Bertolt Meyer
2010-Apr-03 17:20 UTC
[R] Multilevel model with lme(): Weird degrees of freedom (group level df > # of groups)
Hello everyone, I am trying to regress applicants' performance in an assessment center (AC) on their gender (individual level) and the size of the AC (group level) with a multi-level model: model.0 <- lme(performance ~ ACsize + gender, random = ~1 | ACNumber, method = "ML", control = list(opt = "optim")) I have 1047 applicants in 118 ACs: > length(performance) [1] 1047 > length(levels(as.factor(ACNumber))) [1] 118 There are five AC sizes and gender has two levels (coded as -1 for female and 1 for male): > length(levels(as.factor(ACsize))) [1] 5 > length(levels(as.factor(gender))) [1] 2 However, when I examine the model summary, the predictor on the individual level (gender) and the predictor on group level (ACsize) have the same degrees of freedom: > summary(model.0) Linear mixed-effects model fit by maximum likelihood [...] Random effects: Formula: ~1 | ACNumber (Intercept) Residual StdDev: 0.1650112 0.8146622 Fixed effects: performance ~ ACsize + gender Value Std.Error DF t-value p-value (Intercept) 3.0927051 0.24573622 927 12.585467 0.0000 ACsize -0.0568915 0.02782755 927 -2.044431 0.0412 gender 0.1679830 0.02780940 927 6.040510 0.0000 [...] Number of Observations: 1047 Number of Groups: 118 How is it possible that the group-level predictor has a df > than the number of groups? I am a little at a loss here and would appreciate it if someone could explain this to me... What am I missing? Regards, Bertolt -- Dr. Bertolt Meyer Senior research and teaching associate Social Psychology, Institute of Psychology, University of Zurich Binzmuehlestrasse 14, Box 15 CH-8050 Zurich Switzerland bmeyer at sozpsy.uzh.ch tel: +41446357282 fax: +41446357279 mob: +41788966111
Possibly Parallel Threads
- Newbie question: How to use tapply() on several vectors simultaneously
- Help with shading a polygon below a segment of a curve (normal distribution)
- Multilevel package: Obtaining significance for waba within-group correlation?
- vector indexing problem in multilevel data: assigning a specific value to all group members
- how to read the "Sum Sq" - column from summary.aov()