1. Density Curve
A mathematical model for data, providing a way to describe an entire distribution with a single mathematical expression.
An idealized description of the data distribution, representing perhaps the population from which we obtained our sample.

2. Properties of a Density Curve
Always on or above the horizontal axis.
The total area under the curve is exactly 1.
The area under the curve for a given horizontal range is the relative frequency of observations in that range for the idealized (population) distribution.

3. Mean and Median of Density Curves

4. m and s The mean of a density curve is denoted by the Greek letter m.
The standard deviation of a density curve is denoted by the Greek letter s.

5. Normal Distributions Symmetric Single-peaked (i.e., unimodal)
Bell-shaped
Exact form for a particular normal distribution is specified by m and s.

6. The 68-95-99.7 Rule In any normal distribution:
68 % of the individuals fall within 1s of m.
95 % of the individuals fall within 2s of m.
99.7 % of the individuals fall within 3s of m.