Assessment of planning ability: Psychometric analyses on the unidimensionality and construct validity of the Tower of London Task (TOL-F).

The Tower of London (TOL) is widely used to assess planning ability as a prototypical executive function in healthy and clinical populations. Despite its popularity, there is still no consensus on (a) whether the TOL measures a psychometrically unidimensional trait, and (b) how differences in problem structure relate to the difficulty of individual items.Employing the framework of factor analysis and item response theory we investigated these issues of construct validity in a sample of 798 participants (443 female; 16 to 84 years). Participants worked on the TOL-Freiburg version (TOL-F) comprising a set of 24 4- to 6-move problems, which--based on comprehensive cognitive task analyses-systematically differ with regard to several structural problem parameters (minimum number of moves, search depth, goal hierarchy).Results revealed that TOL-F performance is mainly explained by 1 major factor, while further minor factors additionally account for smaller, but possibly still informative, shares of variance. Individual item difficulties can be predicted by the experimentally varied problem parameters to a high degree (r =.89) and can be considered stable across different levels of age, sex, education, and planning ability.These findings demonstrate the TOL-F's construct validity as measuring planning ability in terms of an essentially unidimensional cognitive function while adhering to theory-driven concepts of task difficulty. TOL-F task performance hence represents an accurate, robust, and theoretically grounded estimation of a participant's planning ability. The results further highlight the merit of using established concepts from experimental psychology for improving neuropsychological assessment.