Table 3.

Sample sizes required for 80% statistical power and type 1 error = 0.05 to detect bladder cancer in a cohort study of females for a range of arsenic concentrations.

Arsenic (μg/L)	Excess riska	RRb	Sample size (80% power)
100	0.012	2.28	672
50	0.006	1.64	2,381
20	0.0024	1.26	13,613
10c	0.0012	1.13	52,640
3	0.0004	1.04	462,527

^aEstimates from NRC (2001).

^bBackground [(excess lifetime risk + background lifetime risk) ÷ background lifetime risk. The lifetime risk for women up to age 85 years for bladder cancer, years 2004–2006, is 0.0094 (0.94%) (NCI 2010).

^cCurrent U.S. EPA MCL for arsenic is 10 μg/L (U.S. EPA 2001c).