[R] 2 Sample Confidence Interval - Formatting Data?

[Marc Schwartz]

On Tue, 2007-10-09 at 18:37 -0400, Wayne Aldo Gavioli wrote:
> 
> Hello all,
> 
> I have to run a 2 Sample Confidence Interval on some data; the command for such
> intervals is "confint(...)", but in the help documentation it says that you
> need a "fitted model object" in order to run this command.  What does that
> mean?
> 
> The data is very small, it's:
> 
> x=c(8,12,10,14,2,0,0)
> y=c(-6,0,1,2,-3,-4,2)
> 
> 
> and I want to be able to run a Confidence interval for the difference of two
> means (x-bar minus y-bar).  Do I have to fit these two pieces of data into some
> kind of object?
> 
> 
> A little confused,

Try this, presuming that your data above are not paired:

> t.test(x, y)

        Welch Two Sample t-test

data:  x and y 
t = 3.0621, df = 9.264, p-value = 0.01308
alternative hypothesis: true difference in means is not equal to 0 
95 percent confidence interval:
  2.039893 13.388678 
sample estimates:
mean of x mean of y 
 6.571429 -1.142857 


See ?t.test for more information.


confint(), as per the top of help page which you were reading:

"Computes confidence intervals for one or more parameters in a fitted
model."

Said differently, it computes confidence intervals for the coefficients
in a linear model, not for the difference in means in a two sample test.

HTH,

Marc Schwartz