How to deal with NA in index for Panel data

Question

I want run regression in panel data. There is NA in my index (ID,time). I do not want to delete NA in index.

Result<-plm(TBL~Tang+Prof+LnSALES+MB,data=Panel,model="within",index=c("Firms","Time"))

When I the above code, it returns:

at least one couple (id-time) has NA in at least one index dimension in 
resulting pdata.frame 
to find out which, use e.g. table(index(your_pdataframe), useNA = "ifany")
Error in model.matrix.pFormula(formula, data, rhs = 1, model = model,  : 
NA in the individual index variable

Without delete NA, how can I run panel regression with NA in index? Thanks

Answer 1

Since you are just looking to omit observations where your index is NA, you can do this:

Result <- plm(TBL ~ Tang + Prof + LnSALES + MB,
              data=Panel[which(!is.na(Panel$Firms)), ],
              model="within", index=c("Firms", "Time"))

Explanation

data=Panel[which(!is.na(Panel$Firms)), ] tells plm() to use as data a subset of Panel where Panel$Firms is not NA. Then you cannot have the issue where

at least one couple (id-time) has NA in at least one index dimension in 
resulting pdata.frame