1. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Chapter 13 Experiments and Observational Studies

2. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 2 Observational Studies observational study- a study based on data in which no manipulation of factors has been employed. For example, finding out if playing video games with violent content makes kids more prone to violent behavior.

3. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 3 Observational Studies (cont.) retrospective study- an observational study in which subjects are selected and then their previous conditions or behaviors are determined. A study that makes use of historical information. For example, we find people that are already dead and try to figure out why they die.

4. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 4 Observational Studies (cont.) prospective study an observational study in which subjects are follow to observe future outcomes. For example, we select a group of subjects and sit around and watch them for a decade.

5. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 5 Randomized, Comparative Experiments An experiment is a study design that allows us to prove a cause-and-effect relationship. An experiment: Manipulates factor levels to create treatments. Randomly assigns subjects to these treatment levels. Compares the responses of the subject groups across treatment levels. In an experiment, the experimenter must identify at least one explanatory variable, called a factor, to manipulate and at least one response variable to measure.

6. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 6 Randomized, Comparative Experiments (cont.) In general, the individuals on whom or which we experiment are called experimental units. When humans are involved, they are commonly called subjects or participants. The specific values that the experimenter chooses for a factor are called the levels of the factor. A treatment is a combination of specific levels from all the factors that an experimental unit receives.

7. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 7 The Four Principles of Experimental Design 1.Control: We control sources of variation other than the factors we are testing by making conditions as similar as possible for all treatment groups. 2.Randomize: Randomization allows us to equalize the effects of unknown or uncontrollable sources of variation.

8. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 8 The Four Principles of Experimental Design (cont.) 3.Replicate: is the repetition of an experiment using a large group of subjects. To test a vaccine against a strain of influenza, 10,000 people are given the vaccine and another 10,000 people are given a placebo. Because of the sample size, the effectiveness of the vaccine would most likely be observed.

9. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 9 4.Block : Divide subjects with similar characteristics into blocks, and then within each block, randomly assign subjects to treatment groups. An experimenter testing the effects of a new weight loss drink may first divide the subjects into age categories. Then within each age group, randomly assign subjects to either the treatment group or control group.

10. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 10 In a retrospective or prospective study, subjects are sometimes paired because they are similar in ways not under study. Matching subjects in this way can reduce variability in much the same way as blocking.

11. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 11 Blinding In order to avoid the bias that might result from knowing what treatment was assigned, we use blinding. When every individual in either one of these classes is blinded, an experiment is said to be single-blind. When everyone in both classes is blinded, the experiment is called double-blind.

12. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley A “fake” treatment that looks just like the treatment being tested is called a placebo. Placebos are the best way to blind subjects from knowing whether they are receiving the treatment or not. The placebo effect occurs when taking the null treatment results in a change in the response variable. Placebo controls are so effective that you should use them as an essential tool for blinding whenever possible. The best experiments are usually: randomized, comparative, double-blind, placebo-controlled. Placebos

13. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 13 When the levels of one factor are associated with the levels of another factor, we say that these two factors are confounded. For example, you want to know if Drug X has an effect on causing sleep. The experimenter must take care to design the experiment so that he can be very sure that the subjects in the study fell asleep because of the influence of his Drug X, and that the sleepiness was not caused by other factors. Those other factors would be confounding variables.

14. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 14 Control Treatments Often, we want to compare a situation involving a specific treatment to the status quo situation. A baseline (“business as usual”) measurement is called a control treatment, and the experimental units to whom it is applied is called the control group. For example, a plant is let to grow naturally while another may have the tips of its shoot cut off. The shoot with its tips cut off has no treatment control.

15. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 13- 15 Differences that are larger than we’d get just from the randomization alone are called statistically significant.