On 8/13/05, Ila Patnaik <[EMAIL PROTECTED]> wrote:
> Suppose we observe N individuals, for each of which we have a
> time-series. How do we correctly create a lagged value of the
> time-series variable?
>
> As an example, suppose I create:
>
> A <- data.frame(year=rep(c(1980:1984),3),
> person= factor(sort(rep(1:3,5))),
> wage=c(rnorm(15)))
>
> > A
> year person wage
> 1 1980 1 0.17923212
> 2 1981 1 0.25610292
> 3 1982 1 0.50833655
> 4 1983 1 -0.42448395
> 5 1984 1 0.49233532
> 6 1980 2 -0.49928025
> 7 1981 2 0.06842660
> 8 1982 2 0.65677575
> 9 1983 2 0.15947390
> 10 1984 2 -0.46585116
> 11 1980 3 -0.29052635
> 12 1981 3 -0.27109203
> 13 1982 3 -0.76168164
> 14 1983 3 0.02294361
> 15 1984 3 2.22828032
>
> What I'd like to do is to make a lagged wage for each person, i.e., I
> should get an additional variable A$wage.lag1:
>
> > A
> year person wage wage.lag1
> 1 1980 1 0.17923212 NA
> 2 1981 1 0.25610292 0.17923212
> 3 1982 1 0.50833655 0.25610292
> 4 1983 1 -0.42448395 0.50833655
> 5 1984 1 0.49233532 -0.42448395
> 6 1980 2 -0.49928025 NA
> 7 1981 2 0.06842660 -0.49928025
> 8 1982 2 0.65677575 0.06842660
> 9 1983 2 0.15947390 0.65677575
> 10 1984 2 -0.46585116 0.15947390
> 11 1980 3 -0.29052635 NA
> 12 1981 3 -0.27109203 -0.29052635
> 13 1982 3 -0.76168164 -0.27109203
> 14 1983 3 0.02294361 -0.76168164
> 15 1984 3 2.22828032 0.02294361
>
We can use 'by' to split data frame A by person and to
apply the function f to each such subset of rows. Function f
makes that portion of wage which corresponds to a single
person into a ts class time series so that we can use lag
with it and then we cbind wage together with its lag. From
the cbind'ed result we extract out those times that
correspond to the original series since the example output
only includes those. Note that such extraction has a side
effect of turning wages into a matrix rather than a time
series. We then put every together using cbind(...) once
again and once the 'by' is complete we rbind all rows together.
f <- function(x) {
wage <- ts(x$wage, start = x$year[1])
idx <- seq(length = length(wage))
wages <- cbind(wage, lag(wage, -1))[idx,]
cbind(x[,1:2], wages)
}
result <- do.call("rbind", by(A, A$person, f))
result
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