  Correlations of detection events in photodetectors placed at the opposite
sides of a beam splitter are studied in the frame of classical probability
theory. It is assumed that there is always one photon present during one
elementary measurement (one measurement act). Due to the conservation of
energy, thereis a strict anticorrelation in detections in one elementary
experiment, because the photon cannot excite both of the detectors at the same
time. It is explicitely shown in several examples that the bunching or
anti-bunching of the counts in serieses of elementary single-photon experiments
are governed by the statistical properties of grouping the sequences of the
elementary measurements.
