We use simulation to investigate the joint effects of materiality, evidence extent, evidence nature, and misstatement type on achieved audit risk, i.e., the risk of undetected material financial statement misstatement due to error or fraud. Our primary results are fourfold. First, contrary to conventional audit wisdom, we show that elevating the extent of testing decreases achieved audit risk only under certain conditions and may well increase it. Second, reducing materiality (attempting to perform a more precise audit) can either enhance or jeopardize audit effectiveness. Third, learning about the quality of the internal controls over financial reporting not only can help the auditor to perform an integrated audit, but also helps the auditor to reach better judgments about the extent to which and how evidence from the auditee organization's management and/or information systems may be distorted as a result of misstatement, reducing the risk that the auditor would be misled by such evidence. Fourth, when financial statements are biased intentionally due to fraud, it is especially important for the external auditor to supplement more traditional audit tests with tests that produce evidence that is less likely to be biased by management. Auditors who do not understand these four results run a heightened risk of compromising audit effectiveness.