We introduce a procedure for deciding when a mass-action model is incompatible with observed steady-state data that does not require any parameter estimation. Thus, we avoid the difficulties of nonlinear optimization typically associated with methods based on parameter fitting. Instead, we borrow ideas from algebraic geometry to construct a transformation of the model variables such that any set of steady states of the model under that transformation lies on a common plane, irrespective of the values of the model parameters. Model rejection can then be performed by assessing the degree to which the transformed data deviate from coplanarity. We demonstrate our method by applying it to models of multisite phosphorylation and cell death signaling. Our framework offers a parameter-free perspective on the statistical model selection problem, which can complement conventional statistical methods in certain classes of problems where inference has to be based on steady-state data and the model structures allow for suitable algebraic relationships among the steady-state solutions.I n many branches of science and engineering, one is often interested in the problem of model selection: Given observed data and a set of candidate models for the process generating that data, which is the most appropriate model for that process? Such a situation commonly arises when the inner workings of a process are not completely understood, so that multiple models are consistent with the current state of knowledge. For mechanistic models, e.g., ordinary differential equation (ODE) or stochastic dynamical models, most selection techniques involve parameter estimation, which typically requires some form of optimization, exploration of the parameter space, or formal inference procedure (1, 2). For sufficiently complicated models, however, this task can become infeasible, owing to the nonlinearity and multimodality of the objective function (which penalizes any differences between the data and the model predictions), as well as the high dimensionality of the parameter space (3).Here, we present a framework for the discrimination of massaction ODE models (and suitable generalizations thereof) that does not require or rely upon such estimated parameters. Our method (Fig. 1) operates on steady-state data and combines techniques from algebraic geometry, linear algebra, and statistics to determine when a given model is incompatible with the data under all choices of the model parameters. The core idea is to use the model equations to construct a transformation of the original variables such that any set of steady states of the model under that transformation possesses a simple geometric structure, irrespective of parameter values. In this case, we insist that the transformed steady states lie on a plane, which we detect numerically; if the observed data are not coplanar under the transformation induced by a given model, then we can confidently reject that model.The idea of transformation to coplanarity has been employed before, but previous efforts ...