Specifying which category to treat as the base with 'statsmodels'

Question

In understand that when I have a category variable in a model passed to a statsmodels fit that dummy variables will automatically be generated for the

Answer 1

If you use single quotes to wrap your string, reference's argument needs to be wrapped with double quotes. Very easy mistake to make. I was using single quotes on both.

For example:

'y ~ C(Location, Treatment(reference="China"))'

is correct.

'y ~ C(Location, Treatment(reference='China'))'

is not correct.

Answer 2

You can pass a reference arg to the Treatment contrast, using syntax like

"y ~ C(Location, Treatment(reference='China'))"

http://patsy.readthedocs.org/en/latest/API-reference.html#patsy.Treatment

If you have a better suggestion for naming conventions please file an issue with patsy.