Can one specify a hypothesis to test, for example, a difference between:
This gives the two smooth terms, one for each level of the fixed factor. The
hypothesis() should, then, test if there is a difference (i.e., whether
by=by=TwoLevelFactor is justified or not.
Sorry for taking time to reply to your post. I have not worked much with splines, and there could be
brms specific things here so my hope is that @paul.buerkner can reply.
hypothesis() unfortunately does not work with splines because splines are presented by a whole set of themselves uninterpretable parameters. I suggest you created spline predictions via
posterior_smooths and then do the comparison on a per-draw level before summarizing the difference scores via