第三級差別定價過往文獻的重點在於指出能提升福利的條件，以支持其正當性。本文則試圖以另種角度說明，即使第三級差別定價在某些參數區間內會損及經濟福利，也不表示其正當性就該受到質疑。如果廠商的競爭模式為數量競爭，相對於差別定價，均一定價不再是劣勢策略。當廠商採取均一定價時，必須加總所有市場的需求為單一的總需求，會使市場競爭產生結構性的改變，導致廠商的決策範圍不同於差別定價；同時，市場加總後，廠商可創造壟斷效果，將壟斷力延伸到不具獨占力的市場，使得均一定價有可能成為廠商極大化利潤下的選擇。如果廠商在各個參數區間內基於自利動機下的選擇，無論是差別定價或均一定價，相對於另種選擇，都不利於經濟福利，則差別定價就不見得不具正當性。

In the vast literature of third-degree price discrimination, one focal point of analysis has been on the condition for it to improve social welfare. This research argues that even if price discrimination reduces welfare in some parameter ranges, its validity may still be defensible. When oligopoly firms engage in quantity competition, then relative to price discrimination, uniform pricing is no longer a dominated strategy, because when a firm employs uniform pricing, it aggregates its demand in two separate markets so as to choose its optimal uniform price. Thus, it is able to extend its monopoly power from the market where it is a monopoly to the other market where it faces Cournot competition. Consequently, uniform pricing may be preferred over price discrimination. If a profit-maximizing firm’s choice, whether it is price discrimination or uniform pricing, always results in lower social welfare than the other pricing strategy that is not chosen, then price discrimination should not be less defensible than uniform pricing.