SHAP Baselines for Explainability

Explanations are typically contrastive (that is, they account for deviations from a baseline). As a result, for the same model prediction, you can expect to get different explanations with respect to different baselines. Therefore, your choice of a baseline is crucial. In an ML context, the baseline corresponds to a hypothetical instance that can be either uninformative or informative. During the computation of Shapley values, SageMaker Clarify generates several new instances between the baseline and the given instance, in which the absence of a feature, is modeled by setting the feature value to that of the baseline and the presence of a feature is modeled by setting the feature value to that of the given instance. Thus, the absence of all features corresponds to the baseline and the presence of all features corresponds to the given instance.

How can you choose good baselines? Often it is desirable to select a baseline with very low information content. For example, you can construct an average instance from the training dataset by taking either the median or average for numerical features and the mode for categorical features. For the college admissions example, you might be interested in explaining why a particular applicant was accepted as compared to a baseline acceptances based on an average applicant. If not provided, a baseline is calculated automatically by SageMaker Clarify using K-means or K-prototypes in the input dataset.

Alternatively, you can choose to generate explanations with respect to informative baselines. For the college admissions scenario, you might want to explain why a particular applicant was rejected when compared with other applicants from similar demographic backgrounds. In this case, you can choose a baseline that represents the applicants of interest, namely those from a similar demographic background. Thus, you can use informative baselines to concentrate the analysis on the specific aspects of a particular model prediction. You can isolate the features for assessment by setting demographic attributes and other features that you can't act on to the same value as in the given instance.