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

조회 수: 14(최근 30일)
A Leglep 2020년 11월 11일
편집: A Leglep 2020년 11월 14일
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...
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
##### 댓글 수: 3표시숨기기 이전 댓글 수: 2
John D'Errico 2020년 11월 12일
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?

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### 채택된 답변

Jon 2020년 11월 11일
편집: Jon 2020년 11월 11일
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,'-');
##### 댓글 수: 5표시숨기기 이전 댓글 수: 4
A Leglep 2020년 11월 13일
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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