 The CHSH inequality (8) is a relation between expectation values. An experimental test, therefore, requires empirical estimation of the probabilities of the outcomes of experiments. This estimation involves computing a ratio of event-counts: the number of pair-production events with a certain outcome to the total number of pair-production events. Typically, in experiments involving photons, most of the pairs produced fail to enter the analyzers. Furthermore, some photons that enter the analyzers will fail to be detected; in addition, the detector will occasionally register a detection even when no photon is detected (the rate of occurrence of this is known as the “dark-count”).