national comparisons are irrelevant to the issue of whether New York City public school students have received a sound basic education. The Appellate Division rejected this argument (295 2 at 16). As we have already suggested, the New York Constitution ensures students not an education that approaches the national norm -whatever that may be -- but a sound basic education. Moreover, CFE makes clear that the measure of a sound basic education is educational content -- the set of "basic literacy, calculating, and verbal skills" children acquire and its fit with the goal of productive citizenship (86 2 at 316). Of course, results on a national norm-referenced exam may be translatable into a measure of the skills students must master to have a sound basic education, and we have no cause to doubt that the CTB-R and CAT are designed, as the State argues, to measure mastery of curricula considered important in New York as well as nationally. But during the years reflected in the record, the scores of City schoolchildren on these exams were reported -- as the State admits -- with reference to a norm rather than to achievement levels. The State has not shown how to translate these results into proof that the schools are delivering a sound basic education, properly defined. Thus, while we cannot say that the CTB-R and CAT exam results have no place in the mix of information on outputs, on this record the Appellate Division erred in according primacy to these results. In sum, the Appellate Division improperly relied on the RCTs in that they measure a level of proficiency far below a sound basic education, and, as to exams administered to younger children, it erred in relying on national norm-referenced exam results without evidence tying these results to the constitutional standard. We conclude that the trial court's assessment of exam results, like its assessment of completion rates, better comports with the weight of the credible evidence, and supports its conclusion that, whether measured by the outputs or the inputs, New York City schoolchildren are not receiving the constitutionally-mandated opportunity for a sound basic education. IV. Causation As we noted in CFE, in order to prevail plaintiffs must "establish a correlation between funding and educational opportunity * * * a causal link between the present funding system and any proven failure to provide a sound basic education to New York City school children" (86 2 at 318). The trial court reasoned that the necessary "causal link" between the present funding system and the poor performance of City schools could be established by a