Stat 217 – Review 1 Problems

 

1) Suppose that the observational units in a study are the patients arriving at an emergency room in a given day. For each of the following, indicate whether it can legitimately be considered a variable or not. If it is a variable, classify it as categorical (and if it is binary) or quantitative. If it is not a variable, explain why not.

a. Blood type

b. Waiting time

c. Mode of arrival (ambulance, personal car, on foot, other)

d. Whether or not men have to wait longer than women

e. Number of patients who arrive before noon

f. Whether or not the patient is insured

g. Number of stitches required

h. Whether or not stitches are required

i. Which patients require stitches

j. Number of patients who are insured

k. Assigned room number

 

2) Tennis players often spin a tennis racquet, and observe whether it lands with the logo facing up or down, to determine who serves first.  But is this really a 50-50 process, equally likely to land with the logo facing up or down?   To investigate this, a tennis player spun his racquet 100 times, and he obtained 46 “up” and 54 “down” results.  Does this provide much evidence against believing that spinning the racquet is really a fair 50-50 process? 

a. Produce a graph of these sample results.  Identify the statistic and use an appropriate symbol to represent it.

b. Define the parameter of interest in words.

c. State the null and alternative hypotheses corresponding to this research question.

d. Describe how you could use a coin to conduct a simulation analysis of this study and its result. 

Give sufficient detail that someone else could implement this simulation analysis based on your description.  Be sure to indicate how you would decide whether the observed data provide much evidence against believing that spinning the racquet is really a fair 50-50 process.

e. Which graph below could represent our null distribution? Explain.

 

f. What conclusion do you draw from the null distribution?  Is there convincing evidence against the belief that spinning the racquet this way is a fair 50/50 process?  Explain your reasoning.

 

3) Findings at James Madison University indicate that 21% of students eat breakfast 6 or 7 times a week.  A similar question was asked of a random sample of 159 Cal Poly students. Of the 97 who responded, 35 reported eating breakfast 6 or 7 times a week.  Is this convincing evidence that Cal Poly students have healthier breakfast habits than James Madison students?  More specifically, do more than 21% of all Cal Poly students eat breakfast 6 or 7 times weekly?

a. Define the population of interest and the sample being considered.

b. Define the parameter and the statistic for this study.

c. The p-value turns out to be around 0.001.  What conclusion would you draw from this p-value?

d. Provide an interpretation of this p-value as if to someone not taking a statistics class.

e. If you took another random sample of 159 Cal Poly students, which of your answers to part b would change?

f. What are your thoughts about the fact that only 97 out of the original random sample of 159 responded?

g. Suppose you plan to conduct a new study with a simple random sample of 1,590 Cal Poly students. Explain how you could obtain this sample.

h. Would this new sample size address the issue you identified in part f?

i. How would you expect this p-value in part c to change if of the 1,590 Cal Poly students you sample 36% reported eating breakfast 6 or 7 times a week (larger, smaller, or about the same)? Explain (without finding a new p-value!).

 

4) Return to the tennis racquet study:

a. Do you expect the normal approximation (aka Central Limit Theorem) to apply to this study?

 

The following computer output determines a 95% confidence interval for the probability a racquet lands up based on these results.

b. Interpret this 95% confidence interval in context.

c. How do you expect the confidence interval to change if the sample size was 200 and the sample proportion was still .46?

d. Based on your analyses, would it be legitimate to conclude that the probability a spun tennis racquet lands up is .5?  Discuss both the validity of this conclusion based your analyses and what it means to say “probability” in this context.

 

5) A sample of twenty Dordt College students is taken, four of whom (4/20=20%) say they study at least 35 hours per week during the academic year. At most state universities, the proportion of students who report studying at least 35 hours a week is 10%. We wish to see whether the Dordt sample provides strong evidence that the true proportion of Dordt students who study more than 35 hours a week is more than 10%.

 

Two different approaches were taken in order to yield a p-value.

 

Option #1. 1000 sets of 20 “coin tosses” were generated where the probability of heads was 10%. Out of the 1000 sets of tosses 129 sets had at least 4 heads occur, and so a p-value of 0.129 is obtained, showing little evidence that more than 10% of Dordt students study more than 35 hours a week.

 

Option #2. The Theory-Based Inference applet was used, generating a z-score of 1.49 with a p-value of 0.068, yielding moderate evidence that more than 10% of Dordt students study more than 35 hours a week.

 

One Proportion applet results (Option #1)

 

Theory Based Inference Applet (Option #2)

 

Briefly explain which p-value (Option #1 or Option #2) is more valid and why.

 

Note: Although the results obtained from the One Proportion applet are subject to some variation because it is based on simulation that is NOT the main reason for the discrepancies between the two p-values.

 

6) In a 1993 study, researchers took a sample of people who claimed to have had an intense experience with an unidentified flying object (UFO) and a sample of people who did not claim to have had such an experience (Spanos et al., 1993). They then compared the two groups on a wide variety of variables, including IQ. Suppose you want to test whether or not the average IQ of those who have had such a UFO experience is higher than 100, so you want to test H0: μ = 100 vs. Ha: μ > 100.

a. Identify clearly what the symbol μ represents in this context.

b. Is this a one-sided or a two-sided test? Explain how you can tell.

 

The sample mean IQ of the 25 people in the study who claimed to have had an intense experience with a UFO was 101.6; the standard deviation of these IQs was 8.9.

c. Does this information enable you to check the technical conditions completely? What needs to be true for this procedure to be valid?

d. Calculate the test (standardized) statistic and draw a sketch with shaded area corresponding to obtaining a test statistic as extreme or more extreme than this one observed for the sample of 25 UFO observers.

e. Estimate the value of the p-value from your sketch.

f. Write a sentence interpreting the p-value in the context of this sample and these hypotheses. Summarize the conclusion of your test in context.

 

 

7) The 2004 General Social Survey (GSS) interviewed a random sample of adult Americans. For one question the interviewer asked: “From time to time, most people discuss important matters with other people. Looking back over the last six months–who are the people with whom you discussed matters important to you? Just tell me their first names or initials.” The interviewer then recorded how many names or initials the respondent mentioned. Results are tallied in the following table.

Number of Close Friends

0

1

2

3

4

5

6

Total

Count (Number of Respondents)

397

281

263

232

128

96

70

1467

A histogram of the data:

 mean = 1.987 friends, SD = 1.7708 friends

a. Identify the observational units and variable in the study. Is the variable categorical or quantitative?

b. This distribution is sharply skewed to the             , but a t-interval is still valid. Explain why.

c. Use the 2SD approach to approximate a 95% confidence interval for the mean number of close friends in the population of American adults.

d. Which two of the following are reasonable interpretations of this confidence interval and its confidence level:

e. For one of the incorrect interpretations in part d, explain why it is incorrect.

f. Describe how the interval would change if all else remained the same except

• The sample size was larger.

• The sample mean was larger.

• The sample values were less spread out.

• Every person in the sample reported one more close friend.

 

 

8) A national survey of 47,000 American households in 2006 found that 32.4% of the households included a pet cat. Assume this is a representative sample of American households. Consider the following output.

(a)  Identify the sample and the population in this context.

(b)  Write a one-sentence interpretation of the value z = -4.14 in this context.

(c)   Based on this output, is there convincing evidence that the proportion of all American households that include a pet cat diffs from 1/3?

(d)  Based on this output, is there evidence that the population of all American households that include a pet cat is much different from 1/3?