What's the formula to calculate the standard deviation knowing the mean and a certain probability, but not knowing all the n's.
http://stattrek.com/online-calculator/normal.aspx
For example, ^that calculator only requires mean and a probability of Z to calculate the mean… so how does it do that? What's the formula? And how can I do it by myself in a CAS software during an exam?
Best Answer
The formula for standard deviation is sqrt([sample size][probability of success](1-[probability of success])). To find the sample size from the mean and success rate, you divide the mean by the success rate. If mean=10 and success=0.2, you do 10/0.2 to get your sample size, or 50 in this case. Then you do sqrt(50*0.2*(1-0.2) to get about 2.83. There you go!