Question 2: What is an interaction? How would they test for an interaction between urban-rural living and house dust mite skin sensitisation?
An interaction means that the effect of a predictor variable depends on the value of another, here that house dust mite sensitisation is a much better predictor of wheeze in urban areas than rural areas. They would test for it by including an extra variable in a logistic regression, equal to one for house dust mite sensitive people in urban areas, zero otherwise. Alternatively, if urban is coded one, rural zero, and house dust mite sensitivity is one for sensitive and zero for not sensitive, the interaction variable is found by multiplying these two variables together.
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