This is just a quick post to describe how to calculate p-values for two-variable correlations in Excel. Annoyingly, there is no direct way of doing this. Excel will give you the correlation, but not its associated p-value. It can be done, however, in a slightly roundabout way.
First, calculate the correlation between your groups:
This gives you the sample test statistic r, which can be converted to t with the following formula:
where r is the correlation obtained above and n is your number of observations. Say you have 30 samples for two groups, and a r of 0.5. The calculation to obtain t is then (in excel terms):
Then to assess the significance value associated with this t, simply use the tdist function (Student T distribution output):
=t.dist.2t(t, degrees of freedom)
This gives us results for a two-tailed distribution. Alternatively, the old tdist function can still be used, which requires the user to specify the number of tails (=tdist(t, degrees of freedom, #tails)).
Our calculation thus looks like this:
Which is the p-value for the correlation. Done!