# Polyfit returns the wrong correlation coeffcient when I try do a linear regression

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A LL on 11 Nov 2020
Edited: A LL on 14 Nov 2020
Hello :)
I am trying to do a linear regression for a set of data, but the regression coefficient returned by matlab polyfit don't fit my data.
The data is plot just fine but the linear regression function seems very odd...
Please see the attached figure
I tried to plot with polyval (option 1) and with a linear relation (y=ax+b) (option 2) but it does not make any difference... The regression coefficient returned are just no the good ones...
Can anyone help me?
subwFMA = w10MeanFMA(521:771, 41:161); %sub mean wind speed
subssrdFMA = ssrdMeanFMA(521:771, 41:161); %sub mean solar radiation
%OPTION 1
pFMA = polyfit(subwFMA(:),subssrdFMA(:),1); %find regression coefficient a and b for a linear regression
fFMA = polyval(pFMA,subwFMA(:)); %linear regression function
figure(1)
plot(subwFMA(:),subssrdFMA(:),'o',subwFMA(:),fFMA,'-');
%OPTION 2
polyfitFMA = polyfit(subwFMA(:),subssrdFMA(:),1); %find correletation coefficient a and b for a linear regression
aFMA = polyfitFMA(1,1); %coeff a
bFMA = polyfitFMA(1,2); %coeff b
yssrdFMA = aFMA*subwFMA(:) + bFMA; %linear regression function
figure(2)
plot(subwFMA(:),subssrdFMA(:),'o',subwFMA(:),yssrdFMA,'-');
I also tried to reverse my x and y data in polyfit but it makes things even worse!
Thanks
John D'Errico on 12 Nov 2020
Again, we don't have your data. And it seems like you fit only a small portion of the data. So how can we possibly know what you did wrong?

Jon on 11 Nov 2020
Edited: Jon on 11 Nov 2020
I don't think you are plotting your fitted y values
For example where you have
fFMA = polyval(pFMA,subwFMA(:)); %linear regression function
figure(1)
plot(subwFMA(:),subssrdFMA(:),'o',subwFMA(:),yssrdFMA,'-');
I think you want
yssrdfFMA = polyval(pFMA,subwFMA(:)); %linear regression function
figure(1)
plot(subwFMA(:),subssrdFMA(:),'o',subwFMA(:),yssrdFMA,'-');
and where you have
figure(2)
plot(subwFMA(:),subssrdFMA(:),'o',subwFMA(:),fFMA,'-');
I think you want
figure(2)
plot(subwFMA(:),subssrdFMA(:),'o',subwFMA(:),yssrdFMA,'-');
A LL on 13 Nov 2020
Edited: A LL on 14 Nov 2020
Ok, thanks again for your help!
I will go over my code one more time and if I can not find the error I will post it here. :)
update: I was able to solve my problem. I had a mistake in my code. Thanks again for your help!

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