1. System data read in
sysuse auto, clear
Two, summarize——sum
1. Single data information:
summarize mpg
2. All information
sum
3. Detailed information+detail
sum mpg, detail
4、return list
5. return class, r-class return value
display value
display r(mean)
6. Return to ereturn list——estimation class return value
(1) return
. regress mpg weight length foreign
(2) Enter return value
. ereturn list
（3） avail e(r2)
（4）mat list e(b)
Coefficient - e(b)
Variance - e(V)
7. creturn list - system return value
disp c(current_date) 
disp c(current_time) 
disp c(pi)
pending decimal
disp %13.10f c(pi)