[R] Simple 2-Way Anova issue in R

znd zackdaugherty at mail.com
Mon Nov 9 00:10:43 CET 2009


Hello, I'm new to R and have been following many guides including the two-way
anova (http://www.personality-project.org/r/r.anova.html).  Using that
walkthrough including the supplied data I do str(data.ex2) and receive the
appropriate types of data as follows:
> str(data.ex2)
'data.frame':   16 obs. of  4 variables:
 $ Observation: int  1 2 3 4 5 6 7 8 9 10 ...
 $ Gender     : Factor w/ 2 levels "f","m": 2 2 2 2 2 2 2 2 1 1 ...
 $ Dosage     : Factor w/ 2 levels "a","b": 1 1 1 1 2 2 2 2 1 1 ...
 $ Alertness  : int  8 12 13 12 6 7 23 14 15 12 ...

aov.ex2 = aov(Alertness~Gender*Dosage,data=data.ex2)  

summary(aov.ex2)

Outputs:
              Df  Sum Sq Mean Sq F value Pr(>F)
Gender         1  76.562  76.562  2.9518 0.1115
Dosage         1   5.062   5.062  0.1952 0.6665
Gender:Dosage  1   0.063   0.063  0.0024 0.9617
Residuals     12 311.250  25.938         

However, when I got to use my data that I made in csv format I have to tell
R to interpret my factors which are year and depth as factors...
datafilename="C:/Rclass/hmwk1pt2.csv"                            
data.ex2=read.csv(datafilename,header=T)                              
data.ex2$Year<-as.factor(data.ex2$Year)
data.ex2$Depth<-as.factor(data.ex2$Depth)
data.ex2
str(data.ex2)

This outputs what I would expect:

> str(data.ex2)
'data.frame':   12 obs. of  4 variables:
 $ Year      : Factor w/ 3 levels "1999","2000",..: 1 1 1 1 2 2 2 2 3 3 ...
 $ Depth     : Factor w/ 4 levels "10","15","20",..: 1 2 3 4 1 2 3 4 1 2 ...
 $ Replicate1: num  14.3 15.1 16.7 17.3 16.3 17.4 18.6 20.9 22.9 23.9 ...
 $ Replicate2: num  14.7 15.6 16.9 17.9 16.4 17.2 19.6 21.3 22.7 23.3 ...

But something is not causing my anova to carry through...this is what I
have.

ANOVA = aov(Replicate1~Year*Depth,data=data.ex2)
summary(ANOVA)

which outputs:

> summary(ANOVA)                                          
            Df  Sum Sq Mean Sq
Year         2 143.607  71.803
Depth        3  17.323   5.774
Year:Depth   6   2.587   0.431

There is no F-value or Pr(>F) columns.

I also can't boxplot this correctly, again following the example at that
website above they have:

boxplot(Alertness~Dosage*Gender,data=data.ex2)

which outputs:

http://old.nabble.com/file/p26258684/87o3uicpf6dt4kkdyvfv.jpeg 

My code is:

boxplot(Replicate1~Year*Depth,data=data.ex2)

which outputs:

http://old.nabble.com/file/p26258684/gik02vyhvvbmcvw3ia2h.jpeg 

This is incorrect, it's multiplying my factors but I thought that when I did
the str() on my data it recognized the Year and Depth as factors, not
numbers or integers.

My csv file is:
http://old.nabble.com/file/p26258684/hmwk1pt2.csv hmwk1pt2.csv 

Any help on what is going one would be greatly appreciated because I need to
perform one-way, two-way, nested, and factorial anovas but I first need to
solve this problem before I can continue.


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