# Multiple linear regression to fit data to a third degree polynomial equation with interaction terms

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Gautam 2014 年 4 月 1 日
コメント済み: Gautam 2014 年 4 月 2 日
Hello,
I have data for two independent variables and one dependent variable (obtained from experiment). I need to fit this data using linear regression to a 10 coefficient third degree polynomial equation - for the engineers among you, this is the standard equation for specifying refrigeration compressor performance. This can be done quite easily in EES by entering data into a table and selecting the option "Linear Regression" from the Tables menu. How do I achieve the same in Matlab? My equation is of the form X = C1 + C2.(S) + C3.(D) + C4·(S2) + C5 · (S·D) + C6 · (D2 ) + C7 · (S3 ) + C8 · (D·S2 ) +C9 · (S·D2 ) + C10 · (D3 ) Here, X is the dependent variable and S and D are the independent variables. The numbers next to S and D indicate the power to which they are raised. C1 to C10 are the coefficients that need to be calculated.
Using the 'regress' function gives me a constant of 0, and warns me that my design matrix is rank deficient. Using the curve fitting toolbox (cftool - polynomial option) gives me ridiculous values for the coefficients (p00 = -6.436e15). When I try to input a custom equation in the cftool, it is switching to non-linear regression and asks me to input guess values for the coefficients, which I don't want to do. What other functions are available that I might use to perform this regression and how do I implement them. Any suggestions/ help/ recommendations would be greatly appreciated.
Thanks very much.
Gautam.
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Gautam 2014 年 4 月 1 日
>> X = [-10 101; -10 114; -10 129; -10 144; -5 96; -5 106; -5 119; -5 133; 5 92; 5 97; 5 108; 5 122];
>> Y = [72.03; 67.90; 62.77; 57.55; 88.53; 84.70; 79.91; 73.73; 123.43; 120.91; 115.44; 107.83];
>> x1 = X(:,1);
>> x2 = X(:,2);
>> cftool
I used a custom equation, Y = f(x1, x2) and the custom equation is:
a + b*x1 + c*x2 + d*(x1^2) + e*(x1*x2) + f*(x2^2) + g*(x1^3) + h*(x1^2*x2) + i*(x1*x2^2) + j*(x2^3)
Complex value computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients.
Any help?

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### 採用された回答

Tom Lane 2014 年 4 月 2 日
You have only three distinct values in the first column of your X matrix, so you won't be able to estimate constant, linear, quadratic, and cubic effects for that column. Here's how you can use all terms in the cubic model except x1^3:
fitlm(X,Y,[0 0;1 0;0 1;1 1;2 0;0 2;0 3;1 2;2 1])
Alternatively, if you know that a third order term is appropriate, you will need to collect some data at another value of x1.
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Gautam 2014 年 4 月 2 日
Thank you very much Mr. Stafford for clearing that up for me. I'm a newcomer to Matlab and could not grasp completely the meaning behind the warnings. I tried out fitlm with a much bigger data set and it did give me the exact same values EES did, which has around 2% variation with the published coefficients. I cannot thank you enough.

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### その他の回答 (1 件)

