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.