Calculate the control event rate, or CER, of the experiment. The CER is the measure of a reaction in the control group of the experiment. Divide the number of subjects who experience a reaction by the total number of subjects in the control group. For example, if 50 out of 200 subjects experience a reaction, then the CER is 50/200, which is 0.25 or 25%.
Calculate the experimental event rate, or EER, of the experiment. The EER measures the ratio of reactions given the total number of subjects in the experimental group. For example, in the experimental group, 25 out of 200 experience a reaction, then the EER is 25/200 which is 0.125 or 12.5%.
Subtract the EER from the CER, and then take the inverse of that number to find the NNT. For the example, 0.25 minus 0.125 equals 0.125. Divide that number into 1 to find the inverse, so 1 divided by 0.125 equals 8. The NNT is 8.