From: Peter Perkins <>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Old bug in corrcoef not yet fixed
Date: Thu, 08 Jan 2009 02:19:49 -0500
Organization: The MathWorks, Inc.
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Pasco Alquim wrote:

> Sorry, but what I see in the docs is
> [...]=corrcoef(...,'param1',val1,'param2',val2,...) specifies additional parameters and their values. Valid parameters are the following. 
>   'alpha' A number between 0 and 1 to specify a confidence level of 100*(1 - alpha)%. Default is 0.05 for 95% confidence intervals.
> No mention that, when 'alpha' is provided, one must have 4 argouts

What exactly would you expect the confidence level argument to _do_ if you do not compute the confidence bounds?

> And why should p be the probability for the 95% confidence only? Which is in fact is what it does.

You seem to have a fundamental misunderstanding of what a p-value is.  Choosing a confidence level _in advance_ for a confidence interval has absolutely nothing to do with a p-value, which is the probability of an observed event given an assumed null model.  At best, you could describe the p-value as the smallest significance level at which the null hypothesis "H0: correlation = 0" would have been accepted.

The second output argument from corrcoef is exactly what the help describes it to be:

"Each p-value is the probability of getting a correlation as large as the observed value by random chance, when the true correlation is zero."