Threshold variability in subliminal perception experiments: fixed threshold estimates reduce power to detect subliminal effects.

In studies of subliminal perception, a threshold for awareness is usually estimated for each S (e.g., the stimulus duration that yields chance discrimination performance). This article shows that if the S's true threshold varies randomly from trial to trial, estimation of a fixed threshold has important consequences. Specifically, the estimated threshold approaches the minimum of the distribution of true thresholds, and the power of an experiment to detect subliminal perception is reduced. Because there is much evidence that true thresholds vary across trials, threshold variability may have seriously weakened previous experiments that tested for subliminal perception. Fortunately, it is possible to estimate the maximum power loss resulting from threshold variability and therefore to design an experiment with acceptable power even in the presence of variability.