Drivers of combined assurance in a sustainable development context: Evidence from integrated reports

External assurance engagements performed by auditors and consultants to test sections of organisations' sustainability and integrated reports have become commonplace and are examined in detail by the prior research. Less often considered is how different types of internal and external sources of assurance form part of a combined assurance model that organisations rely on to ensure the integrity of information used for internal decision making and reporting to stakeholders. As a result, this paper offers an important contribution to the academic literature by providing one of the first reviews to deal specifically with the operation of combined assurance. The research considers if firm size and profitability, industry membership and select governance features are associated with the use of more sophisticated combined assurance models for ensuring relevant and reliable integrated reporting. Multiple regression, complemented by supplementary analysis, reveals that larger firms that are more profitable and members of environmentally and socially sensitive industries are not necessarily using extensive combined assurance models. These organisations rely on robust assurance methodologies, but the coverage of their combined assurance models is restricted. Similarly, board experience is substituting for the use of complex assurance methodologies. Board size and independence are only associated with a stronger commitment to implementing codