Normal Distribution
The distribution with density proportional to , with mean and variance .
An normal distribution with parameters and is a continuous random variable , denoted , with probability density function
Proof of density validity. We have
Consider . We have
Changing to polar coordintates with and , we have
Since , we have . Hence, is a valid probability density function.
Mean and variance
If with and , then and .
For the mean, write the integral as an odd part plus a constant part:
The first integrand is odd, so its integral vanishes, and the second integral equals ; hence . For the variance, substituting gives
Linear transformations
If and with , then
Indeed, for we have and , so the change-of-variables formula gives
Standardisation and tables
If , then . Writing for the distribution function of the standard normal, its density is , and since while ,
Existing tables of therefore give for any normal random variable; for instance
Sums of independent normals
If and are independent, their moment generating functions multiply:
so : sums of independent normal random variables are normal.
Gaussian random variables allow zero variance
Equivalently to the density definition, a Gaussian random variable is any random variable of the form
When this has density ; allowing also admits the degenerate case .
Related
Stated in
- Definition 4.14 (Normal Distribution)ยง4.5.1 Introduction
