lagVar {epicalc} | R Documentation |
Create a vector of lagged or subsequent value in a long form longitudinal data
lagVar(var, id, visit, lag.unit=1)
var |
variable to create the lag |
id |
subject identification field |
visit |
visit of measurement |
lag.unit |
lag number of visits |
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.
Virasakdi Chongsuvivatwong <cvirasak@medicine.psu.ac.th>
'lag'
## 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),]