/* oct30.ex2                                 */
 /*                                           */
 /* two sample problem from ST430             */
 /*                                           */
 /* Differences in blood pressure             */
 /*                                           */
 /* sbp = systolic blood pressure (lbs/sq in) */
 /* smoke = smoker (yes) or not (no)          */
 /* group = group id (1=nonsmoker, 2=smoker)  */
 /*                                           */
  data a ;
    input sbp smoke $ group ;
 /*                 ^ says character variable */
 cards ;
  133  no  1
  122  no  1
  130  no  1
  148  no  1
  142  no  1
  146  yes 2
  137  yes 2
  162  yes 2
  160  yes 2
  144  yes 2
  173  yes 2
  166  yes 2
  138  yes 2
  140  yes 2
  134  yes 2
 ;
run ;
 /* two sample t-test                         */
  proc ttest data=a ;
   class group ;             * smoke would work in the same way ;
   var sbp ;                 * what variable? ;
run ;
 /* permutation distribution is random assignment */
 /*  of individual to groups -- with same totals  */
  %let krep = 20 ;                            * macro variable ;
  %let m = 5 ;                                * one group ;
  %let n = 10 ;                               * other group ;
  %let mpn = %eval(&m + &n) ;                 * total ;
  data aperm ;
    array pj(&krep) ;
    retain pj1-pj&krep &m _all_ ;             * initial value ;
    keep group k sbp ;
    set a ;
      seed = 5151917 ;
      do k = 1 to &krep ;
      if( ranuni(seed) lt pj(k)/(&mpn-_n_+1) ) then do ;
                     group = 1 ;
                     pj(k) = pj(k) - 1 ;
                                                     end ;
                                                 else group = 2 ;
      output ;                                * make obs ;
      end ;                                   * of loop on k ;
 run ;
 /* now analyze each one                           */
 /*                                                */
 /* first sort by replication counter              */
   proc sort data=aperm ;
     by k ;
 run ;
 /*                                                */
 /* now compute statistic                          */
 * ods listing close ;                        ****** maybe not yet ;
   proc ttest data=aperm ;
     class group ;                                 * two groups, 1 & 2 ;
     var sbp ;                                     * analyze blood p ;
     by k ;                                        * replication ;
     ods output TTests=tds ;
 run ;
   ods listing ;
 /* what's in this ds?                             */
  proc print data=tds (obs=10) ;
    title 'ods dataset' ;
 run ;