JET high energy neutral particle analyzer measures the flux of fast neutrals originating from the plasma core. From this data, the fast ion distribution function fifast , temperature Ti,^fast and density nifast are derived using knowledge of various plasma parameters and of the cross-section for the required atomic processes. In this paper, a systematic sensitivity study of the effect of uncertainties in these quantities on the evaluation of the NPA fifast, Ti,^fast and nifast is reported. The dominant parameter affecting nifast is the impurity confinement time and therefore a reasonable estimate of this quantity is necessary to reduce the uncertainties in nifast below 50 %. On the other hand, Ti,^fast is much less sensitive and can certainly be provided with an accuracy of better than 10 %.