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SAS Textbook Examples
Modeling Longitudinal Data by Robert Weiss
Chapter 12: Missing Data

The data files can be downloaded from http://rem.ph.ucla.edu/~rob/mld/data.html .

Table 12.1, page 367.

*Complete data;
proc mixed data = missingdata1 method = reml noinfo noclprint noitprint;
 class idno;
 model weight = week/ solution notest;
 random intercept/ subject = idno type = un;
run;

The Mixed Procedure

 Covariance Parameter Estimates
Cov Parm     Subject    Estimate
UN(1,1)      idno         714.01
Residual                 10.1683

           Fit Statistics
-2 Res Log Likelihood           735.4
AIC (smaller is better)         739.4
AICC (smaller is better)        739.5
BIC (smaller is better)         741.4

  Null Model Likelihood Ratio Test
    DF    Chi-Square      Pr > ChiSq
     1        384.48          <.0001

                   Solution for Fixed Effects
                         Standard
Effect       Estimate       Error      DF    t Value    Pr > |t|
Intercept      196.46      5.9972      19      32.76      <.0001
week           0.1414     0.08522      99       1.66      0.1002

*MAR;
proc mixed data = missingdata2 method = reml noclprint noitprint noinfo;
 class idno;
 model weight = week/ solution notest;
 random intercept/ subject = idno type =un;
run;

The Mixed Procedure

 Covariance Parameter Estimates
Cov Parm     Subject    Estimate
UN(1,1)      idno         700.56
Residual                  7.9755

           Fit Statistics
-2 Res Log Likelihood           560.5
AIC (smaller is better)         564.5
AICC (smaller is better)        564.6
BIC (smaller is better)         566.5

  Null Model Likelihood Ratio Test
    DF    Chi-Square      Pr > ChiSq
     1        301.54          <.0001

                   Solution for Fixed Effects
                         Standard
Effect       Estimate       Error      DF    t Value    Pr > |t|
Intercept      196.84      5.9377      19      33.15      <.0001
week         -0.05619     0.09671      70      -0.58      0.5631

*MNAR;
proc mixed data = missingdata3 method = reml noclprint noitprint noinfo;
 class idno;
 model weight = week/ solution notest;
 random intercept/ subject = idno type =un;
run;

The Mixed Procedure

 Covariance Parameter Estimates
Cov Parm     Subject    Estimate
UN(1,1)      idno         707.88
Residual                  6.0963

           Fit Statistics
-2 Res Log Likelihood           490.2
AIC (smaller is better)         494.2
AICC (smaller is better)        494.4
BIC (smaller is better)         496.2

  Null Model Likelihood Ratio Test
    DF    Chi-Square      Pr > ChiSq
     1        280.88          <.0001

                   Solution for Fixed Effects
                         Standard
Effect       Estimate       Error      DF    t Value    Pr > |t|
Intercept      196.29      5.9666      19      32.90      <.0001
week          -0.2007     0.08905      60      -2.25      0.0279

Table 12.2, page 370.

proc sql;
 create table schiz as
  select *,
  (max(week) ne 6) as drop_out,
  sqrt(week) as sweek
 from schizophrenia
 group by id
 order by drop_out, id;
quit;

*Pooled;
proc mixed data = schiz method = ml noitprint noclprint noinfo;
 class id ;
 model severity = sweek drug sweek*drug / solution notest;
 random intercept sweek/ subject=id type = un;
run;

The Mixed Procedure

 Covariance Parameter Estimates
Cov Parm     Subject    Estimate
UN(1,1)      ID           0.3687
UN(2,1)      ID          0.02085
UN(2,2)      ID           0.2420
Residual                  0.5778

           Fit Statistics
-2 Log Likelihood              4649.0
AIC (smaller is better)        4665.0
AICC (smaller is better)       4665.1
BIC (smaller is better)        4697.6

  Null Model Likelihood Ratio Test
    DF    Chi-Square      Pr > ChiSq
     3        542.43          <.0001

                   Solution for Fixed Effects
                          Standard
Effect        Estimate       Error      DF    t Value    Pr > |t|
Intercept       5.3480     0.08790     435      60.84      <.0001
sweek          -0.3361     0.06794     435      -4.95      <.0001
drug           0.04634      0.1011     729       0.46      0.6469
sweek*drug     -0.6405     0.07752     729      -8.26      <.0001

*By dropout;
proc mixed data = schiz method = ml noitprint noclprint noinfo;
 by drop_out;
 class id ;
 model severity = sweek drug sweek*drug / solution notest;
 random intercept sweek/ subject=id type = un;
run;

drop_out=0

The Mixed Procedure

 Covariance Parameter Estimates
Cov Parm     Subject    Estimate
UN(1,1)      ID           0.3984
UN(2,1)      ID         -0.01110
UN(2,2)      ID           0.2048
Residual                  0.5600

           Fit Statistics
-2 Log Likelihood              3782.1
AIC (smaller is better)        3798.1
AICC (smaller is better)       3798.2
BIC (smaller is better)        3828.6

  Null Model Likelihood Ratio Test
    DF    Chi-Square      Pr > ChiSq
     3        468.38          <.0001

