Figure 1

In this diagram two dose-response-relationships are plotted whereas E_max denotes the fraction of the maximum effect. Therapy 1 is quadratic (y = 10 x²) and therapy 2 is linear (y = 2,5 x). One needs one dose unit of therapy 1 to obtain 10% of the maximum effect and four dose units of therapy 2 for the same effect; so a combination would yield (in the strict response additive case) 20%. In the case of Loewe-additivity one would analyse as follows: therapy 2 yields the same like one unit of therapy 1. So the effect would be the same as for two units of therapy 1, namely 40%.