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I have a set of unknown values, say 100 cases, with a mean of 10 and a standard deviation of 5. I know that a subgroup of 20 of the cases has a mean of 8 and a SD of 6.

How can I find the SD of the other 80 values? The mean will be ((10*100)-(20*8))/80 = 10.5. Is there a similar way of finding the SD or estimating it?

kstew
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    Hint: think in terms of sums-of-squares. SStotal = (SSwithin_1+SSwithin_2) + SSbetween. You know SStotal, SSwithin_1, and have estimated Mean_2. Knowing now all the 3 means you can compute SSbetween (see, e.g. Note [here](http://stats.stackexchange.com/a/158239/3277)). Then find SSwithin_2. Solved. – ttnphns Oct 11 '16 at 09:22

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