The Requirement That Is Built into the Calculation Of $s _ { X }$

Multiple Choice

The requirement that is built into the calculation of $s _ { X }$ , the requirement that costs one degree of freedom, is that

A) the null hypothesis is µ_l = µ₂ = 0
B) $\hat{ S }$ is always smaller than S
C) variances of the two populations must be equal
D) $\Sigma$ (X- $\bar { X }$ ) =0.