Producing a rolling average of ALL the previous observations per ID in an unbalanced panel data set

Question

I am trying to compute rolling means of an unbalanced data set. To illustrate my point I have produced this toy example of my data:

ID  year  Var   RollingAvg(Var)
1   2000  2     NA
1   2001  3     2
1   2002  4     2.5
1   2003  2     3
2   2001  2     NA
2   2002  5     2
2   2003  4     3.5

The column RollingAvg(Var) is what I want, but can't get. In words, I am looking for the rolling average of ALL the previous observations of Var for each ID. I have tried using rollapply and ddply in the zoo and the plyr package, but I can't see how to set the rolling window length to use ALL the previous observations for each ID. Maybe I should use the plm package instead? Any help is appreciated.

I have seen other posts on rolling means on BALANCED panel data set, but I can't seem to extrapolate their answers to unbalanced data.

Thanks,

M

Answer 1

Using data.table:

library(data.table)
d = data.table(your_df)

d[, RollingAvg := {avg = cumsum(Var)/seq_len(.N);
                   c(NA, avg[-length(avg)])},
    by = ID]

(or even simplified)

d[, RollingAvg := c(NA, head(cumsum(Var)/(seq_len(.N)), -1)), by = ID]

Answer 2

Assuming that years are contiguous within each ID (which is case in the example data) and DF is the input data frame, here is a solution using just base R. cumRoll is a function that performs the required operation on one ID and ave then performs it by ID:

cumRoll <- function(x) c(NA, head(cumsum(x) / seq_along(x), -1))
DF$Roll <- ave(DF$Var, DF$ID, FUN = cumRoll)

The result is:

> DF
  ID year Var Roll
1  1 2000   2   NA
2  1 2001   3  2.0
3  1 2002   4  2.5
4  1 2003   2  3.0
5  2 2001   2   NA
6  2 2002   5  2.0
7  2 2003   4  3.5