I'm interested in calculating standard deviation and have read your web site.
I hope to get your help:
If I have three groups of data A, B and C,
the mean values for A =a, B =b, and C =c,
the standard deviation(SD) for A =SD1, B =SD2, and C =SD3,
then what is the standard deviation for the average value of A, B and C, namely (A+B+C/3)?
Could you please tell me a formula to calculate their SD. If there are more groups of data?
Thank you very much.
I have used your method to calculate my data, but the final standard deviations are much smaller than any standard deviation from every data sets. Is it ok? It is not in the range between max and min values?
If I have two data sets A and B, with means a and b, and standard deviations SDa and SDb, respectively.
Now I need to calculate the ratio:
a/b, then what is the final standard deviation for the ratio?
Thank you again.