def METHOD_NAME(self):
    """Calculating monitored value used for early stopping on validation set (`val_set`).
    This function will be called by `early_stop()` function.
    Returns
    -------
    res : float
        Monitored value on validation set.
        Return `None` if `val_set` is `None`.
    """
    if self.val_set is None:
        return None
    from ...metrics import NDCG
    from ...eval_methods import ranking_eval
    ndcg_100 = ranking_eval(
        model=self,
        metrics=[NDCG(k=100)],
        train_set=self.train_set,
        test_set=self.val_set,
    )[0][0]
    return ndcg_100