Predict out of sample on fixed effects model - r

Question

Let's consider model :
library(plm)
data("Produc", package = "plm")
model <- plm(pcap ~ hwy + water, data = Produc, model = 'within')
To calculate fitted value of the model we just need to use :
predict(model)
However, when trying to do this out of sample :
predict(model, newdata = data.frame('hwy' = 1, 'water' = 1))
Will get error :
Error in crossprod(beta, t(X)) : non-conformable arguments
Which is quite strange for me because this code will work for any model expect 'within'. I search that there is a function fixef which do predictions on fixed effect model but unfortunately - only in sample.
So : Is there any solution how can we predict out of sample on fixed effect model ?

Answer 1

Just delete intercept for model :
model <- plm(pcap ~ 0 + hwy + water, data = Produc, model = 'within')
predict(model, newdata = data.frame('hwy' = 1, 'water' = 1))
3.980911

Answer 2

Regarding out-of-sample prediction with fixed effects models, it is not clear how data relating to fixed effects not in the original model are to be treated, e.g., data for an individual not contained in the orignal data set the model was estimated on. (This is rather a methodological question than a programming question).
The version 2.6-2 of plm now allows predict for fixed effect models with the original data and with out-of-sample data (see ?predict.plm).
Find below an example with 10 firms for model estimation and the data to be used for prediction contains a firm not contained in the original data set (besides that firm, there are also years not contained in the original model object but these are irrelevant here as it is a one-way individual model). It is unclear what the fixed effect of that out-of-sample firm would be. Hence, by default, no predicted value is given (NA value). If argument na.fill is set to TRUE, the (weighted) mean of the fixed effects contained in the original model object is used as a best guess.
library(plm)
data("Grunfeld", package = "plm")
# fit a fixed effect model
fit.fe <- plm(inv ~ value + capital, data = Grunfeld, model = "within")
# generate 55 new observations of three firms used for prediction:
# * firm 1 with years 1935:1964 (has out-of-sample years 1955:1964),
# * firm 2 with years 1935:1949 (all in sample),
# * firm 11 with years 1935:1944 (firm 11 is out-of-sample)
set.seed(42L)
new.value2 <- runif(55, min = min(Grunfeld$value), max = max(Grunfeld$value))
new.capital2 <- runif(55, min = min(Grunfeld$capital), max = max(Grunfeld$capital))
newdata <- data.frame(firm = c(rep(1, 30), rep(2, 15), rep(11, 10)),
year = c(1935:(1935+29), 1935:(1935+14), 1935:(1935+9)),
value = new.value2, capital = new.capital2)
# make pdata.frame
newdata.p <- pdata.frame(newdata, index = c("firm", "year"))
## predict from fixed effect model with new data as pdata.frame
predict(fit.fe, newdata = newdata.p) # has NA values for the 11'th firm
## set na.fill = TRUE to have the weighted mean used to for fixed effects -> no NA values
predict(fit.fe, newdata = newdata.p, na.fill = TRUE)
NB: When you input a plain data.frame as newdata, it is not clear how the data related to the individuals and time periods, which is why the weighted mean of fixed effects from the original model object is used for all observations in newdata and a warning is printed. For fixed effect model prediction, it is reasonable to assume the user can provide information (via a pdata.frame) how the data the user wants to use for prediction relates to the individual and time dimension of panel data.