[R] One way ANOVA with NO model

[Adaikalavan Ramasamy]

Suppose you have 6 groups (A, B, C, D, E, F) and you measured the weight
of 5 individuals from each group. Therefore you have 30 weight
observations in total.

You wish to test if the mean of the response variable is different for
each of the groups. 
[ i.e. the null hypothesis is that all 6 groups means are the same. ]


Lets simulate some data first:

 grp <- gl(6, k=5, labels=LETTERS[1:6])
 grp
  [1] A A A A A B B B B B C C C C C D D D D D E E E E E F F F F F
 Levels: A B C D E F

 set.seed(1)                    # for reproducibility only
 w <- runif(30, min=40, max=75) # weights 
 w <- round(w, digits=1)


Let us first calculate the group means:

   tapply(w, grp, mean)
       A     B     C     D     E     F
   56.24 62.36 55.54 63.54 55.34 53.94

The group means are close, except for possibly group B and D.


You can do a formal testing by regressing the response (weight) to its
predictors (group). You will need to use the lm() function in R.

   fit <- lm( w ~ grp )

 
You can get a summary of the fit by 

   summary(fit)
   ...
   Coefficients:
               Estimate Std. Error t value Pr(>|t|)
   (Intercept)   56.240      4.725  11.903 1.48e-11 ***
   grpB           6.120      6.682   0.916    0.369
   grpC          -0.700      6.682  -0.105    0.917
   grpD           7.300      6.682   1.093    0.285
   grpE          -0.900      6.682  -0.135    0.894
   grpF          -2.300      6.682  -0.344    0.734
   ...

This simply says that the intercept is strongly NOT zero. Based on the
p-values, one can roughly summarise that none of the groups appear to be
different.


Another useful tool is the ANOVA test which tests if the between group
variations are larger than average within group variation.

   anova(fit)
   Analysis of Variance Table

   Response: w
             Df  Sum Sq Mean Sq F value Pr(>F)
   grp        5  411.15   82.23  0.7367 0.6033
   Residuals 24 2678.79  111.62

This says that there is no significant variation between the groups.

Hope this helps.

Regards, Adai



On Fri, 2006-03-10 at 11:24 -0500, Jason Horn wrote:
> I'd like to do a simple one-way ANOVA comparing the means of 6  
> groups.  But it seems like the only way to do an ANOVA in R is to  
> specify some sort of model, where there is an outcome or dependent  
> variable that is a function of independent variables (linear model).   
> But I don't have a linear model, I just want to do a simple ANOVA  
> (and f-test) to compare the means.  How do I do this?  My stats  
> skills are basic, so please bear with me.
> 
> Thanks for any ideas...
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
>