Table 1. Required numbers of cases in a case–control study of childhood asthma and traffic air pollution.

	Odds ratio = 1.5		Odds ratio = 1.7
Proportion living near roads	3%	4%	3%	4%
Power
90%	2320	1766	1297	989
80%	1684	1282	943	719
70%	1292	984	725	553
60%	1000	762	563	430
50%	765	583	432	330