An approximate F-test based on the Kenward-Roger approach.
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A number or a vector of the beta of the hypothesis, e.g. L beta=L betaH. betaH=0 if modelSmall is a model not a restriction matrix.
If larger than 0 some timing details are printed.
Additional arguments to print function
object must be fitted with restricted maximum
likelihood (i.e. with
REML=TRUE). If the object is fitted with
maximum likelihood (i.e. with
REML=FALSE) then the model is
REML=TRUE before the p-values are calculated. Put
differently, the user needs not worry about this issue.
An F test is calculated according to the approach of Kenward and Roger
(1997). The function works for linear mixed models fitted with the
lmer function of the lme4 package. Only models where the
covariance structure is a sum of known matrices can be compared.
largeModel may be a model fitted with
lmer either using
smallModel can be a model
lmer. It must have the same covariance structure as
largeModel. Furthermore, its linear space of expectation must be a
subspace of the space for
largeModel. The model
can also be a restriction matrix
L specifying the hypothesis L
β = L β_H, where L is a k X p matrix and
β is a p column vector the same length as
The β_H is a p column vector.
Notice: if you want to test a hypothesis L β = c with a k vector c, a suitable β_H is obtained via β_H=L c where L_n is a g-inverse of L.
Notice: It cannot be guaranteed that the results agree with other implementations of the Kenward-Roger approach!
This functionality is not thoroughly tested and should be used with care. Please do report bugs etc.
Ulrich Halekoh, S<c3><b8>ren H<c3><b8>jsgaard (2014)., A Kenward-Roger Approximation and Parametric Bootstrap Methods for Tests in Linear Mixed Models - The R Package pbkrtest., Journal of Statistical Software, 58(10), 1-30., http://www.jstatsoft.org/v59/i09/
Kenward, M. G. and Roger, J. H. (1997), Small Sample Inference for Fixed Effects from Restricted Maximum Likelihood, Biometrics 53: 983-997.
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(fmLarge <- lmer(Reaction ~ Days + (Days|Subject), sleepstudy)) ## removing Days (fmSmall <- lmer(Reaction ~ 1 + (Days|Subject), sleepstudy)) anova(fmLarge,fmSmall) KRmodcomp(fmLarge,fmSmall) ## The same test using a restriction matrix L <- cbind(0,1) KRmodcomp(fmLarge, L) ## Same example, but with independent intercept and slope effects: m.large <- lmer(Reaction ~ Days + (1|Subject) + (0+Days|Subject), data = sleepstudy) m.small <- lmer(Reaction ~ 1 + (1|Subject) + (0+Days|Subject), data = sleepstudy) anova(m.large, m.small) KRmodcomp(m.large, m.small)
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