This article develops an analysis about the statistical nature of the Pondered Deviation from Reference (PDrs) index for knock detection in internal combustion engines. The statistical analysis proved that the PDrs index is a statistic whose probability distribution function follows a log-normal distribution with a mean and variance of 1.015 and 0.0103, respectively. The distribution and the probability knock threshold of the PDrs index is independent of engine operating conditions, types of engines, and fuels used. Through Monte Carlo's simulations were stated that the probability knock threshold is equal to 1.64. Thereby, engines run with abnormal combustion if the PDrs is greater than 1.64, whereas if PDrs ≤ 1.64, engines run under normal combustion.