Most experimental psychologists investigating memory have presented material of one sort or another to subjects and required them to recognize or recall it subsequently. The criterion of success is the recall or recognition of the correct material. Correctness is determined by the experimenter; in the recall task, the degree of correctness required is usually verbatim; that is, the subject has to recall the material in the form in which the experimenter presented it. More recently, some psychologists have required subjects to produce verbal or other behaviour which is derived from their general store of past experience rather than from a particular presentation by the experimenter. In this case there are seldom 'correct' responses. The objective is to obtain evidence concerning the structure of the subject's knowledge.