"The terms balanced and unbalanced are often used to describe whether a panel dataset is missing
some observations. If a dataset does not contain a time variable, then panels are considered
balanced if each panel contains the same number of observations; otherwise, the panels are unbalanced.

When the dataset contains a time variable, panels are said to be strongly balanced if each panel contains the
same time points, weakly balanced if each panel contains the same number of observations but not the same
time points, and unbalanced otherwise."

平衡就说明每一个截面上的数据观测值数量都是一样的，没有缺失观测值。