Always compare yields to inflation

Note $Y_t^m$ the yield of the bond of maturity $m$ bought at time $t$, $P_{t-1}$ the CPI at previous time, $P_t$ the CPI at current time and compute:

\begin{equation}
Y^m_{t} - \frac{P_{t}-P_{t-1}}{P_{t-1}}
\end{equation}

Note: This is one very narrow definition of inflation, yet we use because everyone does to make decisions. We should probably use another name.