Certain safety factor

The Certain Safety Factor is the ratio of the lethal dose to 1% of population to the effective dose to 99% of the population (LD1/ED99). This is a better safety index than the LD50 for materials that have both desirable and undesirable effects, because it factors in the ends of the spectrum where doses may be necessary to produce a response in one person but can, at the same dose, be lethal in another.

$$\mbox{Certain safety factor} = \frac{\mathrm{LD}_{1}}{\mathrm{ED}_{99}}$$