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 life-table method
Synonyms of risk
Cumulative incidence
Cumulative incidence rate
Cumulative incidence proportion
Odds
Odds = p/(1-p)
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 follow-up
1)Ignore and estimate risk as proportion
2)Take shortest period of follow-up
3)Never allow - always do fixed period studies
What to with variable follow-up
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 person-time methods with 3 assumptions
1)Incidence is constant over time
2)Incidence is homogenous among persons in the population
3)Loss to follow-up or censoring is not associated with D
Incidence eqn if assumptions hold
I=(# of events)/(total person-time)
2 equations for estimating risk based on incidence
I must be constant
1)Risk(t) = 1-exp(-I*t)
2)If I*t < 0.1: Risk(t)=I*t
For non-constant I
Use survival analysis (life tables)

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