# What is synergism or antagonism?

(this post is a follow up to What is an interaction?)

In the experiment for Figure 1 of the motivating source article, the researchers were explicitly interested in measuring any synergistic effects of hac and indole on the response. What is a synergistic effect? If hac and indole act independently, then the response should be additive – the HAC+Indole effect should simply be the sum of the independent HAC and Indole effects. A HAC+Indole effect that is larger than the sum of the two independent effects is evidence of a synergistic (positive) interaction. A HAC+Indole effect that is less than the sum of the two independent effects is evidence of an antagonistic (negative) interaction.

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