RDP 2005-05: Underlying Inflation: Concepts, Measurement and Performance Equation

\begin{matrix} \frac{1}{{\hat{σ}}_{t}^{2}} \sum_{i = 1}^{N} w_{i t} (π_{i t} - {\hat{π}}_{i}^{*}) = 0 & for t = 1, \dots, T . \end{matrix}