use 0624b\1,clear
collapse (mean) gpa , by(year)
list
use https://www.stata-press.com/data/r16/college,clear
collapse (mean) mean_gpa=gpa, by(year)
list
use https://www.stata-press.com/data/r16/college,clear
collapse (mean) gpa hour, by(year)
list
*使用collapse分组计算统计量
*collapse能够将变量数据按照对应组别转换为多种统计量
h collapse
sysuse auto,clear
*根据维修次数(rep78)计算price的平均值
collapse (mean) price, by (rep78)
★说明:原始数据会丧失
sysuse auto,clear
*根据维修次数(rep78）计算price的平均值,并将新变量命名为pricemean
collapse (mean) pricemean=price, by (rep78)
sysuse auto,clear
★可以同时计算多个变量和多个统计量
collapse (count) n=price (mean) price mpg, by(rep78)
l
*如何判断两列数据是否相同
sysuse auto,clear
egen a = diff (price mpg)
browse a price mpg
