An analysis of the errors made by a skilled female typist led to the identification of error factors that often act conjointly. Substitution errors indicated that she had acquired a cognitive map of the keyboard and then controlled her fingers by spelling the to-be-typed words covertly. Accordingly, some typographical errors were attributed in part to errors of inner speech. Intrusion errors revealed both habit factors and a response perseveration tendency. A handedness effect and omission errors indicated response error factors, and transpositions indicated temporal error factors. The skilled typist was thus viewed as having learned to inhibit error tendencies, and errors such as antedating response and doubling were interpreted in terms of disinhibition.At midcentury, Lashley (1951Lashley ( , 1961 published a seminal article concerning the serial order of behavior. Prior to that time, the generally accepted view was that of a behavior chain in which each response, in turn, produces feedback that serves as the cue for the next response (see, e.g., Hull, 1931;James, 1890;Washburn, 1916). Lashley (1951Lashley ( , 1961 marshaled a number of arguments against that view, at least for behaviors such as copy typewriting in which a finite set of responses can be emitted rapidly in a variety of orders. He concluded that skilled performance could be understood only in terms of a hierarchical system containing motor programs that enable a sequence of responses to be performed without the aid of feedback.For example, Lashley (1951) mentioned the doubling error, in which a wrong letter is doubled when typing a word containing a double letter (e.g., eeror). According to Lashley, "the set to repeat may be displaced ... the order is dissociated from the idea" (p. 118). This basic concept was captured by Rumelhart and Norman (1982) in a simulation of a skilled typist; in their model, the program contained a double schema that could be transposed as a result of noise leading to doubling the wrong letter. Although he was preceded by Fendrick (1937), Lashley (1951 set the stage for hierarchical theories of performance (e.g.,