Group Variances into Sum of Squares
GMAT glossary · Playbook
Definition
Convert each group variance into an internal sum of squares, add those pieces, then rebuild overall variance if needed.
Group Variances into Sum of Squares in plain English
Pull Group Variances into Sum of Squares when a variance or SSD ask splits into within-group pieces. Clues include Sum of squares, within-group variance, SSD. Blueprint: Convert each group variance into an internal sum of squares. Add those pieces, then rebuild the overall variance if needed. Why it beats the long way: Averaging variances as if they were means ignores group size and internal SS. Micro-example: Group A (n=4) has variance 2, so internal SS =2· 3=6 (using n-1 if that is the stem's convention). Group B (n=6) variance 3 gives SS =15. Add SS pieces before rebuilding an overall variance; averaging the two variances as if they were means ignores group size.
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