The interaction index, isobologram and the appropriate contour of constant response of a dose response surface each offer essentially equivalent information regarding departures from additivity in a chemical combination. The benefit of relating the interaction index and isobolograms to a contour of a fitted dose response surface is that departures from additivity can be related to parameters of a statistical model. This permits an assessment of the statistical significance of the parameters and conclusions regarding the nature of departures from additivity which account for the variability inherent in the experiments used to generate the data. Relationships between statistical models and experimental designs can be exploited to yield economical designs for studying chemical combinations.
