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The Wigner-Ville spectrum is the only conjoint time-frequency representation of the second order properties of non-stationary random signals which is real-valued and compatible with linear filtering and modulation. Pseudo Wigner estimators are known to form a class of estimators being more flexible than short time periodograms: They give uncorrelated estimates for appropriately spaced neighboured frequencies allowing separately to measure the degree of non-stationarity for each band of frequencies. Using the model of quasi stationary processes, their stability and precision are defined as a measure of their character of non-stationarity. The performances are then tested on simulated data and successfully applied to the problem of classifying biological time-keeping facilties.