11 Normal Distribution

11.1 Learning objectives

  • Define and understand Normal random variables.

11.2 Normal distribution

11.2.1 Definition

The normal distribution is defined by two parameters:

  • \(\mu\): population mean
  • \(\sigma\): population standard deviation

It is just a quirk of the normal distribution that the parameters that define it correspond to its mean and standard deviation. This will not generally be true for most other distributions that we will deal with.

11.2.2 Probability distribution

The normal distribution is continuous, and the continuous probability distributions are called probability density functions (pdfs).

The following equation defines the pdf of a normal distribution:

\[ f(x) = \frac{1}{\sigma \sqrt{2\pi}} e^{-\frac{1}{2}{\frac{(x - \mu)^2}{\sigma}}} \]

11.2.2.1 Normal distribution: function of \(\mu\) and \(\sigma\)

Lets have a look at how the normal distribution changes with different mean and variance.

11.2.2.2 The standard normal (Z)

The standard normal distribution is a normal distribution with zero mean and unit variance (\(\mu=0\), \(\sigma^2=1\)). It is also called the \(Z\) distribution. It looks like this: