In assessing whether the controls are operating effectively, is it necessary for the auditor to re-perform the

Question:

In assessing whether the controls are operating effectively, is it necessary for the auditor to re-perform the work of the control itself? For example, if someone tests for the correctness of computations and initials the bottom of a document to indicate that such a control procedure has been performed, is it necessary for the auditor to re-perform? Explain the rationale for your response.