proc sort data=aaa out=aaa1;
by id visit;
run;

data wanted;
set aaa1;
by id visit;
lagx=lag(x);
if first.id then call missing(lagx);
if lagx ne . then mean=mean(x,lagx);
if first.id then y=0;
  y+mean;
if last.id;
keep id y;
run;

proc sort data=aaa;
by id;
run;
data b;
set aaa;
by id;
if first.id then temp=0.5*x;
else if last.id then temp=0.5*x;
else temp=x;
run;
proc sql;
create table total as
select id,sum(temp) as y
  from b
   group by id ;
  quit;

最后的total就是你想要的结果

方法太多了