Mathematics

Polynomial Regression

Based on the plot, it is possible that the data can be modeled by a polynomial function

The unknown coefficients a₀, a₁, and a₂ can be computed by doing a least squares fit, which minimizes the sum of the squares of the deviations of the data from the model. There are six equations in three unknowns,

represented by the 6-by-3 matrix

• X = [ones(size(t))  t  t.^2]
  
  X = 
      1.0000         0         0
      1.0000    0.3000    0.0900
      1.0000    0.8000    0.6400
      1.0000    1.1000    1.2100
      1.0000    1.6000    2.5600
      1.0000    2.3000    5.2900
  

The solution is found with the backslash operator.

• a = X\y
  
  a =
      0.5318
      0.9191
    - 0.2387
  

The second-order polynomial model of the data is therefore

Now evaluate the model at regularly spaced points and overlay the original data in a plot.

• T = (0:0.1:2.5)';
  Y = [ones(size(T))  T  T.^2]*a;
  plot(T,Y,'-',t,y,'o'), grid on
  
  
  

Clearly this fit does not perfectly approximate the data. We could either increase the order of the polynomial fit, or explore some other functional form to get a better approximation.

Regression and Curve Fitting

Linear-in-the-Parameters Regression

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