\name{lagVar}
\alias{lagVar}
\title{Create a vector of lagged or subsequent value}
\description{Create a vector of lagged or subsequent value in a long form longitudinal data}
\usage{lagVar(var, id, visit, lag.unit=1)
}
\details{Data must be in long format having variable to create the lag, id and visit.

The variable 'visit' must be the number of visit, with step = 1.

The default value of lag.unit is 1. When the number is negative, the next measured is created instead. }

\arguments{
	\item{var}{variable to create the lag}
	\item{id}{subject identification field}
	\item{visit}{visit of measurement}
	\item{lag.unit}{lag number of visits}
}
\author{Virasakdi Chongsuvivatwong
	\email{ <cvirasak@medicine.psu.ac.th>}
}
\seealso{'lag'}
\examples{
## Identification of the tree that became smaller during followup
data(Sitka, package="MASS")
use(Sitka)
table(Time)
visit <- Time
pack()
recode(visit, as.numeric(names(table(Time))), 1:5)
lag1.size <- lagVar(var=size, id=tree, visit=visit, lag=1)
data.frame(tree=tree, time=Time, visit=visit, size=size, lag1.size=lag1.size) [1:20,]
# Answer
data.frame(Time, tree, size, lag1.size) [which(lag1.size > size),]

# Alternatively
next.size <- lagVar(size, tree, visit, lag=-1)
data.frame(tree=tree, time=Time, size=size, next.size=next.size) [1:20,]
data.frame(Time, tree, size, next.size) [which(size > next.size),]

}
\keyword{database}