CODEWITHSUNDEEP
rregressionlogistic-regressionordinal
Carl
Carl

Reputation: 5779

Understanding errors from ordinal logistic regression

I am using MASS::polr to run ordinal logistic regressions, but I am getting a lot of errors that I am hoping people can enlighten me about.

First if I run this the function fails to find starting values:

MASS::polr(as.ordered(cyl)~mpg+gear,mtcars)

So if I specify starting values, I get an error from optim stating 'non-finite value supplied by optim':

MASS::polr(as.ordered(cyl)~mpg+gear,mtcars,start=c(1,1,1,1))

After reading some R-help, and previous stack overflow questions about this, the response is usually that there is something wrong with the data i.e. the response variable has a category with relatively few values, but in this instance I don't see anything wrong with mtcars.

Any guidance on how to diagnose, and deal with issues in data that will impact MASS::polr would be appreciated.

Regards

Upvotes: 0

Views: 1133

Answers (1)

Gregor Thomas
Gregor Thomas

Reputation: 146070

Going on a scavenger hunt through ?polr, the starting values are to be specified "in the format c(coefficients, zeta)". Looking lower, we see that zeta is "the intercepts for the class boundaries.". In the Details section, we can see that the zeta values must be ordered:

zeta_0 = -Inf < zeta_1 < ... < zeta_K = Inf

([sic], as that presumably should be a < Inf at the end.) So you need the second zeta value to be greater than the first. This works, for example:

MASS::polr(as.ordered(cyl) ~ mpg + gear, mtcars, start = c(1, 1, 1, 2))

Upvotes: 2

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