[R] linear contrasts for trends in an anova

[ONKELINX, Thierry]

Dear Colm,

Use lm() instead of aov() and your problem is solved. The parameter of Group.L is the linear trend along your groups. Group.Q the quandratic trend.

HTH,

Thierry

summary(lm(Mean_1 ~ Group*eventType, data=doi)) 
Call:
lm(formula = Mean_1 ~ Group * eventType, data = doi)

Residuals:
     Min       1Q   Median       3Q      Max 
-2.88504 -0.74862 -0.01771  0.92154  2.62031 

Coefficients:
                       Estimate Std. Error t value Pr(>|t|)    
(Intercept)              5.1360     0.3610  14.229 3.41e-13 ***
Group.L                  2.7761     0.6252   4.440 0.000172 ***
Group.Q                 -0.6375     0.6252  -1.020 0.318091    
eventTypestops          -2.0464     0.5105  -4.009 0.000515 ***
Group.L:eventTypestops  -2.3123     0.8842  -2.615 0.015175 *  
Group.Q:eventTypestops   1.9637     0.8842   2.221 0.036049 *  
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 

Residual standard error: 1.398 on 24 degrees of freedom
Multiple R-squared: 0.6357,	Adjusted R-squared: 0.5598 
F-statistic: 8.375 on 5 and 24 DF,  p-value: 0.0001073 




----------------------------------------------------------------------------
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek
team Biometrie & Kwaliteitszorg
Gaverstraat 4
9500 Geraardsbergen
Belgium

Research Institute for Nature and Forest
team Biometrics & Quality Assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium

tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

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~ Sir Ronald Aylmer Fisher

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~ Roger Brinner

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-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Namens Colm Connolly
Verzonden: vrijdag 18 december 2009 16:13
Aan: r-help at stat.math.ethz.ch
Onderwerp: [R] linear contrasts for trends in an anova

Hi everybody,

I'm trying to construct contrasts for an ANOVA to determine if there is a significant trend in the means of my groups.

In the following example, based on the type of 2x3 ANOVA I'm trying to perform, does the linear polynomial contrast generated by contr.poly allow me to test for a linear trend across groups?

doi=data.frame(
  Group=c(
    rep(1, 5),  rep(2, 5),  rep(3, 5), rep(1, 5),  rep(2, 5),  rep(3, 5)),
  Mean_1=c(
    rnorm(5, mean=3, sd=1), rnorm(5, mean=5.7, sd=1.2), rnorm(5, mean=7, sd=1.3),
    rnorm(5, mean=3.5, sd=1), rnorm(5, mean=3, sd=1.2), rnorm(5, mean=4.2, sd=1.7)),
  eventType=c( rep("errors", 15), rep("stops", 15))) doi$Group=factor(doi$Group, labels=c("Control", "Short", "Long"), ordered=T)

## construct contrasts
contrasts(doi$Group)=contr.poly(levels(doi$Group))

model1=aov(Mean_1 ~ Group*eventType, data=doi)
model1.summary=summary(model1)
print(model1.summary)
print(summary.lm(model1))

Thanks in advance,

Regards,
--
Dr Colm G. Connolly
Institute of Neuroscience
The Lloyd Building
University of Dublin
Trinity College, Dublin 2, Éire
Fax: +353-1-671-3183

Please note that electronic mail to, from or within the Trinity College Dublin, may be the subject of a request under the Freedom of Information Act.




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