MATLAB: Help with basic fitting

curve fittingfitinterceptlinelinear fitpolynomialslope

Hello,I would like to fit a curve with a line and retrieve slope and intercept of this linear fit. Let x and y be the vectors which define the curve, which is already similar to a line; I plot x vs y, then from the plot window I select Tools –> Basic fitting –> Linear fit, then I press 'Show equation' so that from the expression y=mx + q I retrieve both the slope m and the intercept q. The problem is that I want to obtain immediatly m and q from the Matlab code, without doing it manually as discussed above. How can I do this ? Thanks

Best Answer

Try

 [p,S] = polyfit(x,y,1)

The two elements of p are the coefficients m and q you are looking for.

Related Solutions

MATLAB: What is an accurate function for linear regression

Which Toolboxen do you have? There's fit in the Curve Fitting TB and LinearModel.fit in the Statistics Toolbox. polyfit is certainly just as accurate; for such simple fitting and wants, the coefficients are returned directly and Rsq is easily computed from the definition as 1-SSE/SST

ADDENDUM

>> x=1:10; x=x(:); y=randn(size(x));  % sample data
>> b=polyfit(x,y,1)                   % fit....
b =
  0.2412   -0.7021
>> plot(x,y,'-*)
>> yhat=polyval(b,x);      % evaluate
>> hold all
>> plot(x,yhat)
>> mn=mean(y);
>> dot(y-mn,y-mn)  % definition for SST, total sum squares
ans =
 28.1924
>> SST=var(y)*(length(y)-1)  % Use builtin VAR function
SST =
 28.1924
>> SSE=dot(y-yhat,y-yhat)
SSE =
 23.3944
>> Rsq=1-SSE/SST
Rsq =
  0.1702

Best Answer

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MATLAB: What is an accurate function for linear regression

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