太神奇了，求大神解答，我贴出我的命令，大神帮忙看看为什么每次回归的结果都不一样啊。
*一、数据导入
*1.导入第一张表  资产负债表
clear all
set memory 200m
cd "D:\桌面\stata 论文\数据 2"
import excel 资产负债表.xls,clear firstrow
label data "资产负债表"
label var stkcd "证券代码"
label var accper "会计期间"
label var fixedasset "固定资产"
label var asset "资产"
label var liability "负债"
gen tangibility=fixedasset/asset
label var tangibility "固定资产比例"
gen size=ln(asset)
gen lev=liability/asset
label var size "企业规模"
label var lev "财务杠杆"
sort stkcd
save 资产负债表,replace

*2.导入第二张表 账面市值比
clear
import excel 账面市值比.xls,clear firstrow
label var stkcd "证券代码"
label var accper "会计期间"
rename indcd industry
label var industry 行业
label var booktomaket 账面市值比
gen growth=1/booktomaket
label var growth 成长性
sort stkcd
save 成长性,replace

*3.导入第三张表 盈利能力
import excel 盈利能力.xls,clear firstrow
label var CFFO 经营活动现金流量净额
sort stkcd
save 盈利能力,replace

*4.导入第四张表 资产负债表1
import excel 资产负债表2.xls,clear firstrow
label var sloan 短期借款
label var lloan 长期借款
label var debetpayable 应付债券
sort stkcd
save 资产负债表2,replace

*5.导入第五张表
import excel 审计费用.xls,clear firstrow
label var auditcost 审计费用
sort stkcd
save 审计费用,replace

*.导入第六张表 资产负债表3
import excel 资产负债表3.xls,clear firstrow
sort stkcd
save 资产负债表3,replace

*二.合并表
use 资产负债表2,clear
merge m:m stkcd using 资产负债表
keep if _merge==3
drop _merge
gen DFR=(sloan+lloan+debetpayable)/asset
label var DFR 债务融资率
save 主表,replace

use 盈利能力
merge m:m stkcd using 主表
keep if _merge==3
drop _merge
gen profitability=CFFO/asset
label var profitability 盈利能力
save 主表,replace

use 主表,clear
merge m:m stkcd using 成长性
keep if _merge==3
drop _merge
save 主表,replace

use 主表,replace
merge m:m stkcd using 审计费用
keep if _merge==3
drop _merge
save 主表,replace

use 主表,replace
merge m:m stkcd using 资产负债表3
keep if _merge==3
drop _merge
gen ldebt=lliability/liability
save 主表,replace


*删除缺失值
drop Typrep booktomaket CFFO sloan lloan debetpayable fixedasset asset liability Accper
egen mis = rowmiss(_all)
drop if mis
gen year=substr(accper,1,4)
drop accper
destring year,replace
keep if year>2011
save 主表,replace

*三.缩尾处理
use 主表,clear
sort year
winsor2 growth tangibility size DFR profitability auditcost lev ldebt,replace by(year)
save 主表,replace

*四.回归
use 主表,clear
duplicates  report stkcd year
duplicates drop stkcd year,force
des
destring stkcd,replace
destring year,replace
des
xtset stkcd year
xi:xtreg ldebt auditcost growth tangibility size profitability lev i.year i.industry