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Predict Out-of-Sample Responses of Subtrees

This example hows how to predict out-of-sample responses of regression trees, and then plot the results.

Load the carsmall data set. Consider Weight as a predictor of the response MPG.

load carsmall
idxNaN = isnan(MPG + Weight);
X = Weight(~idxNaN);
Y = MPG(~idxNaN);
n = numel(X);

Partition the data into training (50%) and validation (50%) sets.

rng(1) % For reproducibility
idxTrn = false(n,1);
idxTrn(randsample(n,round(0.5*n))) = true; % Training set logical indices
idxVal = idxTrn == false;                  % Validation set logical indices

Grow a regression tree using the training observations.

Mdl = fitrtree(X(idxTrn),Y(idxTrn));
view(Mdl,'Mode','graph')

Figure Regression tree viewer contains an axes and other objects of type uimenu, uicontrol. The axes contains 30 objects of type line, text.

Compute fitted values of the validation observations for each of several subtrees.

m = max(Mdl.PruneList);
pruneLevels = 0:2:m; % Pruning levels to consider
z = numel(pruneLevels);
Yfit = predict(Mdl,X(idxVal),'SubTrees',pruneLevels);

Yfit is an n-by- z matrix of fitted values in which the rows correspond to observations and the columns correspond to a subtree.

Plot Yfit and Y against X.

figure;
sortDat = sortrows([X(idxVal) Y(idxVal) Yfit],1); % Sort all data with respect to X
plot(sortDat(:,1),sortDat(:,2),'*');
hold on;
plot(repmat(sortDat(:,1),1,size(Yfit,2)),sortDat(:,3:end));
lev = cellstr(num2str((pruneLevels)','Level %d MPG'));
legend(['Observed MPG'; lev])
title 'Out-of-Sample Predictions'
xlabel 'Weight (lbs)';
ylabel 'MPG';
h = findobj(gcf);
axis tight;
set(h(4:end),'LineWidth',3) % Widen all lines

Figure contains an axes. The axes with title Out-of-Sample Predictions contains 6 objects of type line. These objects represent Observed MPG, Level 0 MPG, Level 2 MPG, Level 4 MPG, Level 6 MPG, Level 8 MPG.

The values of Yfit for lower pruning levels tend to follow the data more closely than higher levels. Higher pruning levels tend to be flat for large X intervals.

See Also

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