This is an approximate table of probabilities which relates dispersion units above or below the mean value to the total amount of the sample that is contained outside that region for values less than zero and inside the region for values greater than zero.

Table of Probabilities
-3.0 .001	-2.5 .006	-2.2 .014	-2.0 .023	-1.8 .035	-1.6 .055
-1.4 .081	-1.2 .115	-1.0 .158	-0.8 .212	-0.6 .274	-0.4 .345
-0.2 .421	0.0 0.500	+0.2 .579	+0.4 .655	+0.6 .725	+0.8 .788
+1.0 .841	+1.2 .885	+1.4 .919	+1.6 .945	+1.8 .965	+2.0 .986
+2.2 .986	+2.5 .994	+3.0 .999

The use of this table is not complicated. Here is how it works.

Example 1:

You determine that some event lies +0.4 above the mean. What is the probability that an event that large could happen.
• 65.5% of the sample is contained within that value. That means 100 -65.5 or 34.5% is outside of that range.
• The probability of an event at least that large happening is then 34.5% or approximately 1 out of 3.

Example 2

You determine that some event lies -1.6 below the mean. What is the probability that an event this small could happen?
• If you are below the mean, the calculation is a bit easier. -1.6 encloses 5.5% of the distribution. In other words 100 -5.5% = 94.5% of the time, the data values will be higher than -1.6 below the mean.
• The probability of an event at least this small is therefore 5.5% or about 1 out of 20.