Dear John,
Apologies for not providing reproducible example. I just tried with a plm
example but ran into the same issue;
library(plm)
data("Produc", package = "plm")
zz <- plm(log(gsp) ~ log(pcap) + log(pc) + log(emp) + unemp, data = Produc,
index = c("state","year"))
Ef.hd <- Effect(c("pc", "emp", "unemp"), zz)
Error in UseMethod("droplevels") :
no applicable method for 'droplevels' applied to an object of class "NULL"
What am I doing wrong? Thanks again.
Sincerely,
Milu
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