External auditors are continually facing intense pressure to be more efficient in conducting audits without compromising quality and effectiveness. Optimal utilization of internal audit work can improve both the efficiency and the effectiveness of external audits and can enhance the value of internal auditors to the client organization.

The objective of this study is to understand how the three factors (objectivity, work performance, and competence of the internal auditors) identified by auditing standards and by prior research interact in determining the strength of the internal audit function. Most prior studies have attempted to understand external auditors' rank ordering of the importance of the three factors, without an explicit attempt to model the interactions among the factors. The results from these studies are mixed and inconclusive. Hence, prior studies have not produced a consensus about how external auditors seem to weight and combine these factors in order to make assessments relating to the strength of the internal audit function.

This study employs analytical methods based on Bayesian probability to model external auditors' evaluation of the internal audit function. Specifically, models based on multistage (cascaded) inference theory are developed and analyzed using numerical sensitivity analysis. The modeling contribution is significant in the sense that it is the only study that provides a theoretical model for the decision process. Results reveal that the importance of the three factors varies with the type of evidence (convergent or conflicting) observed and is contingent on the interrelationships among the three factors. A major conclusion of this study is that in the Bayesian context, it is futile to attempt a ranking of the factors since no single factor will dominate under all conditions. The study also provides avenues for future research and for improving the guidance provided by professional auditing standards that relate to the evaluation of internal audit work.

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