Epidemiology: 4) Exposure - Definition of Groups
Terms
undefined, object
copy deck
- Definition of groups
- Appropriate definition, classification and measurment of expsure. Analogous to randomization in CT
- Examples of exposure groups
-
Genetics
Age
Demographics
Lifestyle
Geographoc
Environmental
Disease
Meds
Devices
Education
Policies - 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