I’m having trouble understanding what kind of data and analyses the colon : and slash / operators would be used for. They’re briefly discussed in the multilevel modeling vignette/PDF:
Multiple grouping factors each with multiple group-level effects are possible. Usually, group contains only a single variable name pointing to a factor, but users may also use g1:g2 or g1/g2 if both g1 and g2 are suitable grouping factors. The : operator creates a new grouping factor that consists of the combined levels of g1 and g2. The / operator indicates nested grouping structures and expands one grouping factor into two or more when using multiple / within one term. For instance, if the user were to write (1 | g1/g2), brms will expand this to (1 | g1) + (1 | g1:g2).
What’s an example of data & an analysis goal where we would want to use (1 | g1:g2) instead of (1 | g1 + g2)?
What about when we would want to use (1 | g1/g2) instead of (1 | g1:g2) or just (1 | g1 + g2)?