I have a mean and an SD from a set of 20 results but i do not have 20 seperate results. Is there a way I can calculate a new SD if I add one more result to the result set by using the previous Standard deviation?
Many thanks for the replies guys. If you could explain layman terms I would be ever so grateful, I really do not understand the symbols and what they mean. I'm a web developer who has been handed this task. I can work out SD easily from a result set but the above problem is above my level of understanding.
Let be the original standard deviation, be the original mean, and = the number of data points you started with (20 in your case). Let be the new data point that you are now going to include. You are interested in calculating the new standard deviation and mean . The calculation is:
Hope this helps.
It's a bit awkward on a pocket calculator.
Take a look at this instead.
(1/22*((22-1)*0.327^2+4.33^2*22+5^2-1/(22+1)*(4.33*22+5)^2))^(1/2) - Wolfram|Alpha