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Epidemiology: 4) Exposure - Definition of Groups


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Definition of groups
Appropriate definition, classification and measurment of expsure. Analogous to randomization in CT
Examples of exposure groups
6 - Exposure checklist
1) External E-
2) Internal E-
3) Zero time
4) Classification
5) Measurement
6) Change w\ follow-up
Internal E-
cohort is single group of subjects with uniform procedures.
Classification (E+ or E-) done at same time (t0).
Can be matched or entire E-
Increasing comparability of E- group
try and restrict E- to subjects most similar to E+ (some covariate that makes them very similar to E+)
Using alternative exposure for E-
careful of counfounding. Can show erranous protective effect. Should try and use plocebo or no exposure.
Temporal E- comparisons: Natural crossover
uses person time of exposure compared to person time of unexposed.
t0 should be at initial intervention..not follow/up
External E-
Group from a different population.
Different procedures to identify and collect data.
3 Fundamental weakness of E-
1)Effect of different procedures cannot be separated from the effect of E.
2)Entire group risk of D may be different (healthy worker effect)
3)Usually E+ data more complete
External E - Comparison at Different institutions
Must take care that patients are treated exactly the same.
External E: Control group membership influenced by D
E- Should have same chance of getting exposure an getting outcome as E+
Zero time
inception cohort - patients should be identified at an early and unambiguous point in the course of their disease. It is a challege for E-
Reference category
reference group with which others are compares in calculating measured effect (often lowest quintile of exposure).
3 ways to Constructing Categories
1)Intrinsically meaningful divisions
2)Percentiles (if no a priori H)
3)Don't torture the data

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