‍从总体X中抽取容量为2的样本<img src="https://static.zxtiku.com/wangke/imgs/topic/5595375d3d448264cca889d475046faf.png"/>,设总体均值<img src="https://static.zxtiku.com/wangke/imgs/topic/c20b6b3cb8b2eefbf975d9e6506732d1.png"/>及方差<img src="https://static.zxtiku.com/wangke/imgs/topic/8ad44998ab2778026120ca62c2aefe55.png"/>均存在，在估计<img src="https://static.zxtiku.com/wangke/imgs/topic/c20b6b3cb8b2eefbf975d9e6506732d1.png"/>时，‍<img src="https://static.zxtiku.com/wangke/imgs/topic/7296f0a614cb28238cbc03e9d6593e79.png"/>比<img src="https://static.zxtiku.com/wangke/imgs/topic/7b9474bacd2893bb55846ff07ca67653.png"/>有效。‎