Binary cross-entropy is used as a training loss for click and purchase predictions in search. These losses measure model performance, but they do not necessarily capture the business outcome the system needs. section 5.3.2
When click or purchase data is imbalanced, focal loss is an alternative: it down-weights easy examples so training focuses more on harder ones. Keep the loss in context with other measures; a classification loss may not reflect whether users are happy or spam is controlled appropriately. section 5.3.2 section 9.1.3
Search predictions
Train on clicks and purchases with binary cross-entropy, then assess outcomes with measures such as click-through rate and conversion rate. section 5.3.2


















