\begin{equation}
  \begin{aligned}
    & \mathcal{J}(\Theta) := \sqrt{ \frac{1}{\vert \mathcal{I} \vert} \sum_{(i,j)\in \mathcal{I}} \bigg( \widehat \sigma_{i,j}^{mid} - \widehat \sigma_{i,j}(\Theta) \bigg)^2},
  \end{aligned}\label{calibration_error}
\end{equation}
