st: Problem with ttest in stata 8/9?

["Steven Proud" <steven_proud@hotmail.com>]

Dear Statalisters

Apologies if this has been covered before.


I have become slightly confused whilst trying to use ttest in stata with an 
unequal variance in each sample.

the command ttest x y, unequal unpaired in stata calculates the t-statistic 
as follows

t=(mean(x)-mean(y))/sqrt[((standard deviation(x))^2/nx)+(standard 
deviation(y))^2/ny)]

Now, I believe that this may give biased results, as we do not know the true 
value of the population standard error, and so estimate it from standard 
deviation(x), so we should estimate it as  [standard deviation (x)]/(nx-1)

Therefore, should the t-statistic not be calculated

t=(mean(x)-mean(y))/sqrt[((standard deviation(x))^2/(nx-1))+(standard 
deviation(y))^2/(ny-1))]
?

Thanks,
Steven Proud

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