1. Understanding Observational Studies Lecture 9 Section 3.4 Fri, Feb 3, 2006

2. Types of Observational Studies Retrospective study – The subjects are grouped by the response variable. Then the explanatory variable is measured for each group and compared. Retrospective study – The subjects are grouped by the response variable. Then the explanatory variable is measured for each group and compared. Prospective study – The subjects are grouped by the explanatory variable. Then the response variable is measured for each group and compared. Prospective study – The subjects are grouped by the explanatory variable. Then the response variable is measured for each group and compared.

3. Example Example 3.7, p. 160 – High-fiber Diet Cuts Heart-attack Risk in Men. Example 3.7, p. 160 – High-fiber Diet Cuts Heart-attack Risk in Men.

4. Example 1260 16740 High-fiber diet 2375 22625 Low-fiber diet Heart attack No heart attack Population #1 p 1 = 0.07 Population #2 p 2 = 0.095

5. Example Describe the design of this study had it been set up as a retrospective study. Describe the design of this study had it been set up as a retrospective study.

6. Example 12602375 1674022625 High-fiber diet Low-fiber diet Heart attack No heart attack Population #1 p 1 = 0.653 Population #2 p 2 = 0.575

7. Prospective Studies In a prospective study, the explanatory variable’s value is often its present value (but not always). In a prospective study, the explanatory variable’s value is often its present value (but not always). The value of the response variable is determined by tracking the subjects over weeks, months, or years into the future. The value of the response variable is determined by tracking the subjects over weeks, months, or years into the future.

8. Retrospective Studies In a retrospective study, the response variable’s value is again often its present value (but not always). In a retrospective study, the response variable’s value is again often its present value (but not always). The value of the explanatory variable is determined by looking at past records or through interviews of subjects. The value of the explanatory variable is determined by looking at past records or through interviews of subjects.

9. Difficulties in Observational Studies In an observational study, In an observational study, The researchers cannot control the explanatory variables. The researchers cannot control the explanatory variables. The researchers cannot control the confounding variables. The researchers cannot control the confounding variables. In the previous example, is it plausible to believe that more of those on a low-fiber diet also smoke? or did not get adequate exercise? In the previous example, is it plausible to believe that more of those on a low-fiber diet also smoke? or did not get adequate exercise?

10. Evidence of Causation The following are evidence (not proof) of causation. The following are evidence (not proof) of causation. The same association between the explanatory and response variables occurs in a variety of situations. The same association between the explanatory and response variables occurs in a variety of situations. There is a plausible explanation of how the explanatory variable could affect the response variable. There is a plausible explanation of how the explanatory variable could affect the response variable. There is no equally plausible third factor that could be affecting both the explanatory and the response variables. There is no equally plausible third factor that could be affecting both the explanatory and the response variables.

11. BAD Monograms Do Bad Monograms Spell R.I.P.? Do Bad Monograms Spell R.I.P.? Do Bad Monograms Spell R.I.P.? Do Bad Monograms Spell R.I.P.?