Add Satterthwaite df for regression coefficients #35
Description
Add a function that calculates Satterthwaite degrees of freedom for regression coefficient estimates or linear contrasts thereof. See Giesbrecht & Burns (1985), Eq. 3.3, or Kuznetsova et al. (2017), Appendix A.. For models that can also be estimated with lme4::lmer(), check df results against the results of lmerTest.
Giesbrecht FG, Burns JC (1985). “Two-Stage Analysis Based on a Mixed Model: LargeSample Asymptotic Theory and Small-Sample Simulation Results.” Biometrics, 41(2), 477–486. doi:10.2307/2530872.
Kuznetsova A., Brockhoff P.B. and Christensen R.H.B. (2017). "lmerTest Package: Tests in Linear Mixed Effects Models." Journal of Statistical Software, 82(13), pp. 1–26. doi: 10.18637/jss.v082.i13.