Frequency formulas and coefficient of variation

For frequencies, SS=∑fx²-(∑fx)²/∑f. Population variance=SS/∑f; sample variance=SS/(∑f-1). Replace x by class midpoints for grouped estimates. With assumed mean A and d=x-A, SS=∑fd²-(∑fd)²/∑f. CV=100×SD/mean for positive ratio-scale means. A lower CV means less relative variation, not automatically better performance. CV is unstable near zero and often inappropriate when zero is arbitrary.