# Problems with implementing cross-validation for OLS, PLSR and PCR

I am new to regression methods. I am creating Multiple Linear Regression, Partial Least Squares Regression and Principal Component Regression models for my dataset, and I am a bit confused with the results.

I have 60 predictors (or features) in my data, and 71 observations. Based on what I haver read (and understood), PLSR should give the best results. PCR should give results that are close to PLSR, and MLR is the most simplistic and which should give the worst results. Is my conclusion correct?

Because in my dataset, MLR and PLSR work very very similarly whereas PCR gives a much better prediction result. I have done a comparison of the methods, watching how the RMSE of the cross-validated methods changes with the number of features introduced for the MLR and PLSR methods, and with the number of PCs used for the PCR method. While MLR and PLSR suffer from overfitting very early, PCR keeps improving its prediction accuracy while the number of PCs is increased.

This is what I am doing to compute PCR fit and prediction errors (Matlab code):

PCALoadings_all=PCALoadings;
PCAScores_all=PCAScores;

for ik=1:length(PCAVar) %add PCs one by one

PCAScores= PCAScores_all(:,1:ik);

%Function 'regress' needs a constant column so add it
PCAScores = [PCAScores, ones(length(PCAScores(:,1)),1)];
[betaPCR,~,~,~,statsPCR]  = regress(labels-mean(labels), PCAScores); %Create regression model

betaPCR=betaPCR(1:end-1,:); %remove constant effect

size_PCR= find(betaPCR~=0, 1, 'last'); %find 0-value coefficients

%To make the PCR results easier to interpret in terms of the original
%spectral data, transform to regression coefficients for the original, uncentered variables.

%Create model
betaPCR_fit = [mean(labels) - mean(norm_feature_matrix)*betaPCR; betaPCR];
yfitPCR = [ones(d1,1) norm_feature_matrix]*betaPCR_fit; %compute fitted labels

%Cross-validate
predictMat=[];
for test_idx=1:length(labels)

testix = test_idx; %One value to test
trainix = setdiff(1:length(labels),testix); %All the other values to train

%Feature matrix and labels to train
X=norm_feature_matrix(trainix,:);
y=labels(trainix);

%Feature matrix and labels to test
X_test=norm_feature_matrix(testix,:);
y_test=labels(testix);

%compute beta with training data
betaPCR_CV = [mean(y) - mean(X)*betaPCR; betaPCR];
%prediction with test data
yCV_PCR = [1 X_test]*betaPCR_CV;

predictrow = [y_test, yCV_PCR]; % actual vs predicted
predictMat = [predictMat; predictrow];

end

RMSE_fit_PCR(ik) = sqrt(mean((yfitPCR - labels).^2)); %Compute RMSE for regression fit
RMSE_prediction_PCR(ik) = sqrt(mean((predictMat(:,1) - predictMat(:,2)).^2)); %Compute RMSE for prediction
%plot regressions

end


Do you think I'm doing something wrong? Did I misunderstood something?

• @Xian, I am wondering why you tagged this Q with [self-study]? This does not look like a "routine question from a textbook" to me. Nov 3, 2015 at 11:56
• How many components do you use in the PLS regression to make the second subplot? Nov 3, 2015 at 12:30
• A varying number, from 1 to 60, as I do in the other two models... Nov 3, 2015 at 12:34
• I suspect for PCR that you have some mistake in your code, such that the prediction error actually is the fit error once again. Nov 3, 2015 at 13:09
• Looking briefly at your code, it looks very wrong, as @Soren suspected. You seem to be computing PCA on the whole dataset and then when you do cross-validation loop you use the same betaPCR for each iteration. This is not how it's supposed to work. You should be redoing PCA for each cross-validation iteration, i.e. for each training set you do the PCA and regression and then apply the resulting PCR model to the test set. Next iteration - next model fit. Etc. Nov 3, 2015 at 15:14

Ok, I think I've got it...

These are my new curves (which I think are much more logical):

In fact, there were several mistakes in my code:

1.-In the PLSR part, I was changing the number of features instead of the number of PCs in each iteration. Nonsense.

2.-My Cross-Validation process in all of them was wrong. I was implicitly using the values to test when creating the Cross-Validation model, as @amoeba remarked.

The PCR code is now like this:

  for ik=1:length(PCAVar) %add PCs one by one

PCAScores= PCAScores_all(:,1:ik);

%Function 'regress' needs a constant column so add it
PCAScores = [PCAScores, ones(length(PCAScores(:,1)),1)];
[betaPCR,~,~,~,statsPCR]  = regress(labels-mean(labels), PCAScores); %Create regression model

betaPCR=betaPCR(1:end-1,:); %remove constant effect

size_PCR= find(betaPCR~=0, 1, 'last'); %find 0-value coefficients

%To make the PCR results easier to interpret in terms of the original
%spectral data, transform to regression coefficients for the original, uncentered variables.

%Create model
betaPCR_fit = [mean(labels) - mean(norm_feature_matrix)*betaPCR; betaPCR];
yfitPCR = [ones(d1,1) norm_feature_matrix]*betaPCR_fit; %compute fitted labels
RMSE_fit_PCR(ik) = sqrt(mean((yfitPCR - labels).^2)); %Compute RMSE for regression fit
end

%Cross-validate
predictMat=[];
error_pcnum=[];
for test_idx=1:length(labels)

testix = test_idx; %One value to test
trainix = setdiff(1:length(labels),testix); %All the other values to train

%Feature matrix and labels to train
X=norm_feature_matrix(trainix,:);
y=labels(trainix);

%Feature matrix and labels to test
X_test=norm_feature_matrix(testix,:);
y_test=labels(testix);

predictrow_pcnum=[];
predictMat_pcnum=[];
for pcnum = 1:length(PCAVar)
PCAScores= PCAScores_all(:,1:pcnum);
[betaPCR,~,~,~,~]  = regress(y-mean(y), PCAScores); %Create regression model

%compute beta with training data
betaPCR_CV = [mean(y) - mean(X)*betaPCR; betaPCR];
%prediction with test data
yCV_PCR = [1 X_test]*betaPCR_CV;

predictrow_pcnum = [y_test, yCV_PCR]; % actual vs predicted
predictMat_pcnum = [predictMat_pcnum; predictrow_pcnum];
end
error_pcnum =[error_pcnum, (predictMat_pcnum(:,1) - predictMat_pcnum(:,2))];
predictMat = [predictMat; predictMat_pcnum'];
end
error_mean_bypcs=mean(error_pcnum.^2,2);
RMSE_prediction_PCR = sqrt(error_mean_bypcs); %Compute RMSE for prediction
[~,min_RMSE_nfeat]=min(RMSE_prediction_PCR); %find the minimum RMSE

%RMSE_prediction_PCR(ik) = sqrt(mean((predictMat(:,1) - predictMat(:,2)).^2)); %Compute RMSE for prediction
%plot regressions

predictMat=predictMat(:,min_RMSE_nfeat);
predictMat = reshape(predictMat,[2,length(predictMat)/2])';
with_vs_without_CV(labels, yfitPCR, predictMat, 3, sup_id);
legend(strcat(num2str(min_RMSE_nfeat) ,' PCs used in the PCR model'),'Location','southeast')


Thank you to everyone for your collaboration!

(And if you think I am still wrong, please, let me know...)

• I haven't verified your code, but the figure looks definitely much more reasonable now. Nov 3, 2015 at 21:33