA students who feel this way is approximately:

Choose one answer.

A. (24%, 46%)

B. (32%, 38%)

C. The sample size is too small to compute a confidence interval.

D. (27%, 43%)

3. According to Terry Prachett, the short unit of time in the multiverse is the New York second, defined as the time interval between the light turning green and the cab behind you honking. A magazine took a poll of 100 New Yorkers and found that 90 people agree with that statement wholeheartedly. Which of the following is a 90% confidence interval for the proportion of people who agree with that statement?

Choose one answer.

A. 0.9 +\- 0.50

B. 0.9 +\- .05

C. 0.9 +\- .03

D. 0.9 +\- .06

XIII. Multinomial Experiments and Contingency Tables

References

• SOCR
• EBook
• UCLA Statistics Moodle

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