  In this paper, we present a consistent procedure to assess the significance
of gravitational wave events observed by laser interferometric gravitational
wave detectors based on the background distribution of detection statistic. We
propose a non-parametric method to estimate $p$-value. Based on the estimated
$p$-values, we propose a new procedure to assess the significance of a
particular event with $q$-value which is the minimum false discovery rate that
can be attained when calling the event significant. The $q$-value gives us a
criterion on the significance of events which is different from $P_{\rm astro}$
which is used in the LIGO-Virgo analysis and in other analysis. The proposed
procedure is applied to the 1-OGC and 2-OGC catalogs [2][3]. For most of the
events which were claimed significant in [2] and [3], we also obtain the same
results. However, there are differences in the significance for several
marginal events. Since the proposed procedure does not require any assumptions
on signal and noise, it is very simple and straightforward. The procedure is
also applicable to other searches for gravitational waves whose background
distribution of detection statistic is difficult to know.
