一支典型的股票型基金在其投資組合中包含多檔股票,雖然基金具有分散風險的效果,但過度的多角化會使潛在的成本增加,影響基金績效。每支基金均有無形的監督成本、管理成本、人力成本及有形的交易成本,若股票型基金的持有股票數目太多,反而會使成本上升,投信公司會將多付出的成本移轉至投資人身上。本研究探討股票基金「最適」持有股票數目應為多少,才能真正且有效降低投資組合風險,藉由建立共同基金最適股票數目模型來找出最適持有股票數目,利用多元迴歸之分析方法,透過一階微分以達績效最大下之最適持有股票數目。實證的結果發現,國內股票型基金以夏普指標為應數下,最適持有股票數目為39支股票,而以崔納指標為應變數下,最適持有股票數目為35支股票。另外,本研究也透過隨機取樣來進行相同研究步驟,研究結果為最適持有股票數目為41支股票,故本研究與國內股票型基金經理人平均持有股票數目為38支到45支股票相去不遠,最適持有股票數目約在35支到41支股票之間,能形成基金績效最大的投資組合。
A stock mutual fund consists many kinds of stocks. Although a well-diversified portfolio can reduce risk, however, overdiversification could bring some potential costs. This study posits that there is an optimal number of stocks for each mutual fund, reflecting the trade-off between diversification benefits versus transaction and monitoring costs. This study finds a significant quadratic relation between number of stock holdings and risk-adjusted returns for Taiwan equity mutual fund portfolios during 1995-2007. Regression analysis shows that the optimal number of stock holdings is from 35 to 41.