多层线性模型Multilevel Models，
SPSS,SAS,STATA,R比较，
四个最常用的统计比较，
包括各种模型的code。

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其中stata的code：
.egen sesmeans = mean(ses), by(id)
.gen centses = ses - sesmeans
.xtmixed mathach || id: , var
.xtmixed mathach meanses sector || id: , var
.gen ses_mses=meanses*centses
.gen ses_sect=sector*centses
.xtmixed mathach meanses sector centses ses_mses ses_sect || id: centses,
var cov(un)



Syntax of SAS：
PROC SQL;
CREATE TABLE hsb2 AS
SELECT *, mean(ses) as meanses,
ses-mean(ses) AS cses
FROM hsb
GROUP BY id;
QUIT;
The syntax for estimating the empty model is the following:
PROC MIXED COVTEST DATA=hsb2;
CLASS id;
MODEL mathach = /SOLUTION;
RANDOM intercept/SUBJECT=id
RUN;
PROC MIXED COVTEST DATA=hsb2;
CLASS id;
MODEL mathach = meanses sector /SOLUTION;
RANDOM intercept/SUBJECT=id;
RUN;
PROC MIXED COVTEST DATA=hsb2;
CLASS id;
MODEL mathach = meanses sector cses meanses*cses sector*cses/solution;
RANDOM intercept cses / TYPE=UN SUB=id;
RUN;