Final answer:
Socio-economic class in the study acts as a confounding variable that can influence the relationship between breast-feeding and the reduced incidence of stomach problems in babies.
Step-by-step explanation:
In the study mentioned, socio-economic class represents a confounding variable that is associated with both the exposure (breast-feeding) and the effect (stomach problems). A confounding variable is one that influences both the independent variable and the dependent variable, causing a spurious association. In this case, the higher socio-economic class of the mothers who breast-feed may be related to better overall child health outcomes, including fewer stomach problems, due to factors like better hygiene and less crowding, rather than the breast-feeding alone.
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