Learning by Doing and Audit Quality*

Abstract In this study, we present a nonstrategic, dynamic Bayesian model in which auditors' learning on the job and their choice of professional services jointly affect audit quality. While performing audits over time, auditors accumulate client‐specific knowledge so that their posterior beliefs about clients are updated and become more precise (that is, precision is our surrogate for audit quality) — what we call the learning effect. In addition, auditors can enrich their knowledge accumulation by performing nonaudit services (NAS) that, in fact, may influence clients' managerial decisions — what we call the business advisory effect. This advisory effect permits auditors to anticipate and to learn about changes in clients' business models, which in turn improves their advisory capacity. These dual “learning” and “advisory” effects are interdependent and mutually reinforcing. The advisory effect of NAS may increase or reduce auditors' engagement risk. We show that large professional fees can induce auditors to provide NAS that increase engagement risk and diminish audit quality. However, when NAS reduce engagement risk and increase audit quality, auditors may provide NAS without charging clients. The feature that distinguishes our study — the interdependence between the learning and advisory effects — provides new insight into the trade‐off between audit fees and audit quality. Consequently, our analysis helps explain why the scope of the audit has evolved over time and why the boundaries between audit and NAS are constantly shifting. A recent example of such a shift is that the Sarbanes‐Oxley Act adds control attestation to audits for public companies traded in U.S. markets.