Because of the aforementioned data limitations, together with the complexity of the rules, simulations of Medicaid eligibility will almost invariably be incomplete, and even very good simulations may exclude as much as one-fifth of the eligible population. This may provide the strongest argument against substituting administrative estimates for survey estimates of participants in order to calculate participation rates. If the deficiencies of the eligibility simulation can be matched to the eligibility categories reported in the Medicaid statistics, however, it may be possible to construct an administrative count of participants that is reasonably consistent with the eligibility simulation. For example, we have noted that the medically needy component of Medicaid is the most difficult to simulate, and many analysts make little attempt to do so. Medically needy children under 21, where “children” are defined by the nature of their eligibility rather than their age, are reported in the annual statistics released by HCFA(now known as CMS), and therefore they could be subtracted from an administrative count of participants to yield a numerator that could be used to calculate a Medicaid participation rate that excluded the medically needy from both the numerator and denominator.