plm: duplicate couples

Question

I am working with panel data in R, and I want to run a two way panel fixed effects model on my data set so I used plm:

plm1 <- plm(rprofit_acre~dd89+prcp_0410, data=data, effect="twoways", model="within")

and I am getting the error message

"Error in pdim.default(index[[1L]], index[[2L]]) : duplicate couples (id-time)"

I used the suggestion in the error message to see if there are any duplicates of state+year combination with the line

any(table(data$state,data$year)!=1)

and the result was TRUE, meaning that there aren't any duplicates of state+year combination.

Answer 1

The suggestion in the error message (which you do not reprint fully) is actually table(index(your_pdataframe), useNA = "ifany"). If it prints something differently on your end, use a current version of the package.

You modified that and test for any combinations which appear less or more than once (by != 1). If that yields TRUE, you cannot infer there are not doublicate couples, rather the opposite is true (as there cannot be less than 1 combinations, there must be more than one).

So, you can use the suggestion from the error message to identify the doublicate combinations (your expression just tells you that there are some).