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# Linear regression operator - 'greedy' feature selection option

Member, University Professor Posts: 49 Maven
Can someone please help with more details on how the "greedy" feature selection in Linear Regression operator works?

In the Optimize Selection operator, the two greedy algorithms (forward selection and backward elimination) are clearly specified. However, in the case of the Linear Regression operator, it is not yet clear which algorithm/approach is used in case of the built-in "greedy" feature selection dropdown option for the Linear Regression operator.

I could not find any elaboration on this in the documentation for Linear Regression operator. Also, I checked the source code for this option. According to it -
This class implements an internal forward selection for the linear regression. It uses the Akaike Criterion that is maximized roundwise. Each round the attribute minimizing the akaike criterion is deselected.

I am trying to understand the exact model selection process happening here and the description and the code is a bit unclear to me.

1. The above statement says internal forward selection. But the later sentence "..attribute minimizing the akaike criterion is deselected." suggests backward elimination. Which one is it? Can someone elaborate on this?

2. How is AIC criterion computed here? A couple of sources (link1 and link2) suggest AIC for linear regression as n x ln(SSE / n) + 2 (k + 1) where n is the number of observations and k is the number of predictors (one is added for the intercept term). The source code states -

`akaike = (numberOfExamples - numberOfUsedAttributes) + 2 * numberOfUsedAttributes;`
This is a bit confusing as well. Any insights would be appreciated.

## Answers

• Member, University Professor Posts: 391 Unicorn
edited December 2020
When you peek into the operator's source code (yes everybody can do so), you will find the following (slightly cryptic) comment: "This class implements an internal forward selection for the linear regression. It uses the Akaike Criterion that is maximized roundwise. Each round the attribute minimizing the akaike criterion is deselected."
• Member, University Professor Posts: 391 Unicorn
edited December 2020
What it means is that it maximises AIC, it considers all attributes as candidates for addition and removes one candidate at a time, based on the lowest overall AIC when they are added (sort of as a test), iteratively! AIC is a relative criterion, so it can only be used to compare one model vs the other - so we will not know AIC of the model with an added attribute until it is added!
• Member Posts: 1 Learner I
The model with the smallest Akaike Information Criterian value should be selected.
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