R help - Oct 2006 - help_aov

Hi,

I am trying to run an analysis of variance using R.
in my data table "x" is a continuous variable lengthof 200 and
"p" is a
categorical variable also of length 200 and p is anyone of three categories 
1,2 or ,3.
if I run
summary(aov(x~p,data=test))
I get
Response: x
             Df  Sum Sq Mean Sq F value    Pr(>F)
p           1  3174.7  3174.7  42.749 5.175e-10 ***
Residuals 198 14704.2    74.3
---
and if I run
summary(aov(x~as.factor(p), data=test)) # I get

Response: x
                    Df  Sum Sq  Mean Sq  F value   Pr(>F)
as.factor(p)      2  3175.7  1587.8   21.275  4.31e-09 ***
Residuals       197  14703.2    74.6


Can anyone kindly explain the difference. How will it affect -correct way to 
run - if it is a two-way anova where I have a second categorical variable 
"sex" - male or female.

Many Thanks


Yours sincerely,

Bragadeesh

"Thanjavur Bragadeesh" <bragadeesh02 at hotmail.com> writes:
> Hi,
> 
> I am trying to run an analysis of variance using R.
> in my data table "x" is a continuous variable lengthof 200 and
"p" is a
> categorical variable also of length 200 and p is anyone of three categories
> 1,2 or ,3.
> if I run
> summary(aov(x~p,data=test))
> I get
> Response: x
>              Df  Sum Sq Mean Sq F value    Pr(>F)
> p           1  3174.7  3174.7  42.749 5.175e-10 ***
> Residuals 198 14704.2    74.3

If you get 1 DF for a variable with three different values, then it is
not a categorical variable from R's point of view, but a quantitative
one.  So you need x~factor(p) here like you have below. Or convert "p"
to a factor before the analysis.
> and if I run
> summary(aov(x~as.factor(p), data=test)) # I get
> 
> Response: x
>                     Df  Sum Sq  Mean Sq  F value   Pr(>F)
> as.factor(p)      2  3175.7  1587.8   21.275  4.31e-09 ***
> Residuals       197  14703.2    74.6
> 
> 
> Can anyone kindly explain the difference. How will it affect -correct way
to
> run - if it is a two-way anova where I have a second categorical variable 
> "sex" - male or female.
> 
> Many Thanks
> 
> 
> Yours sincerely,
> 
> Bragadeesh
> 
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