Measurements of psychometric functions at thresholds of single flashes, flash trains and gated sinusoids are analysed. The stimuli are foveal, have a diameter of 1° and a dark surround. In single flashes it is found that the S.D. of the probability density function divided by the 50 per cent threshold increment (Crozier quotient) is constant over a background intensity of 5 decades and a flash duration of 3 decades. This quotient does not differ much between the subjects and is 0.25 on an average. In flash trains with large flash intervals, the 50 per cent threshold drops as a function of the number of flashes/train due to increase in the chance of detection. The relative drop is found at discrete background levels to have the same course. Gated sinusoids also show the same course as a function of the number of peaks. On the basis of a deterministic model used in preceding parts, the invariance of the Crozier quotient can be resolved into one simple stochastic system property. Incorporation into an existing psychophysical model leads to a simple and comprehensive formula for the spread in the 50 per cent thresholds. The threshold course with flash trains and gated sinusoids can be forecast from the stochastic parameters of single flashes. An estimate of the effect on the ratio of flash to flicker sensitivity based on the actual data is in agreement with earlier experimental data.