多品質廠商產品差異化之分析

Abstract

垂直商品差異化的文獻多假定每個廠商提供單一品質的產品。然而，實際上我們可看到個產業中，每個廠商通常同時生產多種不同等級的產品。因此，本篇論文的目的為探討當廠商可提供多於一種品質的產品時，多品質廠商間產品差異化，與品質競爭的特性。 在第二章中，兩個廠商進行兩階段賽局，第一階段中，兩個廠商同時選擇其產品個數（最多二個），與各個產品的品質；第二階段中，兩個廠商開始進行數量競爭。我們首先發現，當均衡為獨占時，獨占廠商必定只提供一種品質的產品。其原因為引入第一個品質的利潤遠高於次引入的品質所帶來的利潤，因此若獨占廠商有誘因引入第二個品質，則另一廠商必會進入市場。此外，我們也證明，當均衡結果為雙占時，兩廠商會生產多種不同品質的產品，且兩廠商提供產品的品質彼此有交錯。這個不對稱的均衡結果與假定增加產品種類不需成本時所得到的對稱均衡相當不同。在此不對稱的均衡中，我們可發現高品質市場裡的品質差異化幅度小於低品質市場；且相鄰產品若是由同一廠商提供，其品質差異化幅度大於由不同廠商提供的情況。 在第三章中，兩個廠商進行兩階段賽局，第一階段中，兩個廠商同時選擇其產品個數（最多二個），與產品的品質；第二階段中，兩個廠商進行價格競爭。與第二章結果相同的是，獨占廠商必不會提供多種不同品質的產品。然而，當均衡結果為雙占時，兩廠商會生產多種不同品質的產品，且兩廠商提供產品的品質完全不交錯，一廠商所生產品質皆高於另一廠商生產之品質。同時，我們發現相鄰產品若是由同一廠商提供，其品質差異化幅度小於由不同廠商提供的情況。此外，我們發現當每單位產出的成本與品質無關，僅每個產品的固定成本與產品品質相關，雙占廠商則不會提供多於一種不同品質的產品。因廠商投入一固定成本後，之後每單位產出與產品品質無關，所以廠商較無誘因再投入固定成本生產一個低品質產品。 在第四章中，我們分析一個兩階段競爭。面對一群品質願付價格異質的消費者，廠商可提供多種品質相近的產品。模型中，產品的個數、品質、價格與市場大小皆為內生決定。結果發現，當與品質相關的單位成本為品質的遞增函數時，各廠商會提供一個連續的品質區段。也就是每個廠商會提供多種不同的品質。這個均衡結果與當市場大小為外生給定時，每個廠商只會提供一個單一品質的均衡結果有很大的不同。此外，相較於每個廠商只能提供單一品質，我們也發現：若允許廠商提供多種不同品質，廠商競爭更激烈、利潤下降，而消費者剩餘與社會福利增加。 第五章總結，並提供本研究之延伸議題。

The literature on vertical product differentiation often makes the assumption that any one firm can produce only one product. Although this is analytically convenient, in reality, firms normally produce and sell multiple differentiated products. Therefore, the purpose of this dissertation is to investigate the product differentiation between firms as each of them may produce multiple quality-differentiated products. Chapter 2 proposes a Cournot model of two-stage competition to examine the characteristics of vertical product differentiation in a multiproduct duopoly. Firms choose the number of products/varieties (with a maximum of two) and their qualities, and then compete in quantities. We first show that, in the equilibrium of a natural monopoly, the monopolist must sell a single product. Moreover, in the equilibrium of a multiproduct duopoly, there are asymmetric equilibrium outcomes with non-segmented patterns of differentiation. They are in sharp contrast with the symmetric outcomes when increasing the number of products is assumed to be costless. We also find that the product differentiation in the high-quality market is smaller than that in the low-quality one, and the product differentiation within a firm is greater than that between firms. In Chapter 3, we characterize the equilibrium of a two-stage game where firms simultaneously choose the number of varieties (with a maximum of two) and associated qualities at the first stage, and compete in prices at the second stage. The result also shows that a natural monopolist never sells multiple varieties. However, in the equilibrium of a multiproduct duopoly, there are always segmented patterns of vertical differentiation. Sandwich, enclosure and interlacing configuration never emerge, because each firm has incentive to reduce the number of products facing head-to-head competition with the rival, in order to avoid keen price competition. The result also shows that the product differentiation of neighboring varieties between firms is greater than that within a firm. In addition, we find that the emergence of multi-quality equilibrium crucially depends on the nature of costs of quality. While the cost of quality falls on the fixed production costs (rather than the variable costs), each firm always produces a single quality of products. In Chapter 4, we investigate a two-stage competition in a vertical differentiated industry, where each firm produces an arbitrary number of similar qualities and sells them to heterogeneous consumers. The number of products, qualities, prices, and the extent of the market coverage are endogenously determined. We show that, when unit costs of quality improvement are increasing and quadratic, each firm has an incentive to provide an interval of qualities. The finding sharply contrasts to the single-quality outcome when the market coverage is exogenously determined. We also show that allowing for an interval of qualities intensifies the level of competition, lowers the profit of each firm, and raises the consumer surplus and the social welfare in comparison to the single-quality duopoly. Chapter 5 concludes and provides some extensions of this research.