# how to average SDs

• Apr 15th 2008, 05:58 AM
321
how to average SDs
I have a sample with two individuals and each is in a different population. One individual is x SDs above the one mean and the other is y SDs above the other mean. How do I take an average of x and y?
• Apr 15th 2008, 07:23 AM
CaptainBlack
Quote:

Originally Posted by 321
I have a sample with two individuals and each is in a different population. One individual is x SDs above the one mean and the other is y SDs above the other mean. How do I take an average of x and y?

What do you want this avaerage for, or rather what are you going to do with it?

RonL
• Apr 15th 2008, 11:59 AM
321
What I want to do is take a correlation coefficient, which I have, and multiply by the average standard deviation to compute a predicted regression towards the mean. I have data points on some variable for both individuals, but I need them to come from two separate population with different means. To make the problem clear, let each score 130 on some measurement. One comes from a population with a mean of 100 and the other from a population with a mean of 85. Let both populations have an SD of 15. One individual scores +2 SD and the other +3. I want an average of how high they score above the mean..
• Apr 15th 2008, 12:01 PM
321
If the answer is add the SDs and divide by two, this will be quite a dumb question.