Over the past decades, many studies used global outcome measures like the IQ when reporting cognitive outcome of pediatric brain tumor patients, assuming that intelligence is a singular and homogeneous construct. In contrast, especially in clinical neuropsychology, the assessment and interpretation of distinct neurocognitive domains emerged as standard. By definition, the full scale IQ (FIQ) is a score attempting to measure intelligence. It is established by calculating the average performance of a number of subtests. Therefore, FIQ depends on the subtests that are used and the influence neurocognitive functions have on these performances. Consequently, the present study investigated the impact of neuropsychological domains on the singular “g-factor” concept and analysed the consequences for interpretation of clinical outcome. The sample consisted of 37 pediatric patients with medulloblastoma, assessed 0–3 years after diagnosis with the Wechsler Intelligence Scales. Information processing speed and visuomotor function were measured by the Trailmaking Test, Form A. Our findings indicate that FIQ was considerably impacted by processing speed and visuomotor coordination, which leaded to an underestimation of the general cognitive performance of many patients. One year after diagnosis, when patients showed the largest norm-deviation, this effect seemed to be at its peak. As already recommended in international guidelines, a comprehensive neuropsychological test battery is necessary to fully understand cognitive outcome. If IQ-tests are used, a detailed subtest analysis with respect to the impact of processing speed seems essential. Otherwise patients may be at risk for wrong decision making, especially in educational guidance.Electronic supplementary materialThe online version of this article (doi:10.1007/s11060-017-2582-x) contains supplementary material, which is available to authorized users.