Expected value, i.e. the probability of each possible result of a discrete random variable is multiplied by the sum of its test results.
If the random variable X obey the mathematical expectation of a [mu], a normal distribution and variance σ ^ 2, denoted by N (μ, σ ^ 2), a normal distribution probability density function of the expected value of [mu] determines its position, its standard difference σ determines the amplitude distribution. When μ = 0, σ = 1 is a normal distribution of standard normal distribution.