add_references_pos
The position where a reference should be added.
Sometimes you don't want the reference to be at the top, for instance
if you have age groups then you may have < 25, 25-39, 40-55, > 55 and
you have the reference to be 25-39 then you should set the reference
list for age_groups
as add_references_pos = list(age_groups = 2)
so that you have the second group as the position for the reference.
reference_zero_effect
Used with references, tells if zero effect
is in exponential form, i.e. exp(0) = 1
, or in regular format,
i.e. 0 = 0
(can be set to any value)