The use of rating scale isosensitivity data to test a discrete, two‐state model for binary choice detection experiments is shown to be inappropriate unless new assumptions are made. It is argued that, whatever assumptions are added, rating‐scale isosensitivity curves will be different from binary‐choice isosensitivity curves. The suggestion is made that a multistate model may be more appropriate than a two‐state model for rating experiments, but the resulting isosensitivity curves appear to be too similar to those of signal‐detectability theory to make rating‐scale data useful for deciding between the theories.