The derivations of human exposure limits for laser radiation rely heavily upon experimental ocular injury studies. The limits are derived by committees of ophthalmic experts through a review of all available threshold data and an understanding of mechanisms of laser/tissue interaction. A major point of discussion in this derivation process relates to the level of uncertainty of the threshold of injury. An indication of the level of uncertainty relates to the slope of the transformed dose-response curve, or the "probit plot" of the data. The most cited point on the probit plot is the exposure that represents a 50% probability of injury: the ED-50. This value is frequently referred to as the "threshold," even though some experimental damage points exist below this "threshold." An analysis of any number of example data sets reveals that the slope in most experiments cannot be explained by biological variation alone. The optical, thermophysical, and biological factors influencing the probit plot are critically analyzed to provide guidance for deriving exposure limits. By theoretically modeling an experiment, small errors in focus are shown to produce a substantial change in the ED-50 and the slope of the probit plot.