Premise: Wilder and his colleagues are cautious about their results also because the data do not make clear whether it was the anesthesia that contributed to the children's learning deficits, or whether it was an underlying condition that may have required surgery and precipitated the learning problems. Of the more than 5,000 babies studied, 593 needed at least one surgery and just over 100 infants needed more than two before age 3. There may be something unusual about this population of children that could have made them vulnerable to learning problems and required they undergo surgery and anesthesia.
Hypothesis: Some babies were required to have surgery.