数据如下,如何实现每家公司仅保留cha值为0的上下各两行观测值(即仅要-4、 -3、0、2、3;-3、-1、0、1、2)?谢谢
Inst dual Treat_Post rongdate cha
.512848 0 0 2010 -4
.570212 0 0 2010 -3
.399854 0 0 2010 0
.498935 0 1 2010 2
.383895 0 1 2010 3
.426367 0 1 2010 4
.566829 0 1 2010 5
.531317 0 1 2010 6
.680776 1 1 2010 7
.756215 0 1 2010 8
.700878 0 1 2010 9
.630195 0 1 2010 10
.304788 0 0 2013 -7
.321428 0 0 2013 -6
.258634 0 0 2013 -5
.412071 0 0 2013 -3
.506493 0 0 2013 -1
.54188 0 0 2013 0
.370434 0 1 2013 1
.404242 0 1 2013 2
.394799 0 1 2013 3
.418461 0 1 2013 5