Relation of the frame in a portable rod-and-frame apparatus to judgements of perceived verticality

To assess the effect of the frame on the mean error in the portable rod-and-frame 18 subjects were tested once with frame present and once without it. 11 frame-dependent subjects produced smaller mean error without the frame; 7 frame-independent subjects were not significantly affected by removal of the frame. Nyborg's statistical method of differentiating frame-dependent and frame-independent subjects appears validated.