The position effect in tests with a time limit: the consideration of interruption and working speed

The position effect is a possible source of impairment of the structural validity of a test concerning model fit. In the case of tests with a time limit there is even a complication of the situation because of a decreasing number of participants completing the last few items of the test. Therefore, it is assumed that the appropriate representation of the position effect must additionally consider interruption due to the time limit and the effect of working speed. Interruption can be represented by the same latent variable as the position effect whereas the contribution of working speed requires another one. Confirmatory factor models including a representation of the position effect as a linear, quadratic or logarithmic increase were compared with models additionally considering interruption as a logistic decrease or simply as immediate interruption. Furthermore, there were models additionally considering working speed. In the sample of 305 participants the investigation of probability-based covariances made apparent that the modeling of interruption and also working speed substantially improved model fit. The best-fitting model was characterized by a linearly increasing representation of the position effect combined with a logistic decrease in the more difficult items and a contribution due to working speed.