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[i-g-t,07/18] stats: Add a note about the standard deviation derived from unbiased variance

Message ID 1435417696-28115-8-git-send-email-damien.lespiau@intel.com (mailing list archive)
State New, archived
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Commit Message

Lespiau, Damien June 27, 2015, 3:08 p.m. UTC 
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
---
 lib/igt_stats.c | 5 +++++
 1 file changed, 5 insertions(+)
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Patch

diff --git a/lib/igt_stats.c b/lib/igt_stats.c
index 12cabba..bd05550 100644
--- a/lib/igt_stats.c
+++ b/lib/igt_stats.c
@@ -121,6 +121,11 @@  bool igt_stats_is_population(igt_stats_t *stats)
  * [Bessel's correction](https://en.wikipedia.org/wiki/Bessel%27s_correction)
  * to the variance.
  *
+ * Note that even if we manage to have an unbiased variance by multiplying
+ * a sample variance by the Bessel's correction, n/(n - 1), the standard
+ * deviation derived from the unbiased variance isn't itself unbiased.
+ * Statisticians talk about a "corrected" standard deviation.
+ *
  * When giving #true to this function, the data set in @stats is considered a
  * full population. It's considered a sample of a bigger population otherwise.
  *