The pbkrtest package: Parametric Bootstrap, Kenward-Roger and Satterthwaite Based Methods for Tests in Mixed Models ================

What does pbkrtest do for you?

Hypothesis test of fixed effects in mixed models (also called random effects models, hierarchical models etc) is most commonly based on large sample asymptotics: When the amount of information becomes large, a test can be based an a \chi^2-approximation. In small sample cases, this approximation can be very unreliable. The pbkrtest provides alternatives to this approximation. To be specific: For linear mixed models (as implemented in the lme4 package), pbkrtest implements the following tests for fixed effects:

  1. a parametric bootstrap test,
  2. a Kenward-Roger-type F-test and
  3. a Satterthwaite-type F-test.

Moreover, for generalized linear mixed models (as implemented in lme4) and for generalized linear models, pbkrtest also implements a parametric bootstrap test

Documentation

The facilities of the package are documented in the paper by [Halekoh and Højsgaard 2014)] (https://www.jstatsoft.org/htaccess.php?volume=059&type=i&issue=09&filename=paper) Please see citation("pbkrtest") for information about citing the paper and the package. If you use the package in your work, please do cite this paper. Please notice: There are other packages that use pbkrtest under the hood. If you use one of those packages, please do also cite our paper.

We also refer to the Webpage for the package

Online documentation

See https://hojsgaard.github.io/pbkrtest/.

Installation

pbkrtest is available on CRAN and development versions can also be found on Github:

## Install from CRAN:
install.packages('pbkrtest')
## Install from Github: Use the remotes package:
remotes::install_github("hojsgaard/pbkrtest", build_vignettes = TRUE)

Development site

See https://github.com/hojsgaard/pbkrtest.

Brief introduction 

library(pbkrtest)
library(ggplot2)

## Sugar beets: Does suger content depend on harvest time?

beets |> ggplot(aes(x=sow, y=sugpct, group=harvest)) +
    geom_jitter(aes(color=harvest), width=0)


fm0 <- lmer(sugpct ~ block + sow + harvest + (1|block:harvest), data=beets)
fm1 <- update(fm0, .~. -harvest)

## Is there an effect of harvest time?
an <- anova(fm0, fm1)
pb <- PBmodcomp(fm0, fm1)
kr <- KRmodcomp(fm0, fm1)
sa <- SATmodcomp(fm0, fm1)

tidy(an)
#> # A tibble: 2 × 9
#>   term   npar   AIC   BIC logLik deviance statistic    df   p.value
#>   <chr> <dbl> <dbl> <dbl>  <dbl>    <dbl>     <dbl> <dbl>     <dbl>
#> 1 fm1       9 -69.1 -56.5   43.5    -87.1      NA      NA NA       
#> 2 fm0      10 -80.0 -66.0   50.0   -100.       12.9     1  0.000326
tidy(pb)
#> # A tibble: 2 × 4
#>   type    stat    df  p.value
#>   <chr>  <dbl> <dbl>    <dbl>
#> 1 LRT     12.9     1 0.000326
#> 2 PBtest  12.9    NA 0.0300
tidy(kr)
#> # A tibble: 1 × 6
#>   type   stat   ndf   ddf F.scaling p.value
#>   <chr> <dbl> <int> <dbl>     <dbl>   <dbl>
#> 1 Ftest  15.2     1  2.00         1  0.0599
tidy(sa)
#> # A tibble: 1 × 5
#>   type  statistic   ndf   ddf p.value
#>   <chr>     <dbl> <int> <dbl>   <dbl>
#> 1 Ftest      15.2     1  2.00  0.0599

Please find more examples in the other vignettes available at https://hojsgaard.github.io/pbkrtest/.