Understanding Understanding Observational Observational

StudiesStudiesLecture 9Lecture 9

Section 3.4Section 3.4

Fri, Feb 3, 2006Fri, Feb 3, 2006

Types of Observational Types of Observational StudiesStudies

Retrospective studyRetrospective study – The subjects are – The subjects are grouped by the response variable. Then grouped by the response variable. Then the explanatory variable is measured for the explanatory variable is measured for each group and compared.each group and compared.

Prospective studyProspective study – The subjects are – The subjects are grouped by the explanatory variable. grouped by the explanatory variable. Then the response variable is measured Then the response variable is measured for each group and compared.for each group and compared.

ExampleExample

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

ExampleExample

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

1260

16740

High-fiberdiet

2375

22625

Low-fiberdiet

Heart attack

No heart attack

Population #1p1 = 0.07

Population #2p2 = 0.095

ExampleExample

Describe the design of this study had Describe the design of this study had it been set up as a it been set up as a retrospectiveretrospective study.study.

ExampleExample

Describe the design of this study had Describe the design of this study had it been set up as a it been set up as a retrospectiveretrospective study.study.

1260 2375

16740 22625

High-fiberdiet

Low-fiberdiet

Heart attack

No heart attack

Population #1p1 = 0.653

Population #2p2 = 0.575

Prospective StudiesProspective Studies

In a prospective study, the In a prospective study, the explanatory variable’s value is often explanatory variable’s value is often its present value (but not always).its present value (but not always).

The value of the response variable is The value of the response variable is determined by tracking the subjects determined by tracking the subjects over weeks, months, or years into over weeks, months, or years into the future.the future.

Retrospective StudiesRetrospective Studies

In a retrospective study, the In a retrospective study, the response variable’s value is again response variable’s value is again often its present value (but not often its present value (but not always).always).

The value of the explanatory variable The value of the explanatory variable is determined by looking at past is determined by looking at past records or through interviews of records or through interviews of subjects.subjects.

Difficulties in Difficulties in Observational StudiesObservational Studies

In an observational study, In an observational study, The researchers cannot control the The researchers cannot control the

explanatory variables.explanatory variables. The researchers cannot control the The researchers cannot control the

confounding variables.confounding variables. In the previous example, is it In the previous example, is it plausibleplausible to believe that more of to believe that more of those on a low-fiber diet also smoke? those on a low-fiber diet also smoke? or did not get adequate exercise? or did not get adequate exercise?

Evidence of CausationEvidence of Causation

The following are evidence (not The following are evidence (not proof) of causation.proof) of causation. The The same associationsame association between the between the

explanatory and response variables explanatory and response variables occurs occurs in a variety of situationsin a variety of situations..

There is There is a plausible explanationa plausible explanation of how of how the explanatory variable could affect the explanatory variable could affect the response variable.the response variable.

There is There is no equally plausible third no equally plausible third factorfactor that could be affecting both the that could be affecting both the explanatory and the response variables.explanatory and the response variables.

BAD MonogramsBAD Monograms

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