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Re: regression with error bars [message #70812 is a reply to message #70719]

Tue, 04 May 2010 19:19

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

On May 4, 3:57 pm, Meagan Adams <meagan....@gmail.com> wrote:
> Hello
>
> I have two columns of data y (with errors Err+ and Err- for each data
> point) and x (with Err). I would like to find the linear regression
> coefficients A and B (in y=Ax+B) considering the errors in my data
> point. Is their a function in IDL which would do the trick? and the
> regression has to do with the errors in y only but not in x, right?

I don't believe there is a built-in function in IDL which can do this.

You can use MPFIT. The trick is that you will need to be able to form
your own scaled residual. Normally you would have a user function
something like this,
FUNCTION MYFUNCT, P, X=X, Y=Y, ERROR=ERROR
MODEL = F(X,P) ;; User model
RETURN, (Y - MODEL)/ERROR
END

but now you will need to reformulate as,
FUNCTION MYFUNCT, P, X=X, Y=Y, ERROR_PLUS=ERROR_PLUS,
ERROR_MINUS=ERROR_MINUS
MODEL = F(X,P) ;; User model
RESID_PLUS = (Y - MODEL) / ERROR_PLUS ;; Residual for positive
values
RESID_MINUS = (Y - MODEL) / ERROR_MINUS ;; Residual for negative
values
;; Combine the two kinds of residuals
RESID = (RESID_PLUS GT 0)*RESID_PLUS + (RESID_MINUS LT
0)*RESID_MINUS
RETURN, RESID
END

Happy fitting,
Craig

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[Message index]

		regression with error bars By: Meagan Adams on Tue, 04 May 2010 12:57
		Re: regression with error bars By: Craig Markwardt on Tue, 04 May 2010 19:19
		Re: regression with error bars By: Giuseppe Papa on Sat, 22 May 2010 09:44
		Re: regression with error bars By: Craig Markwardt on Sat, 22 May 2010 10:53
		Re: regression with error bars By: Giuseppe Papa on Sat, 22 May 2010 09:29
		Re: regression with error bars By: Craig Markwardt on Sat, 22 May 2010 08:02
		Re: regression with error bars By: Giuseppe Papa on Sat, 22 May 2010 06:34

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