Today I re-run some code in R and was greeted with an error “Error in factorize(formula, data, f_out = TRUE)” and (more specifically) “Unable to find variable to convert to factor.” I immediately suspected the as.factor(x) among the predictor variables to be the culprit, but since this is analysis that has worked a few days earlier (on a different machine), I quickly searched on the web and found nothing. For some reason, in this case the as.factor(x) did not work, and the solution was simple: create a new (factor) variable separately and then run the regression analysis with the new variable. So instead of:
z <- zelig(y ~ x1 + as.factor(x2) + x3, model="normal", data=d)
I first create the new variable:
d$x2_factor <- as.factor(d$x2)
and then run the regression analysis with the new variable:
z <- zelig(y ~ x1 + x2_factor + x3, model="normal", data=d)
I just thought I’d share this in case someone else comes across this error and doesn’t find it obvious what the solution is.
Didier Ruedin 
Thank you! This probably should have been obvious to me but it was not.
This was so clear and helpful.
I only posted this because it wasn’t obvious to me… not until I found the solution.