Replacement series designs have been criticized because they may inaccurately predict the outcome of competition, particularly when species’ vital rates respond to competitor density in strongly nonlinear ways. Here we explain that despite this concern, experiments manipulating frequency can still effectively quantify the strength of niche differences in stabilizing coexistence, the goal of an experiment we proposed in an earlier paper. Niche differences cause species to have greater per capita growth rates when rare than when common, and we demonstrate that this result is robust to variation in total density. We also emphasize that our proposed experimental design does not call for fixing density across species’ frequency gradients, thus differing from a traditional replacement series design. We show that our approach and the more labor-intensive response surface design share the same theoretical foundation and both are apppropriate for quantifying the role of niche differences in stabilizing the dynamics of coexisting species.