R remove rows from panel while keeping the panel balanced

Question

Is there an elegant way to balance an unbalanced panel data set? I would like to start with an unbalanced panel (ie, some individuals are missing some data) and end up with

Answer 1

This is the solution that I use - it makes use of the convenient features (including nice merging abilities) of the data.table package and assumes that your data is already a data.table object. It is relatively simple and hopefully easy to follow. It returns a balanced panel with entries for every unique combination of the "individuals" and the "time periods" - i.e. a panel where there is an observation for every individual in every time period.

library(data.table)
Balance_Panel = function(Data, Indiv_ColName, Time_ColName){
    Individuals = unique(Data[, get(Indiv_ColName)])
    Times = unique(Data[, get(Time_ColName)])

    Full_Panel = data.table(expand.grid(Individuals, Times))
    setnames(Full_Panel, c(Indiv_ColName, Time_ColName))
    setkeyv(Full_Panel, c(Indiv_ColName, Time_ColName))
    setkeyv(Data, c(Indiv_ColName, Time_ColName))
    return(Data[Full_Panel])
}

Sample Usage:

Balanced_Data = Balance_Panel(Data, "SubjectID", "ObservationTime")