Thank you Ricardo for taking the time to look at the puzzling code.
Runing on PyMC v4.4.0 Just to follow up: if I run the code for σ = pm.HalfStudentT(“σ”, nu=30,scale=2000) using 'scale as mentioned does not seem to work, it gives:-
error:TypeError: make_node() got an unexpected keyword argument ‘scale’ .
Also σ = pm.HalfNormal(“σ”,sigma=2000) seems to work.
I think I might be wise to avoid σ = pm.HalfStudentT(“σ”, nu=30,scale=2000) and just use pm.HalfNormal(“σ”,sigma=2000). I Am assuming its ok to parameterizing normal distribution with mu and sigma and half-normal with sigma. thanks again for helping with these challenging puzzles. declan