Probability unit obtained from the standardized normal distribution plotted against the logarithm of the concentration or dose of a substance when a quantal or graded response has been measured. A linear plot provides evidence that the distribution is log-normal. Estimates of the \(\pu{L(E)C_{50}}\) and \(\pu{L(E)D_{50}}\), as well as the standard deviation for the distribution, can then be made.