因應未來退休基金自選的優化，本文重新檢視共同基金以定期定額投資策略的績效與改善。在同時考慮持有期間風險與報酬的角度下，我們以理論結果證實定期定額的跨時風險分散效果有限，持有期間理論期望報酬率與風險變異數會隨著扣款期數的增加而遞增，因此定期定額並不適合長期持有。據此，我們進而以持有期間報酬率可能變動區間做為擬訂定期定額擇時調整加減碼的策略，結合再平衡的概念來優化定期定額長期累積報酬。實證結果發現，融合再平衡的定期定額策略在長期累積中確實有擇時優化的功能，不僅總部位累積報酬較為穩定(波動風險降低)，且能減少其負報酬發生的頻率；其中，擇時出場仍是影響整體績效表現的關鍵。

In order to respond to the coming new policy and manage individual's self-selected investment portfolio in the defined contribution pension plan, we re-examine the performance and potential improvement of the famous Dollar-Cost-Average (DCA) strategy in mutual fund management. By considering both the risk and the return of the holding-period-return at the same time, we show theoretically that the effect of time diversification from implementing DCA is limited since both the expected holding period return as well as its risk, in terms of volatility, will increase with the holding periods, i.e., the deduction numbers. Therefore, from the perspective of risk, DCA along is not a proper investment strategy for long-term holding. We accordingly suggest to use the possible range of future holding period return and formulate a new timing and rebalancing strategy, on top of the DCA, to optimize the long-term cumulative return via unwinding/reloading to an underlying fund when the holding period return hit the upper/lower bounds. Our empirical results indicate that the proposed strategy can indeed lead to relatively stable cumulative return and reduce the occurrence of realized negative return. The additional timing and rebalancing scheme is effective and as a key to enhance the overall investment performance over the traditional DCA approach.

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