## Glossary of Epidemiology: 7) Cohort Analysis - Disease Occurence

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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)