對於與國計民生相關的產業，因需法律特許、具有規模經濟或進入門檻，僅 能容納少數廠商生存，使得這些廠商具有高度的市場支配力。政府基於經濟公 平、社會福利與政策推行的考量，不可能完全放任廠商完全的自由競爭，必要時 有可能會直接採用適當的管制措施以規範廠商的行為，或間接利用公營事業作為 操弄民營廠商行為的策略性工具。
本文觀察國內油品市場的亂象，以台灣中油與台塑石化為例，探討受管制產 業中同時存在公、民營廠商的混合雙占(mixed duopoly)市場結構中，公、民營廠 商進行數量競爭(C)、以公營廠商為領導者的 Stackelberg 競爭(SS)、以民營廠商 為領導者的 Stackelberg 競爭(SP)、價格競爭(B)與公、民營廠商價格勾結(CO)等 不同競爭模式，所得到的產量、利潤、利潤差距與社會福利結果，並就其結果分 別對公營廠商的效率與市場規模進行比較靜態分析與排列這些結果的次序。
本文發現在混合雙占市場中，如政府把公營廠商視為控制民營廠商競爭行為 的政策工具，將消費者效用與公營廠商利潤總和之最大化設定為公營廠商的經營 目標，即使公營廠商較民營廠商無效率，民營廠商也不一定能在所有的競爭模式 中獲得利潤優勢。在 C 與 SS 競爭模式下，只要公營廠商的效率沒有低於民營廠 商太多，公營廠商也能比民營廠商獲得更多的利潤。
透過比較靜態分析，本文也發現當公營廠商提升效率後，在所有競爭模式下 公營廠商的利潤均會上升，公、民營廠商的利潤差距縮小；但 C 與 SS 競爭模式 會讓民營廠商利潤下降，社會福利上升；SP 競爭模式會讓民營廠商利潤與社會 福利同時下降；而 B 與 CO 競爭模式使得民營廠商利潤上升，社會福利下降。如 市場規模擴大，所有競爭模式均會使得公、民營廠商利潤上升，社會福利上升；
惟 C 與 SS 競爭模式會讓利潤差距縮小，SP、B 與 CO 競爭模式使得利潤差距擴 大。
對於扮演管制者的政府而言，如以最大化社會福利作為政策目標，可採 SP 競爭模式，還可兼顧較低市場價格，但因公營廠商利潤最低，將會降低政府的財 政收入。如政府以低價供應受管制商品作為政策目標，可採 SS 競爭模式，但會 造成商品總供應量及社會福利均處於偏低的水準。如政府以充分供應受管制商品 數量作為施政目標，可採 CO 模式，雖可兼顧較高的財政收入，但將使得社會福 利最低，且商品的市場價格偏高。如政府純以財政收入為考量，可採 C 競爭模 式，但會使得受管制商品的市場總供應量最低，且市場價格高昻。最後，政府如 採 B 競爭模式，將會造成受管制商品市場價格兩極化的結果，且因民營廠商的 利潤最高，可能會招致「圖利特定廠商」的批評。
由以上的討論結果可知，各種競爭模式均有其利弊得失，政府在混合雙占的 市場結構中，無法對受管制商品有效的尋找出一個兼顧社會福利、消費者價格期 待與廠商利潤的最佳競爭模式。面對生產同質性商品之受管制產業，政府應採行 何種競爭模式，只能依政治理念睿智抉擇了。
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網站部份：
月平均價格預測：http://tonto.eia.doe.gov/ftproot/steo/may07_base.xls 公開資訊觀測站：http://newmops.tse.com.tw/
台塑石化股份有限公司：http://www.fpcc.com.tw/default.asp 台經院產經資料庫服務系統：
http://tie.tier.org.tw/tie/index.jsp?data_base_id=DB001
台灣中油股份有限公司：http://www.cpc.com.tw/big5/home/index.asp 經濟部能源局：http://www.moeaboe.gov.tw/index.asp?group=b
經濟部能源局—油價資訊管理與分析系統：http://210.69.152.10/oil102/
Energy Information Administration(EIA): http://www.eia.doe.gov/
WTI年價格預測：http://www.eia.doe.gov/oiaf/aeo/excel/aeotab_12.xls
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