add_references
True if it should use the data set to look for
references, otherwise supply the function with a vector with names.
Sometimes you want to indicate the reference row for each group.
This needs to be just as many as the groups as the order identified.
Use NA if you don't want to have a reference for that particular group.
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)