5,4,3,2,9,10,4,12,15,20,9,8,3,9,17

Here, the Mean= 8.66 and Std.Dev= 5.45.

So 68% of this data should lie between 14.12 and 3.2 days (+/-1 Std.dev).

Plotted in excel it looks like this:

Now according to the dataset, 2 appears once (7%). The range between 3 and 15 appears 12 times (86%). Numbers > 17 appears once (7%).

The range between 3 and 15 (about 1 std.dev) does not lie within 68%, but rather 86%. Hence it does not follow the 68, 95, 99% rule.

Any explanation?

thank you.