Title: Isolated neutron stars, their $\gamma$-ray efficiencies and EGRET observations

Abstract: We examine a sample of detected and undetected Isolated Neutron Stars (INSs), selected on the basis of their energy loss and distance, in order to maximize their detection probability. Since the sample we consider encompasses more upper limits than detections, we make use, for the first time in this field of astronomy, of survival analysis procedures through the ASURV software package (Feigelson \& Nelson 1985, Isobe et al. 1986). We show that these techniques lead to a better understanding of the physical processes at work in high-energy emission from INSs. In particular, the recent detection of PSR 1951+32 and upper limits from pulsars pointed but not detected by EGRET show that the $\gamma $-ray efficiency of ISN is not correlated to any simple pulsar parameter.