Suppose that we have a model like the following

stan_lmer(zDiff ~ 1 + (1 | condition) + (1 | subject), data=…)

where ‘condition’ is a within-subject factor with `k`

levels. This may well depend on the specific dataset, but in general how small can `k`

go? Is it OK to build a model when `k`

= 2 or 3 with a typical prior of half_normal or half_student for the scale parameter tau? Would a slightly stronger prior help in this context?