Trading Strategies, Portfolio Monitoring, and Rebalancing

Trading strategies translate goals and constraints of asset management into dynamic, intertemporal, and coherent portfolio decisions. Under special assumptions, myopic portfolio policies are shown to be optimal and constant over time. In general, however, both optimal theoretical portfolios and current portfolio positions are subject to random movements so that periodic monitoring and rebalancing are necessary. Transaction and monitoring costs create a trade-off between the cost of not being at the optimal allocation (tracking error) and the cost of swapping the current portfolio for the optimal one. Optimal rebalancing results in the replacement of the optimal allocation with a no-trade region delimited by rebalance boundaries. The factors influencing the boundaries and the rebalancing decisions can be analytically and numerically explained. Popular rebalancing rules imply a substantial amount of excess trading costs, but they can generate positive net returns in the case of mean-reverting market regimes.