本文主要是藉由重新定義"reported cost"，研究台灣銀行業的成本效率。呆帳提列(provision for loan loss, PLL)是銀行最大的應計項目，可能被提前或遞延認列，甚至因為政府的寬容政策或管制改變等因素而產生大幅波動，進一步地導致總成本也大幅波動，使得過往銀行業成本效率研究中的總成本（本文稱之為"reported cost"）有錯估的可能。本文提出了一個儘可能降低reported PLL波動的方法，讓其在當期認列，成為符合經濟意涵的呆帳提列，本文稱之為"economic PLL''，進而求得當期真正的總成本（本文稱之為"economic Cost"）。實證結果發現，相較於reported cost, economic cost不但高出許多，且利用economic cost所求得的效率水準也明顯不同，顯示呆帳提列的大幅波動，確實會使成本效率的評估產生偏誤。舉例來說，彰化銀行的reported cost為19, 107百萬元，economic cost卻高達41, 389百萬元，使用。economic Cost後的效率排名更由第5名大幅下降到第22名。最後，我們進行規模經濟的評估，以找到銀行最恰當的規模，作為未來改善成本效率的基準。

This paper investigates the cost efficiency of Taiwan's banks by redefining the "reported cost". We argue that because the reported PLL is an accrual item, it fluctuates significantly over time, making the cost also fluctuate. Furthermore, the provision is often decided in a discretionary manner, which is affected by the forbearance policy, sudden regulation changes and more. As a result, reported cost is notorious for its discretionary characteristics. We propose an approach to reduce the fluctuation of reported PLL to the minimum and at the same time fulfill the economic meaning of PLL. That is, we suggest an "economic PLL" to replace the reported PLL to be included in the cost. We found that economic PLL differs substantially from reported PLL. For example, the cost of Chang-Hwa Bank has risen steadily from 19,107 million New Taiwan dollars to 41,389 million New Taiwan dollars, and cost efficiency has degraded from rank 5th to rank 22 th. Results are also consistent with our intuition that the improper estimation of reported cost because of PLL discretion may cause the bias of cost efficiency. Finally, we further specify optimal scale economic as the basis of improving cost efficiency in the future.