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.
Vice versa, variance is standard deviation squared. To calculate standard deviation from variance, only take the square root. In our example, the variance was 200, therefore standard deviation is 14.14. For calculating standard deviation of a data set, first calculate the variance and then find the square root.or sample variance and standard deviation, the only difference is in step 4, where we divide by the number of items less one. In our example we would divide 1,000 by 4 (5 less 1) and get the sample variance of 250. Sample standard deviation would be 15.81 (square root of 250).