There are, however, ways to increase independence among thought trials. Campbell (1962) argued that one implication of Simon's work with problem solving is that variation with a strong classification system is preferable to variation without strong classification (p. 66). A strong classification system, one in which an event clearly falls in one and only one category, is especially helpful when solutions are expected to be nonrandomly distributed. Classifications suggest when thought trials may be variations on the same theme rather than variations on different themes.