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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Peter Perkins
2013-10-11
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.
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Wayne King
2013-10-11
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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