Final answer:
Upon obtaining reasonable assurance through control testing and other procedures, an auditor can reduce the extent of substantive tests and potentially issue an unqualified opinion, assuming financial statements are fair and comply with reporting frameworks.
Step-by-step explanation:
If the auditor has obtained a reasonable level of assurance about the fair presentation of the financial statements through understanding internal control, assessing control risk, testing controls, and analytical procedures, then the auditor can significantly reduce other substantive tests. Nevertheless, the sufficiency and appropriateness of audit evidence should always guide the auditor's judgment regarding whether reductions in substantive testing are appropriate. An unqualified opinion can be issued if the auditor concludes that the financial statements are presented fairly, in all material respects, in accordance with the applicable financial reporting framework. Writing the engagement letter happens at the beginning of the audit process, not after obtaining assurance. Additional tests of controls might not be necessary if a reasonable level of assurance has been achieved unless there is a change in circumstances that would warrant further testing.