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Re: st: Bug in margins


From   Richard Williams <[email protected]>
To   [email protected]
Subject   Re: st: Bug in margins
Date   Fri, 30 Aug 2013 13:27:20 -0500

At 11:53 AM 8/30/2013, you wrote:
On Aug 30, 2013, at 10:58 AM, Joe Canner <[email protected]> wrote:
> The requirement that the original model be specified including "i." is also buried deep in the PDF documentation for -margins- (page 1063 of r.pdf).


Actually, the very first line displayed by -help margins- is

        margins [marginlist] ...

    where marginlist is a list of factor variables or interactions that
    appear in the current estimation results.

A pedant might argue that this should read "a list of factor variables or *their* interactions," but regardless, the information *is* accessible.

That said, the fact that so many ask this question does perhaps suggest that a more prominent note be added to the documentation and/or that the error message in this case be changed to "only factor variables and their interactions are allowed," which is the error message you would have gotten if you had typed

    margins c.lapscopiintent

I like your error messages better. Other possible sources of confusion:

. margins gpa
gpa:  factor variables may not contain noninteger values

True, but gpa wasn't a factor variable to begin with, so it would be better to give the "only factor variables" message.

Even ""a list of factor variables or *their* interactions," is potentially confusing because you can't include i.gender#c.income, i.e. the interactions can only be among the factor variables.

And, for good measure, the interactions don't have to be in the estimation command, e.g.

reg y i.x1 i.x2
margins x1#x2

is legitimate. There is no interaction term in the model but if, say, x1 and x2 both had 2 values, you would get a 4 part classification. So I am not even sure what "their interactions" means. The margins notation looks like interactions but no interactions are needed in the model.

I am sure it is hard to come up with crystal clear explanations for every error. I think the biggest thing would be to get the "only factor variables" message out there more often. The "covariates" wording will be very unclear to many people; if you say "factor variables" people have more of a fighting chance of figuring out what the problem is.




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Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
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