A method is proposed for measuring the prompt decay eigenvalue of the neutron population. It is based on the determination of the covariance of the integrated outputs from two neutron detectors placed in a nuclear reactor, for different values of the integration time interval. The covariance is measured by an analysis of the four types of combined outputs which can occur if only the sign of the signal with respect to its mean is recorded from each detector. In fact, the frequence of every combination ++, −+, −, +− assumes a different value according to the degree of coherence between the two detector counting outputs. The method allows experiments to be made with low-detection efficiency and can be applied also to fast reactor-noise analysis, unlike all the other variance-type procedures. Since the detection of only the sign of the variables is needed, a pulse counter is not indispensable and, therefore, the technique is expected to be applicable even to nonzero power reactors.