I have two lots and only N, mean and SD is reported. Now I would like to put them together to get a combined mean and SD. How do I do that? (My Mathcad is 8 Pro, if you are sending an example). Thanks in forward and Seasons Greetings.
If you only have exact distributional parameters, the formula Bonnie gave is correct. However, if you only have sample data, you should modify her formula for variance by using n1-1 everywhere in the formula instead of n1, and n2-1 instead of n2. (The attached file is supposed to be saved as version 8.)
but the formula given for the variance (sigma squared) of the merged distribution is only valid if the means of the two distributions are equal. For the general case refer to formulae in attched picture.