Advanced Measures of Dispersion

Statistics — Third Secondary

Common mistakes

Comparison aspect	Main idea	Keywords
Dividing the sum of deviations by n	the sum of the deviations from the mean is always zero. The deviations must be squared first	meanalwayszeromust
Confusing the variance with the standard deviation	the variance is the square of the standard deviation and is in squared units. Take the square root to get σ	squaresquaredunitsTake
Using a class limit instead of the midpoint	the midpoint is the average of the two limits of the class	averagelimits
Forgetting the frequency when squaring deviations	in a frequency table each (x − x̄)² must be multiplied by its frequency before adding	tablemustmultipliedbefore
Comparing the spread of two different means using σ alone	use the coefficient of variation instead	coefficientvariationinstead
Believing that adding a constant changes the standard deviation	adding a constant changes the mean only	mean

Advanced Measures of Dispersion · 6 / 7