The application to aviation psychology of Gibson's (4) "ecological approach to visual perception" demands information-rich visual displays to adequately and unambiguously enable a pilot to perform flight maneuvers. Optical information often takes the form of invariant properties of a changing optic array, and functional invariants are defined as psychologically effective optical invariants. Their effectiveness is determined by empirical test, but standard experimental paradigms are shown to be inappropriate for testing the effectiveness of information-rich displays. This is due to the presence of inherent and unavoidable confounding factors that are here termed "secondary independent variables" in contradistinction to the "primary independent variables" manipulated by the experimenter. Recommendations for a new methodology and statistical treatment are offered, and the implications for aviation psychology are discussed.