* Adapted from programs found here https://www.hsph.harvard.edu/miguel-hernan/causal-inference-book/ ;

/***************************************************************
PROGRAM 13.1
Estimating the mean outcome within levels of treatment and confounders
Data from NHEFS
***************************************************************/;
libname causinf "V:\sismid2020";

/* some preprocessing of the data */
data nhefs;
	set causinf.nhefs_book;
	if .<school<9 then education='1. 8th grade or less';
	else if 8<school<12 then education='2. HS dropout';
	else if school=12 then education='3. HS';
	else if 12<school<16 then education='4. College dropout';
	else if school ge 16 then education='5. College or more';
	else education= 'Unknown';
	match=1;
	cens= (wt82 eq .);

	if education ne 'Unknown';
run;

data nhefs_nmv;
	set nhefs;
	if wt82 ne .; * provisionally ignore subjects with missing values for weight in 1982;
run;



/***************************************************************
PROGRAM 13.3
Standardizing the mean outcome to the baseline confounders
Data from NHEFS
***************************************************************/;

/* create a dataset with 3 copies of each subject */  
data onesample ;
  set nhefs_nmv ;
  label interv= "Intervention"; 
  interv = -1 ;    /* 1st copy: equal to original one */
  	output ; 
  interv = 0 ;     /* 2nd copy: treatment set to 0, outcome to missing */
  	qsmk = 0 ;
  	wt82_71 = . ;
  	output ;  
  interv = 1 ;     /* 3rd copy: treatment set to 1, outcome to missing*/
  	qsmk = 1 ;
  	wt82_71 = . ;
  	output ;    
run;

* linear model to estimate mean outcome conditional on treatment and confounders;
* parameters are estimated using original observations only (interv= -1) ;
* parameter estimates are used to predict mean outcome for observations with 
  treatment set to 0 (interv=0) and to 1 (innterv=1);
proc genmod data = onesample;
	class exercise active education;
	model wt82_71 = qsmk sex race age age*age education
				smokeintensity smokeintensity*smokeintensity smokeyrs smokeyrs*smokeyrs
				exercise active wt71 wt71*wt71;
    output out = predicted_mean p = meanY ;
run;

* estimate mean outcome in each of the groups interv=0, and interv=1;
* this mean outcome is a weighted average of the mean outcomes in each combination 
	of values of treatment and confounders, that is, the standardized outcome;
proc means data = predicted_mean mean noprint;
  class interv ;
  var meanY ;
  types interv ;
  output out = results (keep = interv mean ) mean = mean ;
run;

proc print data = results noobs label ;
  title "Parametric g-formula";
  var interv mean ;
run;