The benchmarking tool calculates the population variance rather than the sample variance.

Since benchmarking results represent only a sample of the entire population of
results, it would be better to calculate the sample standard deviation, which
is an unbiased estimator of the population variance.  Also, since the Array.avg
function throws out the min and max, this function should not be used when
calculating the variance.

BUG=none
TEST=none
Review URL: http://codereview.chromium.org/7108006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88379 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/docs/examples/extensions/benchmark/options.html

@@ -162,16 +162,20 @@ Array.avg = function(array) {
   return sum / count;
 }
 
-// Compute the standard deviation of an array
+// Compute the sample standard deviation of an array
 Array.stddev = function(array) {
   var count = array.length;
-  var mean = Array.avg(array);
+  var mean = 0;
+  for (var i = 0; i < count; i++) {
+    mean += array[i];
+  }
+  mean /= count;
   var variance = 0;
   for (var i = 0; i < count; i++) {
     var deviation = mean - array[i];
     variance = variance + deviation * deviation;
   }
-  variance = variance / count;
+  variance = variance / (count - 1);
   return Math.sqrt(variance);
 }