Chapter 6. Experiments in the Real World. Chapter 6 1

Transcription

1 Chapter 6 Experiments in the Real World Chapter 6 1

2 Thought Question 1 Suppose you are interested in determining if drinking a glass of red wine each day helps prevent heartburn. You recruit 40 adults age 50 and older to participate in an experiment. You want half of them to drink a glass of red wine each day and the other half to not do so. You ask them which they would prefer, and 20 say they would like to drink the red wine and the other 20 say they would not. You ask each of them to record how many cases of heartburn they have in the next six months. At the end of that time period, you compare the results reported from the two groups. Give three reasons why this is not a good experiment. Chapter 6 2

3 Experiments: Some Techniques Double-blinding to control experimenter/respondent bias Pairing or blocking to reduce a source of variability in responses the same or similar subjects receive each treatment different from a completely randomized design, where all subjects are allocated at random among all treatments Chapter 6 3

4 Double-Blinded: Case Study Quitting Smoking with Nicotine Patches (JAMA, Feb. 23, 1994, pp ) Variables: Explanatory: Treatment assignment Response: Cessation of smoking (yes/no) Double-blinded Participants do not know which patch they received Nor do those measuring smoking behavior Chapter 6 4

5 Pairing or Blocking: Case Study Quitting Smoking with Nicotine Patches (JAMA, Feb. 23, 1994, pp ) Variables: Explanatory: Treatment assignment Response: Cessation of smoking (yes/no) Blocking? consider men and women separately? could use a matched-pairs design self-pairing? Not if cessation is the goal Chapter 6 5

6 Experiments: Difficulties and Disasters Extraneous variables Confounding variables (in chapter 5) Interacting variables Hawthorne, placebo and experimenter effects Refusals, nonadherers, dropouts Extending the results (generalizing) Chapter 6 6

7 Interacting Variables The problem: effect of explanatory variable on response variable may vary over levels of other variables. The solution: measure and study potential interacting variables. does the relationship between explanatory and response variables change for different levels of these interacting variables? if so, report results for different groups defined by the levels of the interacting variables. Chapter 6 7

8 Interacting Variables: Case Study Quitting Smoking with Nicotine Patches (JAMA, Feb. 23, 1994, pp ) Researchers considered: smoker at home found this to be an interacting variable: Percent quitting Nicotine Placebo Smoker at home 31% 20% No smoker at home 58% 20% other variables: age, weight, depression no interactions found Chapter 6 8

9 Hawthorne, Placebo and The problem: Experimenter Effects people may respond differently when they know they are part of an experiment. The solution: use placebos, control groups, and doubleblind studies when possible. Chapter 6 9

10 Hawthorne, Placebo and Experimenter Effects : Case Study I 1920 s Experiment by Hawthorne Works of the Western Electric Company What changes in working conditions improve productivity of workers? More lighting? Less lighting? Other changes? All changes improved productivity! Workers behaved differently when they knew they were part of a study. Chapter 6 10

11 Hawthorne, Placebo and Experimenter Effects : Case Study II Experimenter Effects in Behavioral Research (Rosenthal, 1976, Irvington Pub., p. 410) Teachers given a list of student names told these were students who would show unusual academic development. IQ was measured at end of year first graders on list: 15 points higher second graders on list: 9.5 points higher older: no striking difference Great expectations = self-fulfilling prophecy students were randomly selected (did not have high IQ) Chapter 6 11

12 Extending the Results ( Can We Generalize? ) The problem: lack of generalizability due to: unrealistic treatments unnatural settings sample that is not representative of population The solution: Researchers should use natural settings with a properly chosen sample. Chapter 6 12

13 Extending the Results : Case Study Does Aspirin Prevent Heart Attacks? (NEJM, Jan. 28, 1988, pp ) Participants were measured in their natural setting (at home) Only healthy male physicians were participants Results may not apply to: male physical laborers women Chapter 6 13

14 Key Concepts Double-Blind Experiment Difficulties and Disasters Experimental Designs Completely Randomized Design Matched Pairs Design Block Design Chapter 6 14