```
fm1 <- lmer(Reaction ~ Days + (Days|Subject), sleepstudy)
## likelihood ratio tests
drop1(fm1,test="Chisq")
## use Kenward-Roger corrected F test, or parametric bootstrap,
## to test the significance of each dropped predictor
if (require(pbkrtest) && packageVersion("pbkrtest")>="0.3.8") {
KRSumFun <- function(object, objectDrop, ...) {
krnames <- c("ndf","ddf","Fstat","p.value","F.scaling")
r <- if (missing(objectDrop)) {
setNames(rep(NA,length(krnames)),krnames)
} else {
krtest <- KRmodcomp(object,objectDrop)
unlist(krtest$stats[krnames])
}
attr(r,"method") <- c("Kenward-Roger via pbkrtest package")
r
}
drop1(fm1, test="user", sumFun=KRSumFun)
if(lme4:::testLevel() >= 3) { ## takes about 16 sec
nsim <- 100
PBSumFun <- function(object, objectDrop, ...) {
pbnames <- c("stat","p.value")
r <- if (missing(objectDrop)) {
setNames(rep(NA,length(pbnames)),pbnames)
} else {
pbtest <- PBmodcomp(object,objectDrop,nsim=nsim)
unlist(pbtest$test[2,pbnames])
}
attr(r,"method") <- c("Parametric bootstrap via pbkrtest package")
r
}
system.time(drop1(fm1, test="user", sumFun=PBSumFun))
}
}
## workaround for creating a formula in a separate environment
createFormula <- function(resp, fixed, rand) {
f <- reformulate(c(fixed,rand),response=resp)
## use the parent (createModel) environment, not the
## environment of this function (which does not contain 'data')
environment(f) <- parent.frame()
f
}
createModel <- function(data) {
mf.final <- createFormula("Reaction", "Days", "(Days|Subject)")
lmer(mf.final, data=data)
}
drop1(createModel(data=sleepstudy))
```

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