The first one is the most basic PDF; namely the uniform distribution $$\begin{equation} p(x) = \frac{1}{b-a}\theta(x-a)\theta(b-x), \tag{1} \end{equation}$$ with $$\begin{equation*} \begin{array}{ll} \theta(x)=0 & x < 0 \\ \theta(x)=\frac{1}{b-a} & \in [a,b]. \end{array} \end{equation*}$$ The normal distribution with $b=1$ and $a=0$ is used to generate random numbers.