The Empirical Rule and Normal Distributions
Normal distribution
The normal distribution is important because many variables studied are normally distributed. For example reading ability, height and weight, to name a few. Knowing data is normally distributed means you can use specific statistical tests.
Even better, the tests work pretty well even if the dist
The normal distribution is symmetrical.
Therefore if 68% of the data fall within 1 standard deviation, this means that half of that falls between the mean minus 1 standard deviation with the other half falling between the mean plus 1 standard deviation. In other words 34% fall 1 standard deviation below the mean and 34%
Percentile Notation
Percentiles separate the data into 100 different parts
Pk (0 < k < 100) is the kth percentile.
P50 is the 50th percentile = Q2 = median.
P90 is the 90th percentile. This number separates the bottom 90% of the data from the top 10%.
Percentile Calculations