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

Objective: 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.

Method: 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: 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.

Conclusion: 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.