# How do I use the empirical rule?

Mar 5, 2015

It can give you quick answers without looking up in tables.

Ususally, you standardise the normal distribution, and then look up its z-value in a table.
If you memorise just a few numbers, you can work quicker.
The numbers are $34 , 13.5 \mathmr{and} 2.5$ meaning:
34% of your values will be within one standard deviation above the mean, and 34% will be within one STD below the mean.
13.5% will be between one and two STD above (and 13.5% will be between one and two STD below)
The remaining two times 2.5% will be outside that.

Below is a graph (from Wikipedia) that summarizes: