# [R] lme degrees of freedoms: SAS and R

Kaiya Liu liu.262 at osu.edu
Fri Mar 31 12:38:49 CEST 2000

```Dear list,

I ran a mixed effect model using R 1.4.1 and SAS 8.0 on the SIMS data found
in the SASmixed package and found that the degrees of freedoms for fixed
effects are very different.
From R, df = n - v -1 where n is total # of observations, v is the # of
or can somebody explain which is correct and why?

Thanks a lot!

Kaiya Liu

-------------------------------------
Here are the codes:

For R:
> formula (SIMS)
Gain ~ Pretot | Class
> data(SIMS)
> fm1SIMS <- lme(Gain ~ Pretot, data = SIMS, random = ~ Pretot | Class,
control = list(msVerbose = TRUE))
> summary (fm1SIMS)
_________________________________
For SAS:

proc mixed data=sims;
class class;
model gain = pretot / solution;
random intercept pretot / subject=class type=un;
run;

Most of the results are comparable except the degrees of freedoms for the
fixed effects:

From R: DF = 3500

Fixed effects: Gain ~ Pretot
Value Std.Error   DF   t-value p-value
(Intercept)  7.066218 0.3631916 3500  19.45589  <.0001
Pretot      -0.187538 0.0163680 3500 -11.45756  <.0001

Number of Observations: 3691
Number of Groups: 190
__________________________________________________________
From SAS: DF = 189

Solution for Fixed Effects

Standard
Effect       Estimate       Error      DF    t
Value    Pr > |t|

Intercept      7.0595      0.3658     189      19.30
<.0001
pretot        -0.1860     0.01610     189     -11.56
<.0001

-----------------------------------------------------------
Kaiya Liu
School of Journalism and Communication
Ohio State University

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