Probably a basic question, but say I have a model Y ~ X1*X2 and both X1 and X2 are continuous. How would I do a directional hypothesis test for the effect of X1, conditioned on the predictor X2 being some arbitrary value, e.g., X2 = 2?
Just plug in
X2 (or whatever value) when computing your predictions.
predict function? If so, how do I then test the hypothesis (e.g., that the effect of X1 > 0)?
X2 = 2:
hypothesis(<model>, "X1 + X1:X2 * 2 > 0")
Of course, that’s beautiful and simple! Thank you.