Probability unit obtained by adding 5 to the normal deviates of a standardized normal distribution of results from a dose response study.

Note 1: Addition of 5 removes the complication of handling negative values.

Note 2: A plot of probit against the logarithm of dose or concentration gives a linear plot if the response follows a logarithmic normal distribution. Estimates of the LD50 and ED50 (or LC50 and EC50) can be obtained from this plot.