The state of a linear system is independent of how hard it is driven. Not so, a nonlinear system. As you sweep a nonlinear system through resonance you change the state of the system. If you want to study a system in a particular state it is advantageous to maintain it in that state. The antagonism between conventional experimental practice, sweeping through a resonance, and the desire to maintain the system in a particular state is mitigated by constant strain analysis. The analysis of a sequence of resonance curves on a Berea sandstone will be used to illustrate application of constant strain analysis. The idea of constant strain analysis generalizes to constant field analysis. Resonance data can be treated so that the field (displacement, strain, velocity, ...) responsible for the nonlinear behavior of a system can be identified.