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Hey all,

I was wondering how to calculate a combined mean and std from two groups. like for example;

group 1: mean 100.95/120, std=8.49

group 2: mean 25.44/30, std=3.21

i figured for the means you could just add 100.95+25.44 = 126.39, but wasn't sure how that works for std? thanks!
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> i figured for the means you could just add 100.95+25.44 = 126.39

When you say "combined mean" I'd ordinarily interpret that to mean "what is the mean if I treat group 1 and group 2 as one large group". If that's what you meant, the answer would not be 126.39. If that's NOT what you mean, you'll need to clarify the circumstances (what does "combined mean" mean to you?)

Also, please don't write your means as fractions. e.g. If you intend "the mean is 100.95 from a sample of size 120" please don't write it that way, it's dangerously ambiguous (e.g. one might as easily interpret 100.95 as the sum and then the mean really is 100.95/120 but you should then write "= 0.84125" so that it again is unambiguous.

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