In the U.S., doctors usually get suspicious when a PSA level is 4.0 ng/mL. But studies show that many men with levels well above 4.0 ng/mL do not have cancer. And many men with prostate cancer have PSA scores well below this level. There are a lot of ways to tweak the PSA test to try to get better results.
The Standard Deviation Formula. In this formula, x is the value of the mean, N is the sample size, and x i represents each data value from i=1 to i=N.. The â symbol indicates that you must add up the sum. (x 1 - x) 2 + (x 2 - x) 2 + (x 3 - x) 2 + (x 4 - x) 2 + (x 5 - x) 2. . . + (x N-x) 2.
Unless otherwise noted, these formulas assume simple random sampling. 1  Sample mean = x = (Ï xi) / n. 2  Sample standard deviation = s = sqrt [ Ï (xi-x)2 / (n - 1) ]. 3  Sample variance = s2 = Ï (xi-x)2 / (n - 1). 4  Variance of sample proportion = sp2 = pq / (n - 1). 5  Pooled sample proportion = p = (p1 * n1 + p2 * n2) / (n1 + n2).