Let S? and 5'3 denote the variances of random samples, of sizes n and m respectively, from two independent distributions which are N011, 0'2) and N012, 0'2), respectively. Note that the two populations have different means but the same variances. You may use the fact that 11,312 + m8; 02 has the distribution X2 with (n + m 2) df to help you to nd a condence interval for the common unknown variance 02