Suppose I'm building a logistic regression model y~ax1 + bx2. How do I know when I may want to think about adding the interaction term, i.e. y~ax1+bx2 +(a*b)x3? Is it purely domain knowledge, or are there some visuals or statistic tests I can try?
My understanding of interaction is when 2 variables combined produce a new affect. i,e: sleeping pills make you sleep, alcohol make you drunk, but sleeping pills * alcohol will make you dead, which is the new affect. Is this right? Does this "new effect" need to be as dramatic as that?