getting degrees of freedom from lmer

Question

I've fit an lmer model with the following (albeit made up output):

Random effects:
 Groups        Name        Std.Dev.
 day:sample (Intercept)    0.09
 sample        (Intercept) 0.42
 Residual                  0.023 

I'd really like to build a confidence interval for each effect using the following formula:

$ \frac{(n-1)s^2}{\chi^2_{\alpha/2, n-1}},\frac{(n-1)s^2}{\chi^2_{1-\alpha/2,n-1}} $

Is there a way to conveniently get out the degrees of freedom?

Answer 1

I would instead just create profile likelihood confidence intervals. They’re reliable, and very easy to calculate using the ‘lme4’ package. Example:

> library(lme4)
> fm = lmer(Reaction ~ Days + (Days | Subject),
            data=sleepstudy)
> summary(fm)
[…]
Random effects:
 Groups   Name        Variance Std.Dev. Corr
 Subject  (Intercept) 612.09   24.740       
          Days         35.07    5.922   0.07
 Residual             654.94   25.592       

You can now calculate the profile likelihood confidence intervals with the confint() function:

> confint(fm, oldNames=FALSE)
Computing profile confidence intervals ...
                               2.5 %  97.5 %
sd_(Intercept)|Subject        14.381  37.716
cor_Days.(Intercept)|Subject  -0.482   0.685
sd_Days|Subject                3.801   8.753
sigma                         22.898  28.858
(Intercept)                  237.681 265.130
Days                           7.359  13.576

You can also use the parametric bootstrap to calculate confidence intervals. Here’s the R syntax (using the parm argument to restrict which parameters we want confidence intervals for):

> confint(fm, method="boot", nsim=1000, parm=1:3)
Computing bootstrap confidence intervals ...
                              2.5 % 97.5 %
sd_(Intercept)|Subject       11.886 35.390
cor_Days.(Intercept)|Subject -0.504  0.929
sd_Days|Subject               3.347  8.283

The results will naturally vary somewhat for each run. You can increase nsim to decrease this variation, but this will also increase the time it takes to estimate the confidence intervals.

Answer 2

Degrees for freedom for mixed-models are "problematic". For reading more on it you can check the lmer, p-values and all that post by Douglas Bates. Also r-sig-mixed-models FAQ summarizes the reasons why it is bothersome:

• In general it is not clear that the null distribution of the computed ratio of sums of squares is really an F distribution, for any choice of denominator degrees of freedom. While this is true for special cases that correspond to classical experimental designs (nested, split-plot, randomized block, etc.), it is apparently not true for more complex designs (unbalanced, GLMMs, temporal or spatial correlation, etc.).
• For each simple degrees-of-freedom recipe that has been suggested (trace of the hat matrix, etc.) there seems to be at least one fairly simple counterexample where the recipe fails badly.
• Other df approximation schemes that have been suggested (Satterthwaite, Kenward-Roger, etc.) would apparently be fairly hard to implement in lme4/nlme,
  (...)
• Because the primary authors of lme4 are not convinced of the utility of the general approach of testing with reference to an approximate null distribution, and because of the overhead of anyone else digging into the code to enable the relevant functionality (as a patch or an add-on), this situation is unlikely to change in the future.

The FAQ gives also some alternatives

• use MASS::glmmPQL (uses old nlme rules approximately equivalent to SAS 'inner-outer' rules) for GLMMs, or (n)lme for LMMs
• Guess the denominator df from standard rules (for standard designs) and apply them to t or F tests
• Run the model in lme (if possible) and use the denominator df reported there (which follow a simple 'inner-outer' rule which should correspond to the canonical answer for simple/orthogonal designs), applied to t or F tests. For the explicit specification of the rules that lme uses, see page 91 of Pinheiro and Bates — this page is available on Google Books
• use SAS, Genstat (AS-REML), Stata?
• Assume infinite denominator df (i.e. Z/chi-squared test rather than t/F) if number of groups is large (>45? Various rules of thumb for how large is "approximately infinite" have been posed, including [in Angrist and Pischke's ''Mostly Harmless Econometrics''], 42 (in homage to Douglas Adams)

But if you are interested in confidence intervals there are better approaches, e.g. based on bootstrap as suggested by Karl Ove Hufthammer in his answer, or the ones proposed in the FAQ.