Pacaud, S., Kalsbeek, J. W. H., & Hoogstraten, J. (1968). Information handling capacity as a function of the degree of meaningfulness in a verbal copying task. In XVIth International Congress of the International Association of Applied Psychology (pp. 547-552). Amsterdam.
Elshout, J. J., & Frijda, N. H. (1967). The origin of problem solving methods. In Symposium on heuristic processen in thinking at the International Congress of Psychology , Moscow, August 1966 Amsterdam: University of Amsterdam.