# Comparing two curve fits (using AIC?)

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James Akula on 12 Jan 2023
Commented: Bjorn Gustavsson on 18 Jan 2023
Hello all,
Let's say for a moment I have some a priori ideas about a family of functions that might best describe a particular data set. After I fit the data with each candidate, I can simply look at the outputs (e.g., RMSE, r²) and chose the one with the best values. However, is there a way to gain some degree of confidence about that selection? For example, let's say I have reason to believe that in a set of data like the below, the true distribution is described by a low-order integer exponent in the function y=b×xⁿ+c. So, I might capture some data and then try to fit it as follows:
% Generate some data
b = 1; % True coefficient
n = 3; % True exponent
c = 0; % True y-intercept
rx = randi(100, [100, 1])/10;
rn = 3 + rand([100, 1])/4 - 0.125;
rc = (rand([100, 1]) * 500) - 250 + c;
ry = b * rx.^(rn) + rc;
% Generate and fit the models
m = @(b, c, n, x) b * x.^n + c;
m1 = @(p, x) m(p(1), p(2), 1, x); % n=1
m2 = @(p, x) m(p(1), p(2), 2, x); % n=2
m3 = @(p, x) m(p(1), p(2), 3, x); % n=3
c1 = lsqcurvefit(m1, [1, 0], rx, ry)
c2 = lsqcurvefit(m2, [1, 0], rx, ry)
c3 = lsqcurvefit(m3, [1, 0], rx, ry)
% Plot the results
x = linspace(min(rx), max(rx));
plot(rx, ry, 'ok', x, m1(c1, x), '-g', x, m2(c2, x), '-r', x, m3(c3, x), '-b')
% Determine which model is most likely to be correct. Use AIC?
In the above example, m3 should generally fit the data best, unless the random number generation is very unlucky, because that's what I set it to.
For those who are familiar with Prism, there I might perform this test by starting an "analysis" and chosing compare, and then selecting "for each data set, which of two equations (models) fits best" and then chosing the "Akaike's Information Criterion" test. After running that on one set of data, I got This is very helpful as it lets me know how certain I am about the fit choice (i.e., 86% sure n=3 is better than n=2). What's the best way to do something similar in MATLAB?

James Akula on 13 Jan 2023
I belive the following functions perform the AIC test, as shown in https://www.mathworks.com/matlabcentral/answers/1893055-comparing-two-curve-fits-using-aic#comment_2561700. p1 is the probablility that the first model is the correct one.
% y = y data
% o = observed
% a = "answer" to curve fit
% m = modeled
% 1/2 = model IDs
df = @(oy, a) numel(oy) - numel(a);
SS = @(my, oy) sum(((my - oy).^2));
DifAIC = @(m1y, m2y, oy, a1, a2) numel(oy) .* ...
log(SS(m1y, oy)./SS(m2y, oy)) + 2 .* (df(oy, a2) - df(oy, a1));
p1 = @(m1y, m2y, oy, a1, a2) ...
1 - exp(DifAIC(m1y, m2y, oy, a1, a2)/2)./...
(1 + exp(DifAIC(m1y, m2y, oy, a1, a2)/2));
James Akula on 13 Jan 2023

Bora Eryilmaz on 12 Jan 2023
The lsqcurvefit returns the residual norm of the fit. The model that has the least residual norm would be the "best" one by this criterion:
% Generate some data
b = 1; % True coefficient
n = 3; % True exponent
c = 0; % True y-intercept
rx = randi(100, [100, 1])/10;
rn = 3 + rand([100, 1])/4 - 0.125;
rc = (rand([100, 1]) * 500) - 250 + c;
ry = b * rx.^(rn) + rc;
% Generate and fit the models
m = @(b, c, n, x) b * x.^n + c;
m1 = @(p, x) m(p(1), p(2), 1, x); % n=1
m2 = @(p, x) m(p(1), p(2), 2, x); % n=2
m3 = @(p, x) m(p(1), p(2), 3, x); % n=3
[c1, resnorm1] = lsqcurvefit(m1, [1, 0], rx, ry);
Local minimum possible. lsqcurvefit stopped because the final change in the sum of squares relative to its initial value is less than the value of the function tolerance.
[c2, resnorm2] = lsqcurvefit(m2, [1, 0], rx, ry);
Local minimum possible. lsqcurvefit stopped because the final change in the sum of squares relative to its initial value is less than the value of the function tolerance.
[c3, resnorm3] = lsqcurvefit(m3, [1, 0], rx, ry);
Local minimum possible. lsqcurvefit stopped because the final change in the sum of squares relative to its initial value is less than the value of the function tolerance.
% Plot the results
x = linspace(min(rx), max(rx));
plot(rx, ry, 'ok', x, m1(c1, x), '-g', x, m2(c2, x), '-r', x, m3(c3, x), '-b') % Determine which model is most likely to be correct. Use AIC?
[~, idx] = min([resnorm1 resnorm2 resnorm3]) % Index of best model
idx = 3
Bjorn Gustavsson on 18 Jan 2023
@James Akula, sure, and my suggestion might work well enough for this (with reasonable certainty) simplified toy-example, but can run into severe problems in more complicated real-world cases. I mainly thought it was worth mentioning as a reminder for the case where someone were to have this specific choise to make. (and I don't need anyone to argue this quandry with - I come down on all three of the two sides in this discussion and have frequent and angry arguments with myself from last week...)

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