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ZixuanZhang
ZixuanZhang
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Normal Distribution

The distribution with density proportional to , with mean and variance .

Definition 4.14 (Normal Distribution)

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