Monitor (GBM) flew, the question of the reality of the high-energy cutoff of this distribution would be settled [@PiranReviewInKouveliotou]. It has; the [$E_{\rm peak}$]{} distribution cutoff for GBM, relative to BATSE, does not change all that much, even though GBM has the medium-energy (200 keV to 40 MeV) range of the BGO detectors to contribute to the spectral fitting. Figure \[BATSE\_GBM\_Epeak\] shows a comparison between the fluence (average spectrum) [$E_{\rm peak}$]{} values between the BATSE and GBM data sets [@GoldsteinCat; @GruberCat], indicating that GBM extends the distribution by including more lower and higher energy values. That is not to say that there are [*no*]{} spectra with large values of [$E_{\rm peak}$]{}, just that the majority of fitted [$E_{\rm peak}$]{} values cluster around a  200 keV value. Certainly, there is no ‘undiscovered country’ of [$E_{\rm peak}$]{}values clustering in the MeV range. We will explore why this might be in this paper.

In order to facilitate understanding of the important role [$E_{\rm peak}$]{} plays in the various observed correlations with other burst observables, we examine herein various factors that can affect the observed values of [$E_{\rm peak}$]{}. First of all, in section 2, we examine the limits of detectability of a single 