Evidence is presented here that P wave amplitudes contain additional information on the Earth's heterogeneity and must be considered in future tomographic interpretations. We analyse the reliability and the variance of teleseismic P wave amplitudes recorded at well calibrated broadband Global Seismic Network (GSN) stations from intermediate to deep earthquakes (depth >46 km). The dataset contains 217 earthquakes with mb between 5.6 and 7.6, from 1993 January to 2000 May. Using pairs of closely located events with similar focal mechanisms as well as data recorded at the closely spaced MOMA seismic array we demonstrate the consistency of observations. We deduce that the magnification of the GSN instruments generally drifts by at most 2 per cent per year, and likely much less. P wave amplitudes have variations due to focusing/defocusing with a standard deviation of at least 38 per cent. This reduces to 19 per cent if only periods in excess of 10 s are considered. Tomographic P wave models with mantle velocity anomalies of the order of 1 per cent are unable to reproduce such large variations.