Table 2. A 2×2 table for the effect of dichotomous exposure E on dichotomous outcome D, restricting to a level of a variable C, given causal relationships shown in Figure 2.

Because C is unaffected by E or D, this is equivalent to simple random sampling; we observe a fixed proportion of individuals regardless of values of E and D (in this case, some fraction f).

	D=1	D=0	Total
E=1	fA	fB	fA+fB
E=0	fC	fD	fC+fD
Total	fA+fC	fB+fD	fN