Hi Mike,
I think you use n when you're estimating a population parameter, and you
use n-1 when you're estimating a sample parameter. So if we're trying to
get a confidence interval for the *sample* mean, we use sqrt(p(1-p)/n, but
when we're talking about the mean of the entire population that the sample
was drawn from, you use n-1 instead. It just depends on whether the thing
in the middle of your CI is a statistic or a population parameter (and
they usually tell you in the question which one they want a CI for!)
Cheers,
Zoe
On Wed, 24 Sep 2003, Michael Richard Kellermann wrote:
Hi all -
I have a question about the discussion we had tonight regarding the
confidence interval versus the sample confidence interval. I understand
that when you have to estimate both the mean and the standard deviation to
calculate the confidence interval, you have to use n-1 rather than n.
The formula that we are using to calculate the confidence interval only
requires us to estimate one parameter, p. Using this method for
calculating confidence intervals, when would we need to use n-1?
Cheers,
Mike
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