P^= 30/50 = 0.6
n=50>30 ,by C.L.T. ,利用Z檢定
樞紐量 Z = (P^-P)/√[p(1-p)/n] ~N(0,1)
P之95% C.I. 為 P^±Z0.025√[p(1-p)/n] =0.6±1.96√[0.6*0.4/50]
→(0.4642 , 0.7358)