Q7Short answer
The mean and standard deviation of a set of n1 observations are x1 and s1, respectively while the mean and standard deviation of another set of n2 observations are x2 and s2, respectively. Show that the standard deviation of the combined set of (n1 + n2) observations is given by n1 ( s1 ) 2 + n2 ( s2 ) 2 n1n2 ( x1 − x2 ) 2 S.D. = + n1 + n2 (n1 + n2 ) 2 STATISTICS 279