I'm trying to understand how the base value is calculated. So I used an example from SHAP's github notebook, Census income classification with LightGBM.
Right after I trained the lightgbm model, I applied
explainer.shap_values() on each row of the test set individually. By using
force_plot(), it yields the base value, model output value, and the contributions of features, as shown below:
My understanding is that the base value is derived when the model has no features. But how is it actually calculated in SHAP?