TeX source:

S_{\mathrm{pke}}=\frac{1}{3} \Phi^{-1}\left\{\frac{1}{2} \Phi\left(\frac{\mathrm{USL}_{e}-\mu_{e}}{\sigma_{e}}\right)+\frac{1}{2} \Phi\left(\frac{\mu_{e}-\mathrm{LSL}_{e}}{\sigma_{e}}\right)\right\}