Dear all,
This is not primarily a mixed models question, so I'll ask it in the
framework of glm(). But I have the same question w/r/t glmer().

In my field, sociolinguistics, researchers have used a software tool for
some thirty years that performs logistic regression assuming categorical
predictors. This software is usually called VARBRUL (the current version of
it is called GoldVarb).

Assume a data file like this:

response pred1 pred2
0 a x
1 a y
1 a x
0 a y
0 b x
1 b y
0 b x
0 b y
1 a /
0 b /

My question is about the behavior of the slash (/) used in the last two
lines. Assume sum contrasts. The software estimates the values of a and b
(which sum to zero) and of x and y (which sum to zero). The interesting part
is that for the last two data points, the predicted values are calculated on
the basis of pred1 only, and pred2 is ignored.

Looking at the various options of na.action, I do not see anything that
would correspond to this. Basically we have NA in a certain predictor column
and we want this predictor ignored for the row in question - we don't want
the whole row omitted.

Any way to accomplish this in R?

Dan

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