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.
1 As a random variable the sample mean has a probability distribution, a mean Î¼X- , and a standard deviation ÏX-. 2  There are formulas that relate the mean and standard deviation of the sample mean to the mean and standard deviation of the population from which the sample is drawn.ind all possible random samples with replacement of size two and compute the sample mean for each one. Use them to find the probability distribution, the mean, and the standard deviation of the sample mean X-.