  Inflation predicts the generation of cosmological perturbations. Usually, the
power spectra for the scalar and tensor modes are calculated with help of the
slow roll approximation. In the case of power law inflation an exact result is
available. We compare the predictions for the cosmic microwave background
anisotropies from the slow roll approximation with the exact results from power
law inflation. We find that the so-called consistency check from the slow roll
approximation, $C_2^T/C_2^S \approx -6.93 n_T$, may differ considerably from
the exact result.
