How can I turn new dummy variables into the same dummy variables I used to create a model?

Sarah asked: 2022-06-13 dummyvar , prediction , forecasting , AI, Data Science, and Statistics , Statistics and Machine Lea

Learn how to transform new data into the same dummy variables used in your original MATLAB model. Ensure consistency & accuracy for reliable predictions. Get th

Expert Answer

Prashant Kumar answered . 2025-07-30 21:32:18

There is no direct way to tell dummyvar to use a previously created dummyvar predictor matrix as a reference. So, in this case you would need to dummy-code each categorical variable separately and then do the same for the test vector. The following example should explain the workflow you would need to follow:

Say you have two categorical variables: Month, with categories, 1 to 12 and LeapYear with categories 0 and 1.

In this case, you would need to use the following code:

Monthcat = categorical(Month);
LeapYearcat = categorical(LeapYear);

dumMonth = dummyvar(Monthcat);
dumLeap = dummyvar(LeapYearcat);

dumPredictorMat = [dumMonth dumLeap]; % This would be the input to train your model

% Test inputs
MonthTest = 4;
LeapYearTest = 1;

% Separately dummy coding these inputs

MonthTestcat = categorical(MonthTest,1:12);
LeapTestcat = categorical(LeapYearTest,0:1);

%%Dummy coding them individually before using the model to predict

dumMonthTest = dummyvar(MonthTestcat);
dumLeapTest = dummyvar(LeapTestcat);

dummyTestMat = [dumMonthTest dumLeapTest];

Now you should be able to use the dummyTestMat as an input to the predict function of the model to generate the corresponding predicted responses.

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