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.
Write the formula for the relative standard deviation as a percentage. It is RSD = (SD/Xbar) * 100, where SD is the standard deviation and Xbar is the mean. Perform the calculation for the mean, which is also called the average. The equation is Xbar = Xsum/N, where Xsum is the sum of all the data points, and N is the total number of points. For a data set consisting of points 1, 3, 3 and 5 write Xbar = (1 + 3 + 3 + 5)/4 = 12/4 = 3. Calculate the standard deviation.