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.
The population standard deviation is calculated using =STDEV(VALUES) and in this case the command is =STDEV(A2:A6) which produces an answer of 0.55. The sample standard deviation will always be greater than the population standard deviation when they are calculated for the same dataset. This is because the formula for the sample standard deviation has to take into account the possibility of there being more variation in the true population than has been measured in the sample. Based on their sample of 5 cars, and therefore using the sample standard deviation, the manufacturers could state with 99% confidence that similar cars will do between 14.57 (MEAN-3SD) and 17.87 (MEAN+3SD) kilometres per litre .