Normal distribution, also called normal distribution, is a probability distribution of continuous random variables. A large number of phenomena in nature, human society, psychology and education are distributed according to formal forms, such as the level of ability and the quality of students' grades.

The normal curve is bell-shaped, with low ends and high middle, which is symmetrical left and right, so people often call it bell-shaped curve.

If the random variable X obeys the normal distribution with a mathematical expectation of μ and a variance of σ 2, it is recorded as N(μ, σ 2). The expected value μ of probability density function with normal distribution determines its position, and its standard deviation σ determines its distribution amplitude. When μ=0 and σ= 1, the normal distribution is standard normal distribution.