I have monthly data of sales for 3 firms (A, B and C) for 10 years (12*5=60 data points per firm). I want to test if the variance of sales in A is more than the variance in B which is more than the variance in C.
I calculated variances for each year. Thus, I had 10 values of variances for each firm. Then what? I am not comfortable with grouping in a year as it is clearly an arbitrary grouping.
How should I look at this problem? What is the ideal way to test? The average sale for the three firms is different, so, should COV be used instead of variance?