在人類生活中，影響個人福祉之外生因子不乏具不確定性者；理性的個人在最適化地作消費選擇時，往往必須根據得到的信息判斷不確定性或風險的程度。個人對風險的錯誤判斷或者誤覺，必然導致非最適的選擇，隱含福利損失。傳統文獻包括消費者剩餘的方法與情況評估，以及晚近藉助對偶理論的研究，似乎都未嘗合理地處理或評估這項誤覺所致的福利損失。本文仍將藉助消費者行爲的對偶架構，以支出函數的觀念定義各種與誤覺有關的福利效果，而足以反映個人眞正的福祉變動。利用弱性互補的假設以及效用函數的性質，應該可以處理在推導支出函數時，由於非價格變動與非最適選擇所致的困難，而足以提出衡量上述福利效果的方法。

An individual usually makes choices under uncertainty since the surrounding factors that influence his well-being are often random. Evidently his incorrect perception or misperception of the riskiness will cause welfare loss due to the resulting nonoptimal choice. The literature, including con­sumer surplus approach, contingent valuation study, and some recent ones using the duality theory, does not seem to have addressed the issue ap­propriately. This paper employs the concept of expenditure function to define the welfare loss in a way that it reflects the underlying change of the indi­vidual's happiness owing to his misperception. The weak-complementarity assumption and the quasi-concavity of utility function make it possible to take care of the difficulty in retrieving expenditure functions arising from the particular type of imposed quantity change and the related nonoptimal decision.