Table 1.

Summary of the causal parameters (in %) over 5000 simulations under simple random sampling of size n={100, 500, 2000} and the authors’ biased sampling scheme of size n=2000.

	Sample average treatment effect
	Min	Median	Max	Variance
n=100 simple	−14.8	−5.8	9.2	0.10
n=500 simple	−10.2	−5.6	0.2	0.02
n=2000 simple	−8.0	−5.5	−2.9	0.006
n=2000 biased	−12.8	−10.4	−8.3	0.005