Thank you for your very comprehensible answer. I a priori know that model y=a*x+0 is right and that I can't get x=constant nor y=constant. I'm comparing performance of data gathering in my data set to another data sets in which performance gathering is characterized by R-squared . The data in data sets are not random but have random error with normal distribution. The error in my data is composed of human and equipment error (normal distribution), appliance error (probably skewed normal distribution) and some other interferences. So I compare model y = a*x + b and y = a*x + 0 to know the magnitude of interferences which I can't influence - the difference of R^2's.
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