Quantitative and qualitative interactions

ICH makes the distinction between quantitative and qualitative interactions. A quantitative interaction refers to the situation where the treatment difference is consistently in one direction (for example A is always better than B), but there are differences in terms of magnitude. A qualitative interaction is where the treatment difference is in a different direction for some centres (for example A is better than B in some centres but B is better than A in other centres). The previous example is an example of a quantitative interaction. Had the A mean in centre 4 been 3.9 and the B mean 5.0, then the treatment difference (A — B) would equal —1.1 and we would have a qualitative interaction. 

ICH E9 (1998) Note for Guidance on Statistical Principles for Clinical Trials  

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