“…A standard algorithm for predicting with expert advice is the Aggregating Algorithm (AA) proposed in Vovk (1990Vovk ( , 1998; this algorithm generalizes, e.g., the Bayesian merging scheme (DeSantis, Markowsky, & Wegman, 1988), the Weighted Majority Algorithm (Littlestone & Warmuth, 1994;Vovk, 1992), and Cover's (Cover & Ordentlich, 1996) universal portfolio algorithm. The AA works well in many situations; it was felt, however, that for the important problem of tracking the best expert (Littlestone & Warmuth, 1994;Herbster & Warmuth, 1995;Freund et al, 1997) the AA was not sufficient, and modifications of the AA or entirely different algorithms were used.…”