The simplest case of a normal distribution is known as the standard normal distribution. This is a special case where μ=0
and σ=1, and it is described by this probability density The factor scriptstyle 1/sqrt{2pi} in this expression ensures that the total area under the curve ϕ(x) is equal to one.The
1
/
2
in the exponent ensures that the distribution has unit variance (and therefore also unit standard deviation). This function is symmetric around x=0, where it attains its maximum value 1/sqrt{2pi}; and has inflection points at +1 and −1.
Authors may differ also on which normal distribution should be called the "standard" one. Gauss defined the standard normal as having variance σ2 =
1
/
2
, that is