The two parameters required for the normal (or Gaussian) distribution are the mean and standard deviation. The normal distribution is an important and widely-used distribution in the field of statistics and probability. All normal distributions are symmetric and are commonly called bell curves.

Normal distributions are frequently used to describe equipment that has increasing failure rates with time. The normal distribution is recommended only if failure times can be expressed as a summation of some other random variables. Although the normal distribution is a handy tool for describing all sorts of different data, it allows observations to be negative. Because parts cannot fail before time t = 0, life data is always positive. As a result, the normal distribution does not usually describe life data very well. Most analysts do not even bother to check for a normal fit because life data that follows the normal distribution also generates good Weibull probability plots.