# Why Use Weight of Evidence?

I had been asked why I spent so much effort on developing SAS macros and R functions to do monotonic binning for the WoE transformation, given the availability of other cutting-edge data mining algorithms that will automatically generate the prediction with whatever predictors fed in the model. Nonetheless, what really distinguishes a good modeler from the rest is how to handle challenging data issues before feeding data in the model, including missing values, outliers, linearity, and predictability, in a scalable way that can be rolled out to hundreds or even thousands of potential model drivers in the production environment.

The WoE transformation through monotonic binning provides a convenient way to address each of aforementioned concerns.

1. Because WoE is a piecewise transformation based on the data discretization, all missing values would fall into a standalone category either by itself or to be combined with the neighbor that shares a similar event probability. As a result, the special treatment for missing values is not necessary.

2. After the monotonic binning of each variable, since the WoE value for each bin is a projection from the predictor into the response that is defined by the log ratio between event and non-event distributions, any raw value of the predictor doesn’t matter anymore and therefore the issue related to outliers would disappear.

3. While many modelers would like to use log or power transformations to achieve a good linear relationship between the predictor and log odds of the response, which is heuristic at best with no guarantee for the good outcome, the WoE transformation is strictly linear with respect to log odds of the response with the unity correlation. It is also worth mentioning that a numeric variable and its strictly monotone functions should converge to the same monotonic WoE transformation.

4. At last, because the WoE is defined as the log ratio between event and non-event distributions, it is indicative of the separation between cases with Y = 0 and cases with Y = 1. As the weighted sum of WoE values with the weight being the difference in event and non-event distributions, the IV (Information Value) is an important statistic commonly used to measure the predictor importance.

Below is a simple example showing how to use WoE transformations in the estimation of a logistic regression.

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