seed=20140124
blocksize = 20
N = 200
set.seed(seed)
block = rep(1:ceiling(N/blocksize), each = blocksize)
a1 = data.frame(block, rand=runif(length(block)), envelope= 1: length(block))
a2 = a1[order(a1$block,a1$rand),]
a2$group = rep(c("Group A", "Group B"),times = length(block)/2)
assign = a2[order(a2$envelope),]
head(assign,40)

blockrand = function(seed,blocksize,N){  
    set.seed(seed) 
    block = rep(1:ceiling(N/blocksize), each = blocksize)
    a1 = data.frame(block, rand=runif(length(block)), envelope= 1: length(block)) 
    a2 = a1[order(a1$block,a1$rand),]
    a2$group = rep(c("Group A", "Group B"),times = length(block)/2)  
    assign = a2[order(a2$envelope),]  
    return(assign)
}

%let N = 200;
%let blocksize = 20;
%let seed = 20140124;
data blocks;
    call streaminit(&seed);
    do block = 1 to ceil(&N/&blocksize); 
        do item = 1 to &blocksize;
            if item le &blocksize/2 then group="Group A";
            else group="Group B";
            rand = rand(‘UNIFORM‘);
            output;
        end;
    end;
run;
proc sort data = blocks; by block rand;run;
proc print data = blocks (obs = 40) obs="Envelope";run;

%macro blockrand(N,blocksize,seed);
data blocks;
    call streaminit(&seed);
    do block = 1 to ceil(&N/&blocksize); 
        do item = 1 to &blocksize;
            if item le &blocksize/2 then group="Group A";
            else group="Group B";
            rand = rand(‘UNIFORM‘);
            output;
        end;
    end;
run;
proc sort data = blocks; by block rand;run;
%mend blockrand;