Can a multidimensional test be evaluated with unidimensional item response theory?

The aim of this study was to evaluate possible consequences of using unidimensional item response theory (UIRT) on a multidimensional college admission test. The test consists of 5 subscales and can be divided into two sections, that is, it can be considered both as a unidimensional and a multidimensional test. The test was examined with both UIRT and multidimensional IRT (MIRT). Simulations were used to examine item and ability parameter recovery when UIRT and MIRT models were used. The results obtained from the college admission test showed that although we get a better model fit when using MIRT instead of UIRT, the difference is small if we compare it with using a consecutive UIRT approach. The results from the simulations indicate that if the test only has between-item multidimensionality, it is probably not harmful to use UIRT instead of MIRT models.