Dear R users,
I am doing custom contrasts with R (comparison of group means).
Everything works fine, but I would like to test the 3 contrasts with and
without a Welch correction for unequal variances.
I can replicate SPSS results when equal variances are assumed, but I do
not manage to test the contrasts when equal variances are not assumed. I
pasted below the results with R and SPSS (at the bottom of my email). R
provides F, while SPSS provides t (F = t squared).
Is there a way to apply the Welch correction to custom contrasts with R
anova function (or another correction for unequal variances, like
Brown-Forsythe), or do I need to use another package when equal
variances cannot be assumed ?
Regards,
Michael
==========================================
# R Syntax
==========================================
# c1 : group 1 vs. group 2.
# c2 : group 3 vs. group 4.
# c3 : group 1+2 vs. group 3+4.
c1 <- c(1, -1, 0, 0)
c2 <- c(0, 0, 1, -1)
c3 <- c(1, 1, -1, -1)
modele.global <- lm(var ~ cond,
data=Plan_CR_4_Modifie)
modele.contrastes <- lm(var ~ C(cond, c1, 1)
+ C(cond, c2, 1)
+ C(cond, c3, 1),
data=Plan_CR_4_Modifie)
anova(modele.global)
anova(modele.contrastes)
------------------------------------------
R Results:
> anova(modele.global)
Analysis of Variance Table
Response: var
Df Sum Sq Mean Sq F value Pr(>F)
cond 3 49 16.3333 7.4973 0.0007824 ***
Residuals 28 61 2.1786
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
> anova(modele.contrastes)
Analysis of Variance Table
Response: var
Df Sum Sq Mean Sq F value Pr(>F)
C(cond, c1, 1) 1 1 1.000 0.4590 0.5036442
C(cond, c2, 1) 1 16 16.000 7.3443 0.0113548 *
C(cond, c3, 1) 1 32 32.000 14.6885 0.0006571 ***
Residuals 28 61 2.179
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
==========================================
* SPSS Syntax - CONTRAST TESTS.
==========================================
ONEWAY
var BY cond
/CONTRAST= 1 -1 0 0 /CONTRAST= 0 0 1 -1 /CONTRAST= 1 1 -1 -1
/STATISTICS DESCRIPTIVES HOMOGENEITY
/MISSING ANALYSIS
/POSTHOC = SCHEFFE LSD BONFERRONI ALPHA(.05).
------------------------------------------
SPSS Results - Contrasts:
Assume equal variances
|--------|-----------------|----------|------|------|---------------|
|Contrast|Value of Contrast|Std. Error|t |df |Sig. (2-tailed)|
|--------|-----------------|----------|------|------|---------------|
|1 |-.50 |.738 |-.678 |28 |.504 |
|--------|-----------------|----------|------|------|---------------|
|2 |-2.00 |.738 |-2.710|28 |.011 |
|--------|-----------------|----------|------|------|---------------|
|3 |-4.00 |1.044 |-3.833|28 |.001 |
--------------------------------------------------------------------|
Does not assume equal variances
|--------|-----------------|----------|------|------|---------------|
|Contrast|Value of Contrast|Std. Error|t |df |Sig. (2-tailed)|
|--------|-----------------|----------|------|------|---------------|
|1 |-.50 |.627 |-.798 |11.603|.441 |
|--------|-----------------|----------|------|------|---------------|
|2 |-2.00 |.835 |-2.397|10.155|.037 |
|--------|-----------------|----------|------|------|---------------|
|3 |-4.00 |1.044 |-3.833|19.431|.001 |
--------------------------------------------------------------------|
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