Normal distribution
MCAT glossary · Psychological, Social & Biological Foundations of Behavior
Definition
The symmetric, bell-shaped probability distribution governed by the empirical rule.
Normal distribution in plain English
The normal distribution is that classic bell-shaped curve you get when tons of random things cluster around an average — most people are near the middle, and fewer and fewer are way out at the extremes (think everyone's height: loads of average-height people, very few giants or very short folks). It's symmetric, so it's a mirror image on both sides of the center. The handy 'empirical rule' says about 68% of everything falls within one standard deviation of the average, about 95% within two, and about 99.7% within three — a standard deviation just being the typical distance from the middle. This is why it's so useful: once you know the average and the spread, you can predict how rare or common any value is.
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