Perl in 21 Days
by David Till
Random Sampling
Assuming that two populations of 40 subjects are identical for some
measurable aspect of their physiology (for instance, systolic blood
pressure), what are the chances that a random sample of five subjects from
each population will reflect this? How reliable is random sampling?
Random Sampling Script
Consider the following Perl script:
<![CDATA[
#!/usr/bin/perl -w
use strict;
my @pop = (62, 65, 66, 72, 80, 75, 67, 73,
79, 69, 70, 64, 64, 65, 67, 81,
61, 82, 68, 59, 83, 77, 73, 74,
66, 67, 77, 78, 82, 80, 73, 68,
67, 73, 84, 58, 75, 76, 72, 68);
my $actual_average = 0;
my $num = @pop;
my $tally = 0;
foreach my $n (@pop){
$tally+=$n;
}
$actual_average = $tally/$num;
print "ACTUAL AVERAGE: $actual_average\n";
srand(time);
my $s1 = 0;
my @g1 = ();
my $i = 0; #index
$tally = 0;
foreach my $r (0..4){
$i = int rand($num);
$tally += $pop[$i];
push(@g1,$pop[$i]);
}
$s1 = $tally/5;
print "SAMPLE: @g1, SAMPLE AVERAGE: $s1\n";
my $s2 = 0;
my @g2 = ();
$tally = 0;
foreach my $r (0..4){
$i = int rand($num);
$tally += $pop[$i];
push(@g2,$pop[$i]);
}
$s2 = $tally/5;
print "SAMPLE: @g2, SAMPLE AVERAGE: $s2\n";
exit;
]]>
Run this sample script several times. Make sure you pause for a couple
seconds between each run of the script since srand needs a new value to
produce unique output.
Notice the use of push to add an item to an array. This is a very useful
trick in a lot of circumstances. One particularly useful context for this
trick is when you are creating tables in a CGI script.
Run the Perl script shown above ten times. Redirect the output to a
file so that it can be easily reformatted into an HTML page. To redirect you
do this:
<![CDATA[
./perlScript.pl >> output_file.txt
]]>