data b(keep=id year qtr);
set a;
mdy=yyq(year,qtr);
format mdy yyq6.;
lag1=lag(mdy);
interval=intck("qtr",lag1,mdy);
if interval=2 then do;
mdy1=intnx("qtr",lag1,1);
year=year(mdy1);
qtr=qtr(mdy1);
end;
if interval=2;
run;
proc append base=a data=b force;run;
proc sort data=a;by id year;run;