Joint European Torus high energy neutral particle analyzer measures the flux of fast neutrals originating from the plasma core. From this data, the fast ion distribution function f(i)(fast), temperature T-i,perpendicular to(fast), and density n(i)(fast) are derived using knowledge of various plasma parameters and of the cross section for the required atomic processes. In this article, a systematic sensitivity study of the effect of uncertainties in these quantities on the evaluation of the neutral particle analyzer f(i)(fast), T-i,perpendicular to(fast), and n(i)(fast) is reported. The dominant parameter affecting n(i)(fast) is the impurity confinement time and therefore a reasonable estimate of this quantity is necessary to reduce the uncertainties in n(i)(fast) below 50%. On the other hand, T-i,perpendicular to(fast) is much less sensitive and can certainly be provided with an accuracy of better than 10%. (C) 2004 American Institute of Physics.