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I know next to nothing about statistics so tried Statistics::Basic because it seemed easy to use. Then I tried Statistics::Descriptive because it has functions that Statistics::Basic lacks. What I did not expect was to get a different result when calculating Standard Deviation using Statistics::Descriptive than when using the other module or a subroutine copied form Yahoo Answers. Am I doing something wrong, or does Statistics::Descriptive have a deviant way of calculating deviations?

```
#!/usr/bin/perl
use strict;
use warnings;
use Statistics::Basic qw(:all);
use Statistics::Descriptive;
my @d = (5,10,5,100,150);
print 'StdDev according to Basic is ', stddev(@d), "\n"; #Basic
my $stat = Statistics::Descriptive::Full->new();
$stat->add_data(@d);
print 'StdDev according to Descriptive is ', $stat->standard_deviation(), "\n"; #Descriptive
print 'StdDev according to subroutine is ', standard_deviation(@d) . "\n";
sub standard_deviation {
my (@numbers) = @_;
#Prevent division by 0 error in case you get junk data
return undef unless ( scalar(@numbers) );
# Step 1, find the mean of the numbers
my $total1 = 0;
foreach my $num (@numbers) {
$total1 += $num;
}
my $mean1 = $total1 / ( scalar @numbers );
# Step 2, find the mean of the squares of the differences
# between each number and the mean
my $total2 = 0;
foreach my $num (@numbers) {
$total2 += ( $mean1 - $num )**2;
}
my $mean2 = $total2 / ( scalar @numbers );
# Step 3, standard deviation is the square root of the
# above mean
my $std_dev = sqrt($mean2);
return $std_dev;
}
```

Gives the following output:

```
StdDev according to Basic is 60.12
StdDev according to Descriptive is 67.2123500556259
StdDev according to subroutine is 60.1165534607565
```