Factors, both acoustic and non-acoustic, which may influence the Most Comfortable Loudness Level, the Uncomfortable Loudness Level, the Acoustic Reflex Threshold and the Threshold of Overload are reviewed. A direct experimental comparison of these measures was made with respect to their absolute values and the between- and within-subject variances. The Most Comfortable Loudness Level was found to have the greatest within- and between-subject variability. The Uncomfortable Loudness Level, although having high between-subject variability, did not have significantly greater within-subject variability than the Acoustic Reflex Threshold. The significance of these results is discussed.