Assurance quality – measures and determinants

**Purpose** – This thesis examines how sustainability assurance quality can be measured and identifies the key determinants that influence its effectiveness. It addresses the lack of a robust empirical proxy for assurance quality by investigating whether material sustainability restatements can serve as an outcome-based measure and by analysing the roles of assurance provider experience and independence. **Design/methodology/approach** – The thesis adopts a three-study design. The first study conducts a bibliometric-systematic literature review to map the evolution of sustainability assurance research and identify existing gaps. The second and third studies use panel data from Public Interest Entities in France, Italy and Spain between 2019 and 2023, where sustainability assurance was mandatory, to examine the effects of provider tenure, provider switching, statutory audit fees and abnormal audit fees on the occurrence of material sustainability restatements. **Findings** – The literature review demonstrates that assurance quality remains conceptually underdeveloped and largely measured through input-based proxies. The empirical analyses show that longer provider tenure is associated with fewer material restatements, indicating that accumulated provider experience enhances assurance quality. Conversely, higher statutory audit fees and abnormal audit fees are positively associated with restatements, suggesting that stronger economic dependence may impair provider independence. Provider experience partially mitigates, but does not eliminate, these adverse effects. **Originality/value** – This thesis introduces material sustainability restatements as an empirical proxy for sustainability assurance quality and provides one of the first comprehensive examinations of its determinants within a mandatory European assurance setting. By integrating provider competence and independence, it offers a more comprehensive framework for understanding and evaluating sustainability assurance quality.