在寡占理论下，推测变差认为一家公司能够感知它对手会因它改变产量或价格作出反应。公司会根据另一个公司产量的变差作出推测，并改变它自己的产量。例如，在经典古诺模型的寡占情形中，假设每个公司都知道其竞争对手的产量，从而选择自己的产量。有时这也被称为 “纳什推测（Nash conjecture）” ，因为它是标准纳什均衡（Nash equilibrium）概念的基础。然而，也可以有替代假设。设想有两家公司生产相同的产品，那么市场价格是由两家公司产量的总和所决定（参考古诺 1838 年关于水的双头垄断分析）。现在设想每家公司的 “伯川德推测（Bertrand Conjecture）” 值是 -1 。这表示如果公司 A 增加它的产量，那么它推测 B 会相应的减少相同的产量，所以这样可以总产量，价格也保持不变不受影响。在伯川德推测（Bertrand Conjecture）中，公司相信市场价格不受它们产量的影响，因为每个公司都相信其他公司会调整自己的产量，从而将总产量保持恒定不变。另一个极端是联合利益（Joint-Profit）最大化的推测值是 +1 。此种情况下，每个公司都相信其他公司会效仿它并对产量作出对等的改变，（由于边际成本（marginal cost）的不变）这让公司的行为表现像单一的垄断（monopoly）供应商。
In oligopoly theory, conjectural variation is the belief that one firm has an idea about the way its competitors may react if it varies its output or price. The firm forms a conjecture about the variation in the other firm's output that will accompany any change in its own output. For example, in the classic Cournot model of oligopoly, it is assumed that each firm treats the output of the other firms as given when it chooses its output. This is sometimes called the "Nash conjecture" as it underlies the standard Nash equilibrium concept. However, alternative assumptions can be made. Suppose you have two firms producing the same good, so that the industry price is determined by the combined output of the two firms (think of the water duopoly in Cournot's original 1838 account). Now suppose that each firm has what is called the "Bertrand Conjecture" of −1. This means that if firm A increases its output, it conjectures that firm B will reduce its output to exactly offset firm A's increase, so that total output and hence price remains unchanged. With the Bertrand Conjecture, the firms act as if they believe that the market price is unaffected by their own output, because each firm believes that the other firm will adjust its output so that total output will be constant. At the other extreme is the Joint-Profit maximizing conjecture of +1. In this case each firm believes that the other will imitate exactly any change in output it makes, which leads (with constant marginal cost) to the firms behaving like a single monopoly supplier.