a=0.05a/2=0.0251-a/2=0.975查正态分布表第列x值1.9第行0.061.9行与0.06列交叉处对应值0.975反过来x=1.96对应概率0.975-1.96对应X0.025或者excel输入=NORMSINV(0.975)=1.959963985u(a/2)=1.96