1. COMPARING TWO PROPORTIONS Chapter 22 Part 1

2. Subscripts Use subscripts when comparing two groups.

3. Subscripts You can use numbers or letters. If you are comparing a group of females to a group of males, you might use subscripts of F and M.

4. Stating the Hypotheses We want to know if the true proportions are different. The null hypothesis would state that they are the same.

5. Check Conditions & Model You must check the conditions for both groups when comparing two proportions.

6. Mechanics: Two-Proportion z-interval

7. Example: A recent survey of 886 randomly selected teenagers (aged 12-17) found that more than half of them had online profiles. Some researchers and privacy advocates are concerned about the possible access to personal information about teens in public places on the Internet. There appear to be differences between boys and girls in their online behavior. Among teens aged 15-17, 57% of the 248 boys had posted profiles, compared to 70% of the 256 girls. List the information that is given using proper subscripts.

8. Example: A recent survey of 886 randomly selected teenagers (aged 12-17) found that more than half of them had online profiles. Some researchers and privacy advocates are concerned about the possible access to personal information about teens in public places on the Internet. There appear to be differences between boys and girls in their online behavior. Among teens aged 15-17, 57% of the 248 boys had posted profiles, compared to 70% of the 256 girls. What’s the standard error of the difference in sample proportions?

9. Example: There appear to be differences between boys and girls in their online behavior. Among teens aged 15-17, 57% of the 248 boys had posted profiles, compared to 70% of the 256 girls. What does a 95% confidence interval say about the difference in online behavior? We can be 95% confident that among teens aged 15-17, the proportion of girls who post online profiles is between 4.7% and 21.3% higher than the proportion of boys who do. There is evidence to suggest that teen girls are more likely to post profiles than are boys the same age.

10. Example: There appear to be differences between boys and girls in their online behavior. Among teens aged 15-17, 57% of the 248 boys had posted profiles, compared to 70% of the 256 girls. What does it mean to say that the difference in proportions is significant? It’s very unlikely that samples would show an observed difference this large due to chance.

11. Today’s Assignment :  Homework #12: page 519 #1, 2, 9-14  Homework #11 due tomorrow (Wednesday)