10.94 Breast-feeding helps IQ? A Danish study of individuals born at a Copenhagen hospital between 1959 and 1961 reported higher mean IQ scores for adults who were breast-fed for longer lengths of time as babies

(E. Mortensen et al., JAMA, vol. 287, 2002, pp. 2365–2371).

The mean IQ score was 98.1 1s = 15.92 for the 272 subjects who had been breast-fed for no longer than a month and 108.2 1s = 13.12 for the 104 subjects who had been breast-fed for seven to nine months.

a. With software that can analyze summarized data, use an inferential method to analyze whether the corresponding population means differ, assuming the population standard deviations are equal. Interpret.

b. Was this an experimental or an observational study?

Can you think of a potential lurking variable?