Accounting as an information system should concentrate more on exogenous errors, as these types of errors play an important role in updating beliefs on the expected value of the company. According to the Bayes’ theorem (1763), the formation of new beliefs is based upon prior knowledge and present learning, and exogenous errors can be learned from information reflected in the accounting system. Therefore, what helps pave the road for accounting system information toward the objective of usefulness, is the presentation of information on variability and not on expected values. Whether or not the accounting system has to go far in recording this variability and whether or not it needs to include market errors as well, requires an investigation on the informativeness of prices available in the market. However, the issue not to be ignored when investigating the usefulness of accounting system information is the effectiveness of endogenous (systematic) errors on information. These errors result from the contrast between the multipurpose objectives of the accounting system and the incompatibility of models applied for the measurement of transactions with economic realities.