degree of freedom approximation Kenward-Roger degree of freedom approximation https://www.jstatsoft.org/article/view/v059i09 Alejandra Gonzalez-Beltran Hanna Cwiek Kenward-Roger Philippe Rocca-Serra The Kenward-Roger method's fundamental idea is to calculate the approximate mean and variance of their statistic and then match moments with an F distribution to obtain the denominator degrees of freedom. https://support.sas.com/documentation/cdl/en/statug/68162/HTML/default/viewer.htm#statug_glimmix_details40.htm https://www.ncbi.nlm.nih.gov/pubmed/9333350 https://www.rdocumentation.org/packages/lmerTest/versions/2.0-36/topics/anova-methods ibrary(lme4) library(pbkrtest) fm1 <- lmer(Reaction ~ Days + (Days| Subject), sleepstudy) get_Lb_ddf(fm1, lme4::fixef(fm1)) ready for release