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 casecontrol study – Haiti – diarrheal disease
Casecontrol
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 

CaseControl Studies 
√ 
No 
Followup Studies 
√ 
√ 
_{}
_{}
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
54 The Rate Ratio and its Characteristics
A rate ratio is a ratio of two average
rates. It is sometimes called an incidence density ratio or a hazard
ratio.
vUnder steady state
conditions, OR from casecontrol study approximates IDR from comparable cohort
study. (Steadystate means no major shift in the demographic makeup of source
population.)
vThis approximation does not
require the rare disease assumption.