                   Solution for Fixed Effects
                          Standard
Effect        Estimate       Error      DF    t Value    Pr > |t|
Intercept       5.2212      0.1091     333      47.84      <.0001
sweek          -0.3934     0.07345     333      -5.36      <.0001
drug            0.2016      0.1226     655       1.64      0.1008
sweek*drug     -0.5386     0.08255     655      -6.52      <.0001


drop_out=1

 Covariance Parameter Estimates
Cov Parm     Subject    Estimate
UN(1,1)      ID           0.2858
UN(2,1)      ID          0.02045
UN(2,2)      ID           0.6921
Residual                  0.5147

           Fit Statistics
-2 Log Likelihood               822.9
AIC (smaller is better)         838.9
AICC (smaller is better)        839.5
BIC (smaller is better)         859.9

  Null Model Likelihood Ratio Test
    DF    Chi-Square      Pr > ChiSq
     3         73.32          <.0001

                   Solution for Fixed Effects
                          Standard
Effect        Estimate       Error      DF    t Value    Pr > |t|
Intercept       5.5337      0.1409     100      39.26      <.0001
sweek          -0.1235      0.1773     100      -0.70      0.4877
drug           -0.1939      0.1779      74      -1.09      0.2791
sweek*drug     -1.1674      0.2239      74      -5.21      <.0001

*Covariate and contrasts;
proc mixed data = schiz method = ml noitprint noclprint noinfo;
 class id ;
 model severity = sweek drug sweek*drug drop_out drop_out*sweek 
   drop_out*drug drop_out*sweek*drug / solution notest;
 random intercept sweek/ subject=id type = un;
 estimate 'int placebo nodrop' intercept 1;
 estimate 'int placebo drop'   intercept 1 drop_out 1;
 estimate 'int drug nodrop'    intercept 1 drug 1;
 estimate 'int drug drop'      intercept 1 drug 1 drop_out 1;
 estimate 'slope placebo nodrop' sweek 1;
 estimate 'slope placebo drop' sweek 1 drop_out*sweek 1;
 estimate 'slope drug nodrop' sweek 1 sweek*drug 1;
 estimate 'slope drug drop' sweek 1 sweek*drug 1 drop_out*sweek 1 drop_out*sweek*drug 1;
run;

The Mixed Procedure

 Covariance Parameter Estimates
Cov Parm     Subject    Estimate
UN(1,1)      ID           0.3612
UN(2,1)      ID          0.01175
UN(2,2)      ID           0.2300
Residual                  0.5768

           Fit Statistics
-2 Log Likelihood              4623.3
AIC (smaller is better)        4647.3
AICC (smaller is better)       4647.5
BIC (smaller is better)        4696.2

  Null Model Likelihood Ratio Test
    DF    Chi-Square      Pr > ChiSq
     3        523.49          <.0001

                        Solution for Fixed Effects
                                    Standard
Effect                 Estimate       Error      DF    t Value    Pr > |t|
Intercept                5.2210      0.1075     433      48.55      <.0001
sweek                   -0.3934     0.07635     433      -5.15      <.0001
drug                     0.2017      0.1208     729       1.67      0.0955
sweek*drug              -0.5386     0.08580     729      -6.28      <.0001
drop_out                 0.3203      0.1864     729       1.72      0.0862
sweek*drop_out           0.2517      0.1593     729       1.58      0.1146
drug*drop_out           -0.3987      0.2270     729      -1.76      0.0794
sweek*drug*drop_out     -0.6348      0.1960     729      -3.24      0.0013

                                 Estimates
                                    Standard
Label                   Estimate       Error      DF    t Value    Pr > |t|
int placebo nodrop        5.2210      0.1075     433      48.55      <.0001
int placebo drop          5.5413      0.1523     729      36.39      <.0001
int drug nodrop           5.4227     0.05511     729      98.39      <.0001
int drug drop             5.7430      0.1944     729      29.54      <.0001
slope placebo nodrop     -0.3934     0.07635     433      -5.15      <.0001
slope placebo drop       -0.1417      0.1399     729      -1.01      0.3115
slope drug nodrop        -0.9320     0.03916     729     -23.80      <.0001
slope drug drop          -1.3150      0.1073     729     -12.26      <.0001

Figure 12.2, page 371.

proc mixed data = schiz method = ml noitprint noclprint noinfo;
 class id ;
 model severity = sweek drug sweek*drug drop_out drop_out*sweek 
   drop_out*drug drop_out*sweek*drug / solution notest outpredm=pred_schiz;;
 random intercept sweek/ subject=id type = un;
run;

proc sql;
 create table pred_schiz2 as
  select mean(pred) as pred,
  drug, drop_out, week
  from pred_schiz
  group by week, drop_out, drug
  order by drop_out, drug, week;
 quit;

data pred_schiz2;
 set pred_schiz2;
 group=compress(drug||drop_out);
  if group = "01" then group = "placebo drop";
  if group = "00" then group = "placebo nodrop";
  if group = "11" then group = "drug drop";
  if group = "10" then group = "drug nodrop";
run; 

goptions reset = all;
 symbol1 value=none interpol=join r=4;
 axis1 order =(0 to 6 by 1 ) label=(a=0 'Week') minor=none;
 axis2 order =(2 to 6 by 1) label = (a=90 'Illness severity') minor=none ;

proc gplot data = pred_schiz2;
 plot pred*week=group /haxis=axis1 vaxis = axis2;
run; quit;