纵向数据也叫做面板数据，Panel data，就是对一组截面进行跟踪调查的数据，见伍德里奇的书。

In a longitudinal study,individuals are followed over a period of time and data are collected for each individual at mutiple time points.These data,which are collected repeatedly over time for each individual,are called longitudinal data.The basic modesl are mixed effects  models.