problem with the graph after calibration

[Ian Evans]

I use imageJ mainly for quantification of westerns. After I input the OD values of an optical density step tablet (see first screenshot)I use the Redbird function to get a calibration curve, however the graph doesn’t display correctly even though the equation of the line seems reasonable (2nd screenshot). Is this a known bug or am I doing something wrong?

Thanks




Dr Ian Evans
Centre for Cardiovascular Biology & Medicine
Department of Medicine
Rayne Building
UCL Medical School
5 University Street
London
WC1E 6JJ

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[Michael Schmid]

Hi Ian,

the Rodbard function works well for ascending functions. Your data are descending.
(by the way, it is named after an NIH scientist with that name, not Redbird.)

In the current version of ImageJ, I did not get a fit at all without modifying the starting parameters. I tried this with the Analyze>Tools>Curve Fitting, but the fit is not very good.
The y = a + b ln(x-c) function works better; see attachments, and it has no problems with descending data.

It seems that your version of ImageJ (which one are you using?) produced at least a somewhat reasonable fit, though clearly not the best fit (I did several retries; my fit in the attachment should be at least very close to the optimum).
The reason for the strange plot is that the function almost diverges at x = 0. Unfortunately, the y = a + b ln(x-c) function is not really much better in this respect; it diverges already at x = 5.2.


[By the way, your post appeared blank and with no attachment in my default mailer; I had to use webmail to see it. Try using a different mailer when posting to the mailing list next time.]

Michael
________________________________________________________________
On Mar 24, 2015, at 18:28, Ian Evans wrote:

> I use imageJ mainly for quantification of westerns. After I input the OD values of
> an optical density step tablet (see first screenshot)I use the Redbird function to
> get a calibration curve, however the graph doesn’t display correctly even though the
> equation of the line seems reasonable (2nd screenshot). Is this a known bug or am I
> doing something wrong?
>
> Thanks
>
>
>
>
> Dr Ian Evans
> Centre for Cardiovascular Biology & Medicine
> Department of Medicine
> Rayne Building
> UCL Medical School
> 5 University Street
> London

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ImageJ mailing list: http://imagej.nih.gov/ij/list.html