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I'd like to be able to calculate the sample variance for a subpopulation [B] given the sample mean and variance for its complement [A] and the overall population [A+B].

I have the sample mean and variance for

  • Overall population [A+B]
  • Subpopulation [A]

I know I can calculate the mean for B as:

  mean[B] = ( mean[A+B] * (n[A] + n[B]) - mean[A] * n[B]) / nB

But I don't know if it's easy to calculate the sample variance for the complement in a similar way.

prototype
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  • Ah, I think it's rearranging teh final line of this answer to solve for s2[n+1:n+m] http://stats.stackexchange.com/questions/43159/how-to-calculate-pooled-variance-of-two-groups-given-known-group-variances-mean?rq=1 – prototype Oct 27 '14 at 20:15
  • vB = ((v0 + m0*m0)*n0 - (vA + mA*mA)*nA)/nB - mB*mB; – prototype Oct 27 '14 at 21:26

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