Although the definition of “mental work” remains elusive, systematic means/ methods should be considered for gaining evidence about interactive language features requiring more/less effort of the human mind. The suggested approach employs a structuring of the. user's conceptual reference spaces into sets of “action primitives”, peculiar to the type of computer-aided task involved. An interactive command language can then be regarded as the range of some transformation on the user's set of action primitives. The nature and efficiency of that transformation, in conjunction with the inherent number of mental association links, are hypothesized to have direct relationships to the level of required mental work. The user's delay or “think time”, expended immediately preceding command utilization, is one measurable quantity that should be useful as a work level indicator.