(PR#7225) propogation of rounding error for t.tests with unequal sample sizes (PR#7225)

My apologies for not including the transcript in the original message:

xx <- rep(2.10,80)
groups.balanced <- as.factor(c(rep("one",40),rep("two",40)))
groups.unbalanced <- as.factor(c(rep("one",44),rep("two",36)))
t.test(xx ~ groups.balanced)

	Welch Two Sample t-test

data:  xx by groups.balanced 
t = 0, df = 78, p-value = 1
alternative hypothesis: true difference in means is not equal to 0 
95 percent confidence interval:
 -4.004247e-16  4.004247e-16 
sample estimates:
mean in group one mean in group two 
              2.1               2.1 

t.test(xx ~ groups.unbalanced)

	Welch Two Sample t-test

data:  xx by groups.unbalanced 
t = -4.1007, df = 54.298, p-value = 0.0001388
alternative hypothesis: true difference in means is not equal to 0 
95 percent confidence interval:
 -1.322366e-15 -4.539911e-16 
sample estimates:
mean in group one mean in group two 
              2.1               2.1 

-----Original Message-----
From: Prof Brian Ripley [mailto:ripley@stats.ox.ac.uk] 
Sent: Monday, September 13, 2004 4:04 PM
To: Tucker-Kellogg, Gregory
Cc: r-devel@stat.math.ethz.ch; R-bugs@biostat.ku.dk
Subject: Re: [Rd] (PR#7225) propogation of rounding error for 
t.tests with unequal sample sizes (PR#7225)


What are you seeing here?  I get identical results in the two cases.

On Mon, 13 Sep 2004 tucker@mpi.com wrote:

# users would benefit from a warning about the behavior in the # 
groups.unbalanced case below.  A propogation of difference is the 
variance # calculation is leading to an apparently significant 
difference in # means, even though the numeric values are all 
identical.  Obviously upon # inspection the difference in means is 
meaningless, but users might be warned # if all of the input data is 
identical

That's not so easy to achieve, given rounding error, but I am 
seeing 0 variance in both cases.

xx <- rep(2.10,80)

groups.balanced <- as.factor(c(rep("one",40),rep("two",40)))
groups.unbalanced <- as.factor(c(rep("one",44),rep("two",36)))

t.test(xx ~ groups.balanced)

t.test(xx ~ groups.unbalanced)



<<insert bug report here>>



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Version:
 platform = sparc-sun-solaris2.8
 arch = sparc
 os = solaris2.8
 system = sparc, solaris2.8
 status =
 major = 1
 minor = 9.0
 year = 2004
 month = 04
 day = 12
 language = R

Search Path:
 .GlobalEnv, package:methods, package:stats, package:graphics, 
package:utils, Autoloads, package:base



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Brian D. Ripley,                  ripley@stats.ox.ac.uk
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