在處理縱向數據（longitudinal data）時綜合指標（summary measures/response feature）也是一個很重要的分析工具，比如一些臨床試驗往往會收集多個時間點上的同一個指標，將多個時間點的該指標總和成一個指標便於試驗的分析
The use of summary measures is one of the most important and straightforward approaches to the analysis of longitudinal data (at least when there are few missing values, and when the repeated measurements on each subject are made at the same time points). If the measurements made on the ith individual in the study are written as the vector x'i = [xi1, . . . , xiT ], then a scalar-valued function f is choosen so that si = f(xi) captures some essential feature of the response over time
