Did anyone else get this warning?
snctmodel <- zelig(RES ~ IMPORT + COST + TARGET +
COOP +
TARGET*COOP, model="oprobit", data=snct)
Warning message:
In function (formula, data, weights, start, ..., subset, na.action, :
design appears to be rank-deficient, so dropping some coefs
What causes this? Does it mean the estimates are wrong?