 But this implies that formula_38 is excluded, namely all possible values of formula_25.Such a result is difficult to interpret because the experiment cannot essentially distinguish very small values of formula_25 from the background-only hypothesis, and thus declaring that such small values are excluded (in favor of the background-only hypothesis) seems inappropriate.To overcome this difficulty Zech suggested conditioning the probability that formula_41 on the observation that formula_42, where formula_43 is the (unmeasurable) number of background events.