Lecture 3 (Ecologic Studies)

Question 1

Ecologic study

Answer 1

• Studies association between exposure and disease where units of analysis are groups (vs individuals)
• Involves assessment of correlation
• Descriptive OR analytic

Question 2

Ecological variables: def and 3 types

Answer 2

• Properties of grps, places, or organizations
  1) Aggregate variables
  2) Environmental variables
  3) Global variables

Question 3

Aggregate variables

Answer 3

-Summary of observations of individuals in grp (mean, median, proportion, rate)

Question 4

Environmental variables

Answer 4

• Physical characteristics of a place
• Usually have analogues at individual level, which vary
• Ex: pollution, rainfall

Question 5

Global variables

Answer 5

• Attribute of a grp, organization, or place for which there is no distinct analogue at the individual level
• Ex: pop density, existence of law, etc.

Question 6

Pros: ecological studies

Answer 6

• Convenient, inexpensive (2ndary data sources)
• Reflects group averages (substantial variability at individual level)
• Allows analysis of exposure’s effect when it doesn’t vary enough at individual level
• Most appropriate level of analysis for certain variables

Question 7

Linear regression analysis

Answer 7

-Finds mathematical model to describe disease variable (y) as a function of exposure variables (x)

y=mx+b or multiple linear regression formula

Question 8

Correlation coefficient

Answer 8

• Measure of association indicating degree to which 2 variables have linear relationship
• 1 < r < +1
• Higher absolute value: stronger linear relationship
• Close to 0: little/no linear relationship between variables
• Used for aggregate and individual level variables

Question 9

Limitations of correlation

Answer 9

• Doesn’t measure strength of non-linear rel
• n<50 pairs: reliability unclear
• Outliers can skew results
• Correlation =/= causation

Question 10

Coefficient of determination

Answer 10

• r2
• Proportion of variance in y explained by x in model
• How good line is at explaining data
• Does not measure strength of slope

Question 11

Ecologic Fallacy/Bias

Answer 11

• Association between variables at ecologic level might not translate to individual level
• Prob: heterogeneity of exposure w/in grps (are those exposed the ones developing disease?)
• Major limitation for making causal inferences