[igt-dev] [i-g-t] tests/kms_setmode: basic Improve accuracy with using of confidence interval

[Anand Moon]

Using confidence interval formula accuracy is estimates approximately
at 95% when {X} is the average of a sample of size n.
Use this confidence interval formula as used in statistics
to estimate the accuracy.

Cc: Dorota Czaplejewicz <dorota.czaplejewicz at collabora.co.uk>
Signed-off-by: Anand Moon <anandx.ram.moon at intel.com>
---
See below link.
https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule#Cumulative_distribution_function
---
 tests/kms_setmode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/kms_setmode.c b/tests/kms_setmode.c
index 92f3ead2..58814a13 100644
--- a/tests/kms_setmode.c
+++ b/tests/kms_setmode.c
@@ -494,7 +494,7 @@ static void check_timings(int crtc_idx, const drmModeModeInfo *kmode)
 	 * which depend on timing, giving the lowest acceptable MTBF of 5.6s
 	 * for 60Hz sampling rate.
 	 */
-	accuracy = 3. * stddev;
+	accuracy = 3. * stddev / sqrt(CALIBRATE_TS_STEPS);
 
 	igt_info("Expected frametime: %.0fus; measured %.1fus +- %.3fus accuracy %.2f%% [%.2f scanlines]\n",
 		 expected, mean, stddev,
-- 
2.25.1