Mathematical Model
An idealized description.
Density Curve
A curve that is always on or above the horizontal axis, and has an area of exactly 1 underneath it.
Median
The point with half the observations on either side.
Mean
The point at which the curve would balance if made of solid material.
Normal Curves
Density curves that are symmetric, single-peaked, and bell-shaped.
Inflection Points
The points at which this change of curvature takes place and are located at distance of standard deviation on either side of the mean.
Empirical Rule
68-95-99.7 rule which describes what percent of observations lie within one, two, and three standard deviations of the mean.
Observation's Percentile
The percent of the distribution that is at or to the left of the observation.
Z-Score
A standardized value of x.
Standard Normal Distribution
The normal distribution N (0, 1) with mean 0 and standard deviation 1.
Z-Score
z = x - u / o
z = x - the mean / standard deviation
Standard Deviation
The distance from the mean to the inflection points on either side.