Epidemiology: 7) Cohort Analysis  Disease Occurence
Terms
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 What/Why is a measure of disease occurence for?

Quantify the likelihood of disease development in a single group.
1)Risk
2)Odds
3)Incidence rates  4 purpose of cohort analysis

1)Measure of the occurence of D
2)Measure the effect of E on the occurrence of D
3)Control for confounding by other differences between E+ and E
4)Test/Identify effect modification  Risk
 The probability that a person who initially does not have the disease will develop it by the end of fixed or defined period
 3 major components of risk

1)Probability (between 0 and 1)
2)simple: calculate proportion who develop D
3)Can use lifetable method  Synonyms of risk

Cumulative incidence
Cumulative incidence rate
Cumulative incidence proportion  Odds

Odds = p/(1p)
p=risk  Incidence Rate
 likelihood of occurerence for binary outcomes that quantifies the instantaneous rate of disease development.
 3 What NOT to do for variable followup

1)Ignore and estimate risk as proportion
2)Take shortest period of followup
3)Never allow  always do fixed period studies  What to with variable followup
 Estimate incidence rate and calculate risk if strict assumptions hold true
 Synonyms for Incidence

1)Incidence rate
2)Incidence density
3)Hazard function
4)Force morbidity/mortality  4 characteristics of Incidence rate

1)Varies between 0 and inf
2)[events/time]
3)Magnitude of rate depends upon time unit
4)often not constant over time  Can use persontime methods with 3 assumptions

1)Incidence is constant over time
2)Incidence is homogenous among persons in the population
3)Loss to followup or censoring is not associated with D  Incidence eqn if assumptions hold
 I=(# of events)/(total persontime)
 2 equations for estimating risk based on incidence

I must be constant
1)Risk(t) = 1exp(I*t)
2)If I*t < 0.1: Risk(t)=I*t  For nonconstant I
 Use survival analysis (life tables)