In this post we prove that SStotal = SSBG + SSWG and νtotal = νBG + νWG, where s2=SStotal/νtotal – variance. These are well known equations, used, say, in ANOVA. They are not obvious although the proof is easy.
So…
Variance decomposition (between group component and within group component)
Decompositions of sum of squares and degree of freedom