Confidence intervals 置信区间 X±2x=xt2。a √n X u-1.645σxμ+1.645σx 90% Samples -1.96σx +196σx 95% Samples -2.580x +2.58σ 99% Samples90% Samples 95% Samples x _ Confidence Intervals 置信区间 x x  − 1.645  + 1.645   x  96 x − 1.96 + 1. x x  − 2.58  + 2.58 99% Samples n X Z X X Z   • =  • X _