32 CHAPTER 1 Introduction to Statistics Treatment Group: Women Bad experimental design: Treat all women subjects and give the men a placebo. (Problem: We don’t know if effects are due to gender or to treatment.) Completely randomized experimental design: Use randomness to determine who gets the treatment and who gets the placebo. Treat all women subjects. Placebo Group: Men Give all men a placebo. Treat these randomly selected subjects and give the others a placebo. (a) (b) Before After Alex Bob Chris Block of Women Randomized block design: 1. Form a block of women and a block of men. 2. Within each block, randomly select subjects to be treated. Matched pairs design: Get measurements from the same subjects before and after some treatment. Treat randomly selected women. Block of Men Treat randomly selected men. (c) (d) FIGURE 1-6 Designs of Experiments ■ Twins: A test of Crest toothpaste used matched pairs of twins, where one twin used Crest and the other used another toothpaste. Rigorously Controlled Design: Carefully assign subjects to different treatment groups, so that those given each treatment are similar in the ways that are important to the experiment. This can be extremely difficult to implement, and often we can never be sure that we have accounted for all of the relevant factors. Sampling Errors In statistics, you could use a good sampling method and do everything correctly, and yet it is possible to get wrong results. No matter how well you plan and execute the sample collection process, there is likely to be some error in the results. The different types of sampling errors are described here. Survey Pitfalls Surveys constitute a huge and growing business in the United States, but survey results can be compromised by many factors. A growing number of people refuse to respond; the average response rate is now about 22%, compared to 36% around the year 2000. A growing number of people are more difficult to reach because they use cell phones (no directories); about 15% of adults now have cell phones and no landlines, and they tend to be younger than average. There are obvious problems associated with surveys that ask respondents about drug use, theft, or sexual behavior, and a social desirability bias occurs when survey respondents are not honest because they don’t want to be viewed negatively by the person conducting the interview. S s c A l f
RkJQdWJsaXNoZXIy NjM5ODQ=