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Re: st: Constraints in mprobit
At 08:16 PM 5/25/2006, Emily Clough wrote:
I can get yours to work, but not mine. Here's an example of what I've
been getting (the "Equation[1] not found" error message is the one I get
consistently):
. constraint 1 [1]ref_Chances=0
Try referring to the equation # rather than the value of the dv, e.g.
constraint 1 [#1]ref_Chances=0
There are different ways of referring to equations (value of y, value
label, equation #), and if one way doesn't work, sometimes another
way will. Stata seems to be a little inconsistent across commands as
to how constraints are implemented, or whether they are even allowed at all.
Incidentally, note that in Rich Gate's example, he used the value
labels of Y when referring to the equations. If he had instead done
the seemingly equivalent
. constraint 1 [2]age = [3]age
. mprobit insure age male nonwhite site2 site3, constraints(1)
(note: constraint number 1 caused error r(303))
you see that he would get an error. (But do the same thing with
mlogit and it runs fine).
As a sidelight, you'll see that there are several little differences
between mlogit and mprobit. e.g. mlogit reports a LR chi-square,
mprobit reports a Wald chi-square. mlogit uses the value labels of y
for the equations, mprobit (sometimes) uses labels like _outcome_1,
_outcome_2. They even differ in how you are allowed to abbreviate
constraints! I don't think Stata was aiming for perfect consistency
between the two commands. mlogit seems to have always had some sort
of privileged status among Stata commands, e.g. it can do things with
equation labels that other commands can't do - and my impression is
that Stata kind of regrets that it did that! Perhaps StataCorp feels
that it is stuck with the way it did mlogit but it doesn't have to do
the same things with new commands.
-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
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