**  A basic SAS merge is an example of deterministic linkage, using the rule that if the variables on  ;
**  the BY statement agree, the link is made:   ;
data links;
merge cases(in=a) hosp(in=b);
   by PHIN;
   if a and b;
run; 

data links;
merge cases(in=a) hosp(in=b);
   by REGNO SEX BYR BMONTH;
   if a and b;

run;

**   It is difficult to code more sophisticated rules into a SAS program, such as the "at least three of five"  ;
**   approach described above. Intrepid programmers at the Centre have taken a crack at it, though;