\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
p_{\rm trial}^{\rm arb}(\phi_{\rm g_a,f,p,e}) = 
{{f_{\rm ideal}}\over{2 \pi}} +
{
{2 \left(1 - f_{\rm ideal}\right)}
\over
{
n_{\rm sub}
\left[
{\rm Erf}\left({{{\rm hi}(i)-\mu(i)}\over{\sigma(i) \sqrt{2}}}\right)
- {\rm Erf}\left({{{\rm lo}(i)-\mu(i)}\over{\sigma(i) \sqrt{2}}}\right)
\right]
\sigma(i) \sqrt{2 \pi}
}
}
\exp\left(-{{(\phi_{\rm g_a,f,p,e} - \mu(i))^2}\over{2 \sigma(i)^2}}\right)
\end{displaymath}
\end{document}