Like combined mean, the combined variance or standard deviation can be calculated for different sets of data. Suppose we have two sets of data containing and  observations with means and , and variances and . If is the combined mean and is the combined variance of observations, then combined variance is given by
           
            It can be written as
           
            Where
The combine standard deviation can be calculated by taking the square root of.

Example:
            For a group of 50 male workers the mean and standard deviation of their daily wages are $ 63 and $ 9 respectively. For a group of 40 female workers these values are $ 54 and $ 6 respectively. Find the mean and variance of the combined group of 90 workers.
Solution:
            Here, ,
                    , ,
            Combined Arithmetic Mean
                                                         
            Combined Variance