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https://issues.apache.org/jira/browse/MATH-377?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12880832#action_12880832
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Phil Steitz commented on MATH-377:
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I guess if you assume normalliy distributed errors, it makes sense, so drop the 
last comment and I am +1 for the change (with definition added to the javadoc).

> weight versus sigma in AbstractLeastSquares
> -------------------------------------------
>
>                 Key: MATH-377
>                 URL: https://issues.apache.org/jira/browse/MATH-377
>             Project: Commons Math
>          Issue Type: Bug
>    Affects Versions: 2.1
>            Reporter: Dimitri Pourbaix
>             Fix For: 2.2
>
>   Original Estimate: 0.02h
>  Remaining Estimate: 0.02h
>
> In AbstractLeastSquares, residualsWeights contains the WEIGHTS assigned to 
> each observation.  In the method getRMS(), these weights are multiplicative 
> as they should. unlike in getChiSquare() where it appears at the denominator! 
>   If the weight is really the weight of the observation, it should multiply 
> the square of the residual even in the computation of the chi2.
>  Once corrected, getRMS() can even reduce
>  public double getRMS() {return Math.sqrt(getChiSquare()/rows);}

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