Suppose we wanted to use data on children of all gestational ages in the study. Suggest a type of analysis that could be used to relate the Bayley score to severe hypothyroxinemia while controlling for gestational age. (Do not actually carry out the analysis.)

To test the primary hypothesis of the study, infants were categorized by gestational age and were designated as having severe hypothyroxinemia if their thyroxine concentration was 2.6 sd below the mean score for the assay that their specimen came from. Thyroxine assays typically were done in batches of 240 specimens; a mean and sd were calculated for each batch, based on a sample size of 240. Children in the study were given the Bayley Mental Development Index at <30 months of age. The Bayley test is a commonly used test of mental development in young children. The results for the subgroup of children with gestational age 30ˆ’31 weeks are shown in Table 11.22.

Table 11.22: Bayley Mental Development Index by presence or absence of severe hypothyroxinemia

 

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Sevore hypothyroxinemia Mean score sd No 106 + 21 88 + 25 138 Yes 17