Rainrate estimates using polarization techniques promise to be more accurate than the 'factor of two' accuracy available from reflectivity (Z) alone. Polarization techniques essentially measure raindrop shape and hence mean size, which, when combined with Z, provides a measure of the drop concentration. The remaining error is due to variability in the shape of the drop spectrum which governs how the mean (high moment) drop size sensed by the radar is related to the lower moment mean drop size appropriate for the precipitation. This variability of the shape of the spectrum is well captured by the value of the index in a normalised gamma distribution. Many recent papers have derived rainfall rates from polarisation parameters using an index close to 1. In this paper we present evidence based on radar observations that the mean index for natural rainfall is closer to 5. Changing the index from 1 to 5 alters the rainfall rates derived from differential reflectivity or differential phase observations by about 30%.