在同時考慮基金過往之報酬率與波動度之狀況下，我們發展出定期定額持有期間報酬率的可能變動區間，並且發現當定期定額持有期間增加，所推導出之預期報酬率變異數雖得以降低，但仍會隨持有期間拉長而不斷累積，顯示定期定額策略之跨時分散風險能力有限，不適合作為長期之投資策略。 我們進一步分析不同類型之基金，是否符合這樣的特性，以及跨類基金在持有期間內於可能的變動區間內的績效表現。研究發現，在所觀察之基金類型中，超過六成以上之基金類型並不適合作為長期定期定額之投資標的，尤其是以成熟國家為主之股票、債券或全球平衡型類別；反觀波動度較大之新興市場股票型基金，如印度股票、拉丁美洲股票等，其報酬率表現有較高之機率優於預期平均報酬率，且其報酬貢獻度多來自於上方風險。 研究結果發現，即便是定期定額投資策略，基金類型的選擇仍扮演重要之角色，投資人甚至可以在各類型中更深入分析每一檔基金之報酬率貢獻來源，挑選出更具有定期定額投資價值之基金。 ;We derived the possible range for dollar-cost average (DCA) holding period return, by considering mutual funds’ historical return and standard deviation at the same time. The volatility of mutual funds indeed diversified as the holding period getting longer, but the risk still accumulate over time. It shows that the time diversification ability of DCA strategy is relatively limited, and gives a thought that this strategy is actually not suitable for long term investment. We go further to investigate whether this characteristic happens in different fund categories. Also, we would like to see what kind of categories may provide better risk and return performance within the possible range we constructed at the very beginning. The research shows that over 60% of the categories under observation is not a good choice for long term investment, especially the developed country stocks and bonds. In the contrary, some categories such as developing country stocks, still can provide higher probability of better DCA performance while having higher volatility. The outcomes of this research reveal that “category selection” is still important when investing by using DCA strategy. Investors who want to have a more precise single fund selection for DCA investment, should also put emphasis on the contribution of return that is above the expected average return.