Analysis of 2x2 Tables and Confounding

m1, m0, n1, and n0 are marginal totals and n is the overall total

Prevalence is the proportion of sampled individuals that possess a condition of interest at a given point in time.

Incidence is the proportion of individuals that develop a condition of interest of interest of a period of time.

The Odds Ratio (OR) of an outcome are the ratio of the probabilty that the outcome occurs to the probability that the outcome does not occur:

OR = ((a / n1) / (c / n1)) / ((b/n0) / (d/n0)) = (a/c) / (b/d) = ad / bc

Risk Ratio (RR), or relative risk, compares the risk of a health event among one group with the risk among another group:

RR = (a / (a + c)) / (b / (b + d)) = (a / n1) / (b /n0)

The OR is always farther from 1 than RR (unless both equal 1). If a rare disease OR ~= RR