In the market for fresh vegetables, for example, prices must balance the supply and demand for the current harvest. By contrast, oil producers and others in the industry can keep supply off the market if they think that its price will rise later, or they can put extra supply on the market if they think the price will fall.
无论何时，石油价格取决于市场参与者对未来供求的预期。预期的作用使石油市场与其他市场十分不同。比如，在新鲜蔬菜市场，价格必须平衡当季收成供求平衡。相反，石油生产商和其他行业参与者可以保持低供给，只要它们认为价格会在日后上升；也可以超额供给市场，只要它们觉得价格会在日后下跌。