LESSON 5

MEASURES OF EFFECT

Present measures of association/effect/impact first and then study designs?

Epi Kept Simple (Epi Info, SAS, SPSS)

Table 8.3. HIV Infection and Intravenous Drug Use (IVDU) in Women entering New York State Prison System.

	Disease
Exposure	HIV+	HIV-
IVDU +	61	75	136
IVDU -	27	312	339
	88	387	475

ActivEpi

	Exposure
Disease	IVDU+	IVDU-
HIV +	61	27	88
HIV -	75	312	387
	136	339	475

All Exams will present tables as shown in ActivEpi

The Odds Ratio

Example of case-control study – Haiti – diarrheal disease

Case-control study - Cannot compute risks, risk ratio. Can compute the odds ratio.

Estimated odds ratio: ratio of the odds of exposure for cases divided by the odds of exposure for controls. Example:

Interpretation of odds ratio?

The odds of exposure for cases was about 3.2 times the odds of exposure for controls.

Calculating the Odds Ratio

Alternative odds formula:

Odds of exposure in cases = a/b

Odds of exposure in controls = c/d

Odds ratio = (a/b) / (c/d) = ad/bc (cross product ratio)

	OR	RR
Case-Control Studies	√	No
Follow-up Studies	√	√

Comparing the Risk Ratio and the Odds Ratio

Note: except when RR=1, the ROR is always further away from the null than the RR

If a disease is "rare", then the risk odds ratio will closely approximate the risk ratio.

Comparison of RR with ROR when disease infrequent

5-4 The Rate Ratio and its Characteristics

The Rate Ratio

A rate ratio is a ratio of two average rates. It is sometimes called an incidence density ratio or a hazard ratio.

Odds Ratio Approximation to Rate Ratio

vUnder steady state conditions, OR from case-control study approximates IDR from comparable cohort study. (Steady-state means no major shift in the demographic make-up of source population.)

vThis approximation does not require the rare disease assumption.