[R] factorial anova

[John Sorkin]

Rick,
I read you data into a data.frame called data.

I sugguest you run the model as follows:
fit<-anova( dosel ~ estado * Bosque, data = data)
summary(fit1)

The results are:

> contrasts(data$Bosque)
         siemprev
deciduo         0
siemprev        1

> contrasts(data$estado)
         pristino
activo          0
pristino        1

> summary(fit1)

Call:
lm(formula = dosel ~ estado * Bosque, data = data)

Residuals:
    Min      1Q  Median      3Q     Max 
-30.160  -2.548   0.312   3.588  21.840 

Coefficients:
                               Estimate Std. Error  t value Pr(>|t|)    
(Intercept)                   5.632e+01  5.769e+00    9.762 3.84e-08 ***
estadopristino                3.453e+01  8.159e+00    4.232 0.000635 ***
Bosquesiemprev                1.249e-15  8.159e+00 1.53e-16 1.000000    
estadopristino:Bosquesiemprev 5.408e+00  1.154e+01    0.469 0.645622    
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 

Residual standard error: 12.9 on 16 degrees of freedom
Multiple R-Squared: 0.7245,     Adjusted R-squared: 0.6729 
F-statistic: 14.03 on 3 and 16 DF,  p-value: 9.615e-05 

You will note that the p values for the interaction and the main effect for Bosqueiemprev are no longer the same.


Feliz ano nuevo!

John


John Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
Baltimore VA Medical Center GRECC and
University of Maryland School of Medicine Claude Pepper OAIC

University of Maryland School of Medicine
Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524

410-605-7119 
- NOTE NEW EMAIL ADDRESS:
jsorkin at grecc.umaryland.edu
>>> Rick Bilonick <rab at nauticom.net> 12/25/05 9:09 PM >>>
On Sun, 2005-12-25 at 23:01 -0300, Petra Wallem wrote:
> Hello  every body, I am trying to do a factorial anova analysis
> following this model:
> 
> model<-anova(lm(responsevariable~factorA*factorB))
> model<-anova(lm(luz$dosel~luz$estado*luz$Bosque))
> 
>  Df Sum Sq Mean Sq F value    Pr(>F)
> estado         1 6931.1  6931.1 41.6455 7.974e-06 ***
> Bosque         1   36.6    36.6  0.2197    0.6456
> estado:Bosque  1   36.6    36.6  0.2197    0.6456
> Residuals     16 2662.9   166.4
> 
> Strange is that the sum of squares of the factor Bosque are identical to
> the SS of the interaction, and are non significant. But when I plot the
> data, the interaction surley is significant...
> 
> my data.frame looks as follows:
> 
> Bosque   estado      lux  dosel
> 1   deciduo pristino  703 88.56
> 2   deciduo pristino  800 90.64
> 3   deciduo pristino  150 95.84
> 4   deciduo pristino  245 87.52
> 5   deciduo pristino 1300 91.68
> 6   deciduo   activo 1900 26.16
> 7   deciduo   activo  840 59.44
> 8   deciduo   activo  323 69.84
> 9   deciduo   activo  112 75.04
> 10  deciduo   activo 1360 51.12
> 11 siemprev   activo  900 41.76
> 12 siemprev   activo  480 65.68
> 13 siemprev   activo  350 78.16
> 14 siemprev   activo  350 37.60
> 15 siemprev   activo  272 58.40
> 16 siemprev pristino  100 94.80
> 17 siemprev pristino   60 95.84
> 18 siemprev pristino   50 97.92
> 19 siemprev pristino  270 94.80
> 20 siemprev pristino  110 97.92
> 
> Dose some body understand what I am doing wrong??? I have been
> navigating at the R site search, but didn't found much posting on
> factorial anova.
> 
> In advance thanks a lot for your comments
> Petra
> 
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It would help if you would use the "dump" function and paste the output
into an e-mail:

> dump("luz","")

Also, it's much easier to use "data=luz" as an argument in the lm
function rather than appending the data frame name to each variable. I
don't think that "model" contains the lm model output. It looks like you
are saving the anova table.

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