* Setup
sysuse auto
contract rep78 foreign
keep if rep78 >=3 & rep78 <=5

tab rep78 foreign [fw=_freq], chi2

* our table as an mlogit
mlogit rep78 i.foreign [fw=_freq]

* a reduced categories model
generate rep3 = rep78~=3
logit rep3 i.foreign [fw=_freq]

* The reduced model is similar
*   to imposing several equality
*   constraints on the mlogit.
* Let's test those constraints.
mlogit rep78 i.foreign [fw=_freq]
test [4=5]
test ([4]_cons = [5]_cons)
test ([4]1.foreign=[5]1.foreign)([4]_cons = [5]_cons)  

* This is a simple model, so
*    we could arrive at a similar
*    test statistic by comparing
*    two crosstabs.
tab rep78 foreign [fw=_freq], chi2
scalar chi2 = r(chi2)
tab rep3 foreign [fw=_freq], chi2
display chi2 - r(chi2)

* We would not want to collapse
*    categories 3 and 4 together.
mlogit rep78 i.foreign [fw=_freq], base(5)
test ([3]1.foreign=[4]1.foreign)