Explain Prostate specific antigen (PSA) not Normal in 60 year olds but the ln(PSA) is approximately Normal with = 0.3 and = 0.8 95% of ln(PSA) falls in 2 = 0.3 (2)(0.8) = 1.9 to 1.3 Thus, 2.5% are above ln(PSA) 1.3; take anti-log of 1.3: e1.3 = 3.67 Since only 2.5% of population has values greater than 3.67 use this as cut-point for suspiciously high results