[R-sig-ME] lme and lmer degrees of freedom (and hence p values) from don't agree . . . Why?????????
Lize van der Merwe
lizestats at gmail.com
Fri Jul 10 21:34:38 CEST 2015
John
I received a very detailed and clear explanation from Ben Bolker and John Westfall through this list on Tuesday.
Lize
-----Original Message-----
From: R-sig-mixed-models [mailto:r-sig-mixed-models-bounces at r-project.org] On Behalf Of John Sorkin
Sent: 10 July 2015 21:22
To: r-sig-mixed-models at r-project.org
Cc: lme4-authors at r-forge.wu-wien.ac.at
Subject: [R-sig-ME] lme and lmer degrees of freedom (and hence p values) from don't agree . . . Why?????????
Forgive me for posting this again, I initially posted several days ago and have yet to hear from anyone. Any help that can be offered would be appreciated!
John
I am posting this message to this list (after posting to R help) at the suggestion of Bert Gunter.
I am trying to fit data from 23 subjects using random effects regression, and am comparing the results of lme and lmer. The point estimates and the SEs are the same in both models, however the degrees of freedom are widely different. lme reports 88 DF, lmer approximately 22. Can someone help me understand why the DFs are not the same? I have
23 subjects, each of whom is studied in up to five different experimental conditions (i.e. Amp). For each condition multiple measurements are made for each subject (i.e. X).
Thank you,
John
# lme: Random intercept, random slope.
cat("********This analysis has 88 degrees of freedom\n") fit0X.new <- groupedData(X~Amp|SS,data=data,order.groups=FALSE)
xx <- lme(fit0X.new,random=~1+Amp)
summary(xx)
cat("\n\n")
# lmer: Random intercept, random slope.
cat("*********This analysis has ~22 degrees of freedom\n") fit0X <- lmer(X~Amp+(1+Amp|SS),data=data)
print(summary(fit0X))
fit0XSum<-summary(fit0X)$coefficients
********This analysis has 88 degrees of freedom Linear mixed-effects model fit by REML
Data: fit0X.new
AIC BIC logLik
331.7688 347.9717 -159.8844
Random effects:
Formula: ~1 + Amp | SS
Structure: General positive-definite, Log-Cholesky parametrization
StdDev Corr
(Intercept) 1.3515911 (Intr)
Amp 2.5619953 -0.366
Residual 0.6139429
Fixed effects: X ~ Amp
Value Std.Error DF t-value p-value
(Intercept) 1.718376 0.3609133 88 4.761188 0
Amp 6.890429 0.5978236 88 11.525856 0
Correlation:
(Intr)
Amp -0.526
Standardized Within-Group Residuals:
Min Q1 Med Q3 Max
-2.2177007 -0.5770388 -0.1249565 0.5247444 4.1150164 Number of Observations: 112 Number of Groups: 23
*********This analysis has ~22 degrees of freedom Linear mixed model fit by REML t-tests use Satterthwaite approximations to degrees of freedom [merModLmerTest]
Formula: X ~ Amp + (1 + Amp | SS)
Data: data
REML criterion at convergence: 319.8
Scaled residuals:
Min 1Q Median 3Q Max
-2.2177 -0.5770 -0.1250 0.5247 4.1150 Random effects:
Groups Name Variance Std.Dev. Corr
SS (Intercept) 1.8268 1.3516
Amp 6.5638 2.5620 -0.37
Residual 0.3769 0.6139
Number of obs: 112, groups: SS, 23
Fixed effects:
Estimate Std. Error df t value Pr(>|t|)
(Intercept) 1.7184 0.3609 21.1150 4.761 0.000104 ***
Amp 6.8904 0.5978 22.0460 11.526 8.37e-11 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 Correlation of Fixed Effects:
(Intr)
Amp -0.526
John David Sorkin M.D., Ph.D.
Professor of Medicine
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology and
Geriatric Medicine
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
John David Sorkin M.D., Ph.D.
Professor of Medicine
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology and
Geriatric Medicine
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
John David Sorkin M.D., Ph.D.
Professor of Medicine
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology and
Geriatric Medicine
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(PThis email message, including any attachments, is for the sole use of
the intended recipient(s) and may contain confidential and privileged
information. Any unauthorized use, disclosure or distribution is
prohibited. If you are not the intended recipient, please contact the
sender by reply email and destroy all copies of the original message.
_______________________________________________
R-sig-mixed-models at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
More information about the R-sig-mixed-models
mailing list