Difference between spearmann correlation and fit command
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Hello all,
I have some data that I want to analyze (EMG of muscles vs subjects' fatigue) in order to find the relation between two measurs.
At first, I used the 'corr' command to calculate the spearman correlation coefficient between the EMG and the fatigue for each subject. Afterwards, I used the 'fit' command to fit a linear model to my data.
To my surprise, in some cases (about 5% of the cases), the spearman correlation coefficient was positive, but the fitted linear model had a negative slope (or vice versa).
Shouldn't they have the same tendency? Or am I doing something wrong?
I'm attaching my matlab code, and excel file with some example data that shows the spoken discrepancy.
Thank you.
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Accepted Answer
Peter Perkins
on 11 Oct 2013
You're (correctly) fitting a linear model with a slope and an intercept. The "model" that a correlation coefficient is equivalent to has no intercept. Look at your figure to see why one is positive and the other negative.
Hope this helps.
More Answers (1)
Wayne King
on 11 Oct 2013
I would not put any faith in the correlation coefficient, the p-value is 0.9 and the reported rho is close to 0.
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