Third grade elementary school children solved tests on mathematical reasoning and numerical facility. Metacognitive skillfulness was assessed through think aloud protocols, prospective and retrospective child ratings, teacher questionnaires, calibration measures and EPA2000. In our dataset metacognition has a lot in common with intelligence, but planning measured with teacher ratings plays a role above and beyond IQ. Moreover, we found that skills are generally related, but that it is more appropriate to assess them separately. In addition, results show the value of an experienced teacher as actual measure of metacognitive planning skills. Our dataset suggests convergent validity for prospective and retrospective child ratings, but no significant relationship with the other metacognitive measures. Metacognitive skillfulness combined with intelligence accounts for between 52.9% and 76.5% of the mathematics performances. The choice of diagnostic instruments highly determines the predicted percentage. Consequences for the assessment of metacognitive skills are discussed.