Sp=(n11)  s12+(n21)s22(n1+n22)

Description:

Math equation. The pooled estimate of the common standard deviation (Sp) is the square root of a quotient. The numerator of the quotient is the sum of first sample size minus 1 times the square of the first sample's standard deviation plus the second sample's sample size minus 1 times the square of the second sample's standard deviation. The denominator for the quotient is the sum of the two sample sizes minus 2.