在現實社會中，產品品質的選擇及研發，一直是廠商很重視的課題。而本文藉由一個簡化的雙占價格競爭模型，來討論市場規模、R&D額外成本對多產品廠商R&D決策的影響，以及後續與均衡產量及利潤的關係。我們發現在獨占市場下，當市場規模越高、產品生產成本和產品品質越低時，相對於低品質產品，廠商將投入較多的製程研發於高品質產品。這也會增加高品質產品的產量。我們另外考慮由一個生產兩樣產品的廠商與另一個生產單一產品的廠商之間的雙占競爭，除了廠商內的競食效應，還多了廠商間的競爭效果，所以兩家廠商的R&D策略會有所改變，並接續影響到其均衡產量及利潤。相對於獨占廠商的結果，我們發現當多產品廠商面臨另一家廠商的競爭時，其兩個產品的R&D投入大小在某些參數值下將發生逆轉。

Product quality choices and innovations have always been important issues for firms. This essay constructs a simple price-competition duopoly model to study the effects of market size and R&D costs on multi-product firms’ process R&D decisions, as well as, the corresponding equilibrium quantities and profits. We find that under monopoly, the R&D investment on the high-quality product is more likely to be larger than the low quality product, the larger the market size, the lower the R&D cost, or the lower the quality parameter. We also consider a duopolistic competition between a firm producing two products (multi-product firm) and a single-product firm. In addition to cannibalization effect between two products produced by the multiproduct firm, there is also the competition effect from the single-product firm. This changes the firms’ R&D decisions and corresponding output levels and profits. We find that under certain parameter values, the results obtained in the case of monopoly reverse.

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