On the basis of Kolmogorov's general theory of chain processes, a formula for a conditional count probability as measured by a certain type of time analyzer is derived and compared with the classical Rossi-α formula. The discrepancy between the Rossi-&aplha; formula and the experimental results reported by several authors is explained. It is shown that this discrepancy is due to the frequently misunderstood function of the time analyzer used. The mathematical technique employed in the paper is well suited for calculating the conditional count probabilities, which might possibly be measured by other types of time analyzers.