1. The Practice of Statistics, 5th Edition Starnes, Tabor, Yates, Moore Bedford Freeman Worth Publishers CHAPTER 10 Comparing Two Populations or Groups 10.1 Comparing Two Proportions

2. Learning Objectives After this section, you should be able to: The Practice of Statistics, 5 th Edition2 DESCRIBE the shape, center, and spread of the sampling distribution of the difference of two sample proportions. DETERMINE whether the conditions are met for doing inference about p 1 − p 2. CONSTRUCT and INTERPRET a confidence interval to compare two proportions. PERFORM a significance test to compare two proportions. Comparing Two Proportions

3. The Practice of Statistics, 5 th Edition3 Introduction Suppose we want to compare the proportions of individuals with a certain characteristic in Population 1 and Population 2. Let’s call these parameters of interest p 1 and p 2. The ideal strategy is to take a separate random sample from each population and to compare the sample proportions with that characteristic. What if we want to compare the effectiveness of Treatment 1 and Treatment 2 in a completely randomized experiment? This time, the parameters p 1 and p 2 that we want to compare are the true proportions of successful outcomes for each treatment. We use the proportions of successes in the two treatment groups to make the comparison.

4. The Practice of Statistics, 5 th Edition4 The Sampling Distribution of a Difference Between Two Proportions To explore the sampling distribution of the difference between two proportions, let’s start with two populations having a known proportion of successes. At School 1, 70% of students did their homework last night At School 2, 50% of students did their homework last night. Suppose the counselor at School 1 takes an SRS of 100 students and records the sample proportion that did their homework. School 2’s counselor takes an SRS of 200 students and records the sample proportion that did their homework.

5. The Practice of Statistics, 5 th Edition5 The Sampling Distribution of a Difference Between Two Proportions Using Fathom software, we generated an SRS of 100 students from School 1 and a separate SRS of 200 students from School 2. The difference in sample proportions was then be calculated and plotted. We repeated this process 1000 times.

6. The Practice of Statistics, 5 th Edition6 The Sampling Distribution of the Difference Between Sample Proportions Choose an SRS of size n 1 from Population 1 with proportion of successes p 1 and an independent SRS of size n 2 from Population 2 with proportion of successes p 2. The Sampling Distribution of a Difference Between Two Proportions

7. The Practice of Statistics, 5 th Edition7 The Sampling Distribution of a Difference Between Two Proportions

8. The Practice of Statistics, 5 th Edition8 The Sampling Distribution of a Difference Between Two Proportions Suppose that there are two large high schools, each with more than 2000 students, in a certain town. At School 1, 70% of students did their homework last night. Only 50% of the students at School 2 did their homework last night. The counselor at School 1 takes an SRS of 100 students and records the proportion that did homework. School 2’s counselor takes an SRS of 200 students and records the proportion that did homework

9. The Practice of Statistics, 5 th Edition9 Confidence Intervals for p 1 – p 2 If the Normal condition is met, we find the critical value z* for the given confidence level from the standard Normal curve.

10. The Practice of Statistics, 5 th Edition10 Confidence Intervals for p 1 – p 2 Conditions For Constructing A Confidence Interval About A Difference In Proportions Random: The data come from two independent random samples or from two groups in a randomized experiment. o 10%: When sampling without replacement, check that n 1 ≤ (1/10)N 1 and n 2 ≤ (1/10)N 2.

11. The Practice of Statistics, 5 th Edition11 Confidence Intervals for p 1 – p 2 Two-Sample z Interval for a Difference Between Two Proportions

12. The Practice of Statistics, 5 th Edition12 Significance Tests for p 1 – p 2 An observed difference between two sample proportions can reflect an actual difference in the parameters, or it may just be due to chance variation in random sampling or random assignment. Significance tests help us decide which explanation makes more sense. The null hypothesis has the general form H 0 : p 1 - p 2 = hypothesized value We’ll restrict ourselves to situations in which the hypothesized difference is 0. Then the null hypothesis says that there is no difference between the two parameters: H 0 : p 1 - p 2 = 0 or, alternatively, H 0 : p 1 = p 2 The alternative hypothesis says what kind of difference we expect. H a : p 1 - p 2 > 0, H a : p 1 - p 2 < 0, or H a : p 1 - p 2 ≠ 0

13. The Practice of Statistics, 5 th Edition13 Significance Tests for p 1 – p 2 Conditions For Performing a Significance Test About A Difference In Proportions Random: The data come from two independent random samples or from two groups in a randomized experiment. o 10%: When sampling without replacement, check that n 1 ≤ (1/10)N 1 and n 2 ≤ (1/10)N 2.

14. The Practice of Statistics, 5 th Edition14 Significance Tests for p 1 – p 2 If H 0 : p 1 = p 2 is true, the two parameters are the same. We call their common value p. We need a way to estimate p, so it makes sense to combine the data from the two samples. This pooled (or combined) sample proportion is:

15. The Practice of Statistics, 5 th Edition15 Significance Tests for p 1 – p 2 Two-Sample z Test for the Difference Between Two Proportions

16. The Practice of Statistics, 5 th Edition16 Inference for Experiments Many important statistical results come from randomized comparative experiments. Defining the parameters in experimental settings is more challenging. Most experiments on people use recruited volunteers as subjects. When subjects are not randomly selected, researchers cannot generalize the results of an experiment to some larger populations of interest. Researchers can draw cause-and-effect conclusions that apply to people like those who took part in the experiment. Unless the experimental units are randomly selected, we don’t need to check the 10% condition when performing inference about an experiment.

17. Section Summary In this section, we learned how to… The Practice of Statistics, 5 th Edition17 DESCRIBE the shape, center, and spread of the sampling distribution of the difference of two sample proportions. DETERMINE whether the conditions are met for doing inference about p 1 − p 2. CONSTRUCT and INTERPRET a confidence interval to compare two proportions. PERFORM a significance test to compare two proportions. Comparing Two Proportions