A. σ2B. σC. μD. x?
A.σ2 B.σ C.μ D.x̅
A.總是大于0 B.總是小于0 C.有時(shí)大于,有時(shí)小于0,取決于數(shù)據(jù)個(gè)體 D.總是等于0