[R] Selecting groups with R

[David Winsemius]

On Aug 21, 2009, at 6:35 PM, jlwoodard wrote:

>
> Thank you David!
>
>
> David Winsemius wrote:
>>
>>
>> How.... did you do the "t-test"?
>>
>>
>
> t.test(Score,Color)

?t,test

t.test expects two numeric vectors, not a numeric vector and a  
grouping indicator.

 > t.test(dataset[dataset$Color=="RED", "Score"], dataset[dataset 
$Color=="WHITE", "Score"] )

	Welch Two Sample t-test

data:  dataset[dataset$Color == "RED", "Score"] and dataset[dataset 
$Color == "WHITE", "Score"]
t = -7.6485, df = 3.298, p-value = 0.003305
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
  -18.143205  -7.856795
sample estimates:
mean of x mean of y
  11.66667  24.66667
>
-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT