diff --git a/inc/math.inc b/inc/math.inc
new file mode 100644
--- /dev/null
+++ b/inc/math.inc
@@ -0,0 +1,107 @@
+<?php /* -*- mode: php; indent-tabs-mode: nil; -*- */
+/*
+ * Copyright 2011 Nathan Phillip Brink <ohnobinki@ohnopublishing.net>
+ *
+ * This file is a part of slate_permutate.
+ *
+ * slate_permutate is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * slate_permutate is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with slate_permutate.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+if (!function_exists('mean'))
+  {
+    /**
+     * \brief
+     *   Calculate the mean of a set of numerical values without
+     *   overflowing stuff.
+     */
+    function mean(array $values)
+    {
+      /*
+       * As the influence of each element reduces with each iteration
+       * in the used algorithm, shuffling the array should give a
+       * better idea of what the actual mean is for larger arrays.
+       */
+      shuffle($values);
+
+      $val = 0;
+      $i = 0;
+      foreach ($values as $value)
+        {
+          $val = $val * $i / ($i + 1)
+            + $value / ($i + 1);
+          $i ++;
+        }
+
+      return $val;
+    }
+  }
+
+if (!function_exists('stddev'))
+  {
+    function stddev(array $values)
+    {
+      $mean = mean($values);
+
+      $squares = 0;
+      $i = 0;
+      foreach ($values as $value)
+        $squares += pow($mean - $value, 2);
+      return sqrt($squares / (count($values) - 1));
+    }
+  }
+
+/**
+ * \brief
+ *   Return the four quartile points of an array of sorted values with
+ *   normal integral indexes.
+ */
+function sp_iqr(array $values)
+{
+  $count = count($values);
+  if (!$count)
+    return array(0, 0, 0, 0);
+  return array(
+               $values[0],
+               $values[(int)($count / 4)],
+               $values[(int)($count / 2)],
+               $values[(int)(3 * $count / 4)],
+               $values[$count - 1],
+               );
+}
+
+/**
+ * \brief
+ *   Remove any `outliers' from an array of values.
+ *
+ * An outlier is defined as any value that falls further than 1.5
+ * standard deviations outside of some sort of inter-quartile range.
+ */
+function filter_outliers(array $values)
+{
+  sort($values, SORT_NUMERIC);
+  $values = array_values($values);
+
+  $stddev = stddev($values);
+  list(, $iqr_min, $iqr_max, ) = sp_iqr($values);
+
+  $min = $iqr_min - 1.5 * $stddev;
+  $max = $iqr_max + 1.5 * $stddev;
+
+  $count = count($values);
+  for ($i = 0; $i < $count; $i ++)
+    if ($values[$i] < $min
+        || $values[$i] > $max)
+      unset($values[$i]);
+  return array_values($values);
+}