I conducted a multilevel analysis using repeated measures across four time points. The model contains intercept, linear slope and quadratic slope. I am interested in examining the extent to which variable A interacts with other identified variables in my model, so I added the interaction terms to intercept, linear slope and quadratic slope to estimate the fixed effects of these interactions. I then eliminated the non-significant interactions and retan the model until the interaction terms in the model were all significant.
My questions are:
When delete non-significant interactions from the model, if the interaction was significant on linear slope but not quadratic slope, should I still keep the non-significant one on quadratic slope? Or should I only keep the significant one on linear slope?
How should I interpret an interaction on linear slope but not quadratic slope (or vice versa)?
Thank you for your response in advance! Any advice is appreciated.