Hi John, very appreciate for your detailed explain and pointing out my misunderstanding of R^2 in regression. Your suggestions really helps me, thanks you very much.
Chi-Fu
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15 Jan 2014
Total Least Squares Method
Mathematical method known as total least squares or orthogonal regression or error-in-variables.
I'm sorry, but you seem to think that even for a normal regression, that R^2 offers some "objective" measure of the regression, that it can offer a pass/fail rule for the regression. This is a classic misimpression I think.
There is no simple measure that I know of (including R^2) that will reliably tell you that a regression model is adequate for a general set of data. This will still be true for the TLS case. Nothing changes in that respect. That applies to a general model, and for a completely arbitrary set of data.
Nothing stops you from constructing some simple measure however, based on a subset of your many sets of data. For example, extract some subset of size N of those sets at random, and perform a careful analysis of the resulting models. Look at the residual plots for every one of those models. Compute various measures of fit (I'll offer a couple of example measures later.)
Choose a large enough subset of models such that there will be at least a few of the regression models that would be deemed inadequate. Now find a decision point for some parameter P, such that it reasonably predicts when the regression would have failed, in YOUR opinion, based on that subset of analyses.
Essentially I m suggesting that you choose some parameter P and a set point such that it offers a way to discriminate between "good" and "bad" models.
I can't tell you in advance that any given R^2 analogue, if it is greater than perhaps 0.975 (or any magic number) that it will offer the discriminant you desire. But the point is that you can choose some parameter, and essential calibrate things so that it might indicate when there MIGHT be a problem. Even so, any single number can be fooled by lack of fit masquerading as noise, but you seem insistent on having a magic measure.
So two such measures that would be appropriate for a TLS model might be:
1. Root Mean Square Total Error.
2. One minus the ratio of singular values. (Or maybe squared singular values. It is early in the morning for me, and my brain is a bit foggy now about which would be the proper analogue.)
So the first measure is simply an analogue of mean square error, so you compute the squared normal distance to the line for each point. Compute the sqrt of the mean of all of those squared distances, so RMSTE.
The second measure I listed is based on the idea of how the SVD works to generate a TLS model itself. You can think of the singular values as measuring the size of the point cloud in each of two directions, along the regression line, and normal to that line. So the ratio of the smaller singular value divided by the larger one will yield a number between 0 and 1. Subtract that from 1, and we get a number that will mimic R^2. A visual way of thinking about it is as if we are measuring the ratio of the axis lengths of an ellipse around your data. If that ellipse around your data formed a perfect circle, then this measure would return 0. If the data fell on a perfect straight line with no error, then this measure would return 1.
I'm now done consulting in the comments, with my apologies to the authors of this submission.
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14 Jan 2014
Total Least Squares Method
Mathematical method known as total least squares or orthogonal regression or error-in-variables.
Hi John,
Thanks for letting me know to trust my eye more than a number. My problem is I have more than ten thousands curves and I want to rule out some data that fit really bad before I start to select data manually. So if there is any objective value that represent the goodness of fitting in TLS, it would be useful to me.
Best,
Chi-Fu
Comment only
14 Jan 2014
Total Least Squares Method
Mathematical method known as total least squares or orthogonal regression or error-in-variables.
@Chi-Fu: The classic R^2 formula is not appropriate for TLS, since it fails to utilize the errors in both variables.
You COULD cobble up a scheme that mimics R^2, in the sense that as it approaches 1, the fit becomes perfect, and at zero the fit is random. However, IF you are a chronic user of R^2 such that you rely on it to know when your fit is adequate, any such measure will not satisfy your internal calibration for the resulting number. You will simply get a number out, but without any meaning behind that number. For example, is 0.9 a good enough value for R^2? For some systems, maybe any R^2 over 0.5 might be as good as we can expect. For others, any R^2 under 0.99 might reflect a worthless model.
Is the error in the model lack of fit? Or is it noise? Looking at the residuals can help you learn things like this, but a simple number will be completely fooled.
In the end, DON'T rely on a simple measure like R^2 anyway. It is a crutch. Instead, look at the fit. Look at the residuals. If the result seems adequate for your purposes, then it is. Your eyes and your brain will provide a far better measure of the fit than any single number.
Look at it another way. I (or any random person) might fit your set of data with a line. Without knowing your goals, without your knowledge of the process, your knowledge of the noise structure, I might deem the fit adequate. Hey, it might look like a reasonable fit to me! Only you know how good of a fit you need to obtain. WHAT ARE YOUR GOALS FOR THIS FIT? Only you can use your knowledge of the system to know if the fit is adequate.
Of course, if you have no knowledge of the process that generated your data, if you have no understanding of that system, if you are just pushing numbers through a formula to get a number out, then you are the wrong person to be doing the job! Don't use a number to provide a substitute for good judgment. If you lack that skill or any basis for judgment, then find someone who has the necessary knowledge.
So throw away the crutch of R^2. Please.
I'll get off my soapbox now, end my rant.
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