Not so fast. Take a closer look at the dose-response curves above. In this idealized example, both dose-response relationships are perfectly described by 4-parameter logistic curves, which implies that the EC50 is the inflection point of the curve. Both of the marketing statements of brand A and brand B are correct, i.e. the EC50 of brand A is at 10 grams per liter, while the EC50 of brand B is at 5 grams per liter. But the overlay plot clearly shows that, if your interest is to kill almost all the bugs (and not only half of them) in your garden, brand A is the better choice. Since the dose-response relationships are dissimilar, comparing the two products using only a single number, e.g. "2" as in "twice as potent", is misleading marketing at best, but, depending on the drug or substance that is being evaluated, can be very dangerous.

__What are "parallel" curves?__

As you've learned in basic geometry, two lines are parallel if their slopes are identical and their intercepts are different. When graphed, the plot of the parallel line looks shifted or translated. In fact, the graphs never intersect. For curves, e.g. a 4-parameter logistic curve, the "identical slope" characterization of parallelism does no longer apply, but the other interpretations of parallelism can be generalized. For example, two 4-parameter logistic curves are said to be parallel, or similar, if their asymptotes and effect rates (their B-parameters) are identical and only their Inflection points (C-parameters) differ. Graphically, the curves look like horizontally translated copies of each other.