Sep 11, 2009, 6:05 PM
Post #1 of 3
Subtraction Error using Perl
I observed the following abnormality while running my PERL program calculating student t test test statistics value. Hope you can help. Thanks in advance.
$t0 = ($mean1 - $mean2)/(sqrt($varp*(1/$n1+1/$n2)));
$mean_diff = $mean1 - $mean2;
I printed the value of the variable in the equation:
print "($mean1 - $mean2) $mean_diff /sqrt($varp*(1/$n1+1/$n2)) = $t0\n";
The results was as below:
$mean1-$mean2 $mean_diff /sqrt($varp*(1/$n1+1/$n2)) = $t0
(0.61 - 0.61) -1.11022302462516e-16 /sqrt(8.40561424617007e-32*(1/39+1/35)) = -1.64465938636703
Which was puzzling. Shouldn’t 0.61 – 0.61 result to 0? This screwed up my test statistics and wonder if any of you can help work around this issue? Do I need to load any additional perl mathematics module to calculate this properly?
I expected $t0 to be 0 since the numerator should be a 0.