Shashank Prasanna 2014 年 4 月 1 日
There are numerous ways to do this. The most easiest of them all is to use the Curve Fitting Tool with the correct options. Try this link for help:
Using the Statistics Toolbox you can do that by specifying the polyijf modelspec:
Another way is to solve a linear system in MATLAB. Assuming X, S and D are vectors in your MATLAB workspace. Create random data and compute the coefficients in C:
S = randn(100,1);
D = randn(100,1),
X = randn(100,1);
M = [ones(length(S),1), S, D, S.^2, S.*D, D.^2, S.^3, D.*S.^2, S.*D.^2, D.^3]
C = M\X
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Gautam 2014 年 4 月 2 日
I'm posting some of the attempts I made below:
X = [-10 101; -10 114; -10 129; -10 144; -5 96; -5 106; -5 119; -5 133; 5 92; 5 97; 5 108; 5 122];
>> Y = [72.03; 67.90; 62.77; 57.55; 88.53; 84.70; 79.91; 73.73; 123.43; 120.91; 115.44; 107.83];
>> x1 = X(:,1);
>> x2 = X(:,2);
>> mdl = fitlm(X,Y,'poly33')
Warning: Regression design matrix is rank deficient to within machine precision. > In TermsRegression>TermsRegression.checkDesignRank at 98 In LinearModel.LinearModel>LinearModel.fit at 868 In fitlm at 117
mdl =
Linear regression model: y ~ 1 + x1^2 + x1*x2 + x2^2 + x1^3 + (x1^2):x2 + x1:(x2^2) + x2^3
Estimated Coefficients: Estimate SE tStat pValue _________ ________ _______ ______
(Intercept) 0 0 NaN NaN
x1 -10.143 5.3752 -1.887 0.19979
x2 -0.3125 1.1432 -0.27335 0.81022
x1^2 5.5413 1.6852 3.2882 0.081361
x1:x2 0.0043522 0.026071 0.16694 0.88277
x2^2 2.6105e-05 0.010348 0.0025228 0.99822
x1^3 0.55158 0.16763 3.2905 0.08126
x1^2:x2 -5.4944e-05 0.00021022 -0.26136 0.81827
x1:x2^2 -8.3639e-05 0.00012017 -0.69599 0.55844
x2^3 -4.12e-06 3.109e-05 -0.13252 0.9067
Number of observations: 12, Error degrees of freedom: 3 Root Mean Squared Error: 0.165 R-squared: 1, Adjusted R-Squared 1 F-statistic vs. constant model: 2.76e+04, p-value = 3.3e-07
Attempt 2:
A = [1 -10 101 100 -1010 10201 -1000 10100 -102010 1030301
1 -5 96 25 -480 9216 -125 2400 -46080 884736
1 5 92 25 460 8464 125 2300 42320 778688
1 -10 114 100 -1140 12996 -1000 11400 -129960 1481544
1 -5 106 25 -530 11236 -125 2650 -56180 1191016
1 5 97 25 485 9409 125 2425 47045 912673
1 -10 129 100 -1290 16641 -1000 12900 -166410 2146689
1 -5 119 25 -595 14161 -125 2975 -70805 1685159
1 5 108 25 540 11664 125 2700 58320 1259712
1 -10 144 100 -1440 20736 -1000 14400 -207360 2985984
1 -5 133 25 -665 17689 -125 3325 -88445 2352637
1 5 122 25 610 14884 125 3050 74420 1815848
];
b = A\Y Warning: Rank deficient, rank = 9, tol = 4.463946e-09.
b =
0
-62.6154
18.9421
-35.6208
2.9627
-0.0958
-3.8184
-0.0236
-0.0136
0.0001
Attempt 3:
sf = fit([x1 x2],Y, 'poly33', 'Robust', 'Bi')
Warning: Equation is badly conditioned. Remove repeated data points or try centering and scaling. > In Warning>Warning.throw at 30 In fit>iLinearFit at 690 In fit>iFit at 396 In fit at 108 Warning: Iteration limit reached for robust fitting. > In Warning>Warning.throw at 30 In fit>iRobustLinearFit at 796 In fit>iFit at 404 In fit at 108
Linear model Poly33:
sf(x,y) = p00 + p10*x + p01*y + p20*x^2 + p11*x*y + p02*y^2 + p30*x^3 +
p21*x^2*y + p12*x*y^2 + p03*y^3
Coefficients (with 95% confidence bounds):
p00 = -2.736e+14 (-6.205e+17, 6.199e+17)
p10 = -2.736e+13 (-6.205e+16, 6.199e+16)
p01 = 0.3112 (-346.1, 346.7)
p20 = 1.094e+13 (-2.48e+16, 2.482e+16)
p11 = -0.009459 (-5.569, 5.551)
p02 = -0.005727 (-3.308, 3.296)
p30 = 1.094e+12 (-2.48e+15, 2.482e+15)
p21 = -0.0001193 (-0.4058, 0.4055)
p12 = -1.846e-05 (-0.02545, 0.02542)
p03 = 1.358e-05 (-0.01007, 0.01009)
Attempt 4:
sf = fit([x1 x2],Y, 'poly33', 'Robust', 'Bi','Normalize','on')
Warning: Equation is badly conditioned. Remove repeated data points or try centering and scaling.
> In Warning>Warning.throw at 30
In fit>iLinearFit at 690
In fit>iFit at 396
In fit at 108
Warning: Iteration limit reached for robust fitting.
> In Warning>Warning.throw at 30
In fit>iRobustLinearFit at 796
In fit>iFit at 404
In fit at 108
Linear model Poly33:
sf(x,y) = p00 + p10*x + p01*y + p20*x^2 + p11*x*y + p02*y^2 + p30*x^3 +
p21*x^2*y + p12*x*y^2 + p03*y^3
where x is normalized by mean -3.333 and std 6.513
and where y is normalized by mean 113.4 and std 16.34
Coefficients (with 95% confidence bounds):
p00 = 2.802e+14 (-4.173e+15, 4.734e+15)
p10 = 1.15e+15 (-1.712e+16, 1.942e+16)
p01 = -6.475 (-15.32, 2.371)
p20 = 1.005 (-3.659, 5.67)
p11 = -1.507 (-6.289, 3.275)
p02 = -0.3495 (-5.084, 4.385)
p30 = -8.362e+14 (-1.413e+16, 1.245e+16)
p21 = -0.2157 (-8.185, 7.754)
p12 = -0.6978 (-12.13, 10.73)
p03 = -0.3148 (-7.039, 6.409)
Is there anything I haven't yet tried out or any mistakes I made? Anything at all would be helpful. Thank you.

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