What is the difference between using aov() and lme() in analyzing a longitudinal dataset?

by

Can anyone tell me the difference between using aov() and lme() for analyzing longitudinal data and how to interpret results from these two methods?

Below, I analyze the same dataset using aov() and lme() and got 2 different results. With aov(), I got a significant result in the time by treatment interaction, but fitting a linear mixed model, time by treatment interaction is insignificant.

> UOP.kg.aov <- aov(UOP.kg~time*treat+Error(id), raw3.42)
> summary(UOP.kg.aov)

Error: id
          Df  Sum Sq Mean Sq F value Pr(>F)
treat      1   0.142  0.1421  0.0377 0.8471
Residuals 39 147.129  3.7725               

Error: Within
            Df  Sum Sq Mean Sq  F value  Pr(>F)    
time         1 194.087 194.087 534.3542 < 2e-16 ***
time:treat   1   2.077   2.077   5.7197 0.01792 *  
Residuals  162  58.841   0.363                     
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

> UOP.kg.lme <- lme(UOP.kg~time*treat, random=list(id=pdDiag(~time)), 
                    na.action=na.omit, raw3.42)
> summary(UOP.kg.lme)
Linear mixed-effects model fit by REML
 Data: raw3.42 
       AIC      BIC    logLik
  225.7806 248.9037 -105.8903

Random effects:
 Formula: ~time | id
 Structure: Diagonal
        (Intercept)      time  Residual
StdDev:   0.6817425 0.5121545 0.1780466

Fixed effects: UOP.kg ~ time + treat + time:treat 
                 Value Std.Error  DF   t-value p-value
(Intercept)  0.5901420 0.1480515 162  3.986059  0.0001
time         0.8623864 0.1104533 162  7.807701  0.0000
treat       -0.2144487 0.2174843  39 -0.986042  0.3302
time:treat   0.1979578 0.1622534 162  1.220053  0.2242
 Correlation: 
           (Intr) time   treat 
time       -0.023              
treat      -0.681  0.016       
time:treat  0.016 -0.681 -0.023

Standardized Within-Group Residuals:
         Min           Q1          Med           Q3          Max 
-3.198315285 -0.384858426  0.002705899  0.404637305  2.049705655 

Number of Observations: 205
Number of Groups: 41

Answer

Based on your description, it appears that you have a repeated-measures model with a single treatment factor. Since I do not have access to the dataset (raw3.42), I will use the Orthodont data from the nlme package to illustrate what is going on here. The data structure is the same (repeated measurements for two different groups – in this case, males and females).

If you run the following code:

library(nlme)
data(Orthodont)

res <- lme(distance ~ age*Sex, random = ~ 1 | Subject, data = Orthodont)
anova(res)

you will get the following results:

            numDF denDF  F-value p-value
(Intercept)     1    79 4123.156  <.0001
age             1    79  122.450  <.0001
Sex             1    25    9.292  0.0054
age:Sex         1    79    6.303  0.0141

If you run:

res <- aov(distance ~ age*Sex + Error(Subject), data = Orthodont)
summary(res)

you will get:

Error: Subject
          Df Sum Sq Mean Sq F value   Pr(>F)   
Sex        1 140.46 140.465  9.2921 0.005375 **
Residuals 25 377.91  15.117                    

Error: Within
          Df  Sum Sq Mean Sq  F value  Pr(>F)    
age        1 235.356 235.356 122.4502 < 2e-16 ***
age:Sex    1  12.114  12.114   6.3027 0.01410 *  
Residuals 79 151.842   1.922

Note that the F-tests are exactly identical.

For lme(), you used list(id=pdDiag(~time)), which not only adds a random intercept to the model, but also a random slope. Moreover, by using pdDiag, you are setting the correlation between the random intercept and slope to zero. This is a different model than what you specified via aov() and hence you get different results.

Attribution
Source : Link , Question Author : biostat_newbie , Answer Author : Wolfgang

Leave a Comment