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 log-linear fit of data		  New Topic  Reply to Topic
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akijitsu

2 Posts
Posted - 01/13/2012 :  12:34:44 PM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
Hello - I hope someone can help with this problem.

I am working with data to follow the equation:

Y=Y0Exp(mX)

where m is gradient
and Y0 the value of Y at X=0 (y axis intercept)

I have plotted some data in log(Y) vs X format. I want to fit this data to find the intercept value Y0.

I am not confident of the answers provided by simply performing a linear fit.

Hideo Fujii

USA
1582 Posts
Posted - 01/13/2012 :  2:26:45 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi akijitsu,

Yes, you can perform the linear fit for the semi-log plot.
Make sure that when you run the linear fit, "Apparent Fit" check box is checked as shown below:



--Hideo Fujii
OriginLab
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akijitsu

2 Posts
Posted - 01/17/2012 :  07:14:52 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi Hideo

Thanks for your reply. I have checked 'apparent fit'. However the intercept value provided by the table is clearly wrong - it gives me a intercept value of -2 While i can clearly see the fitted line is crossing the y axis at about 0.001. Is there another setting which could be causing this error?

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Hideo Fujii

USA
1582 Posts
Posted - 01/17/2012 :  11:01:18 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi akijitsu,

Since it is semi-log scale on Y, i.e., log10(y)=a0+a1*x,
actual intercept should be y(0)=10^a0.

--Hideo Fujii
OriginLab
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cjg

Australia
Posts
Posted - 08/12/2012 :  02:34:46 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
A similar problem on a log/log plot gives a funny answer (see attached).
Certainly not the intercept to the power 10.

Hope I am missing something obvious?

regards

Chris
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Hideo Fujii

USA
1582 Posts
Posted - 08/13/2012 :  10:43:04 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi Chris,

I don't see any strange thing here because the intercept is the Y value at X=1 (i.e., 10^0) - far below your current Y scale.

Does it make sense?

--Hideo Fujii
OriginLab
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