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Eugene Kirpichov commented on MATH278:

(No, the FFT optimization may only be done if the abscissae are an arithmetic progression. Might make sense to include this as a special case in a separate method)
> Robust locally weighted regression (Loess / Lowess)
> 
>
> Key: MATH278
> URL: https://issues.apache.org/jira/browse/MATH278
> Project: Commons Math
> Issue Type: New Feature
> Reporter: Eugene Kirpichov
> Attachments: loess.patch, loess.patch.v2
>
>
> Attached is a patch that implements the robust Loess procedure for smoothing univariate scatterplots with local linear regression ( http://en.wikipedia.org/wiki/Local_regression) described by William Cleveland in http://www.math.tau.ac.il/~yekutiel/MA%20seminar/Cleveland%201979.pdf , with tests.
> (Also, the patch fixes one missingjavadoc checkstyle warning in the AbstractIntegrator class: I wanted to make it so that the code with my patch does not generate any checkstyle warnings at all)
> I propose to include the procedure into commonsmath because commonsmath, as of now, does not possess a method for robust smoothing of noisy data: there is interpolation (which virtually can't be used for noisy data at all) and there's regression, which has quite different goals.
> Loess allows one to build a smooth curve with a controllable degree of smoothness that approximates the overall shape of the data.
> I tried to follow the code requirements as strictly as possible: the tests cover the code completely, there are no checkstyle warnings, etc. The code is completely written by myself from scratch, with no borrowings of thirdparty licensed code.
> The method is pretty computationally intensive (10000 points with a bandwidth of 0.3 and 4 robustness iterations take about 3.7sec on my machine; generally the complexity is O(robustnessIters * n^2 * bandwidth)), but I don't know how to optimize it further; all implementations that I have found use exactly the same algorithm as mine for the unidimensional case.
> Some TODOs, in vastly increasing order of complexity:
>  Make the weight function customizable: according to Cleveland, this is needed in some exotic cases only, like, where the desired approximation is noncontinuous, for example.
>  Make the degree of the locally fitted polynomial customizable: currently the algorithm does only a linear local regression; it might be useful to make it also use quadratic regression. Higher degrees are not worth it, according to Cleveland.
>  Generalize the algorithm to the multidimensional case: this will require A LOT of hard work.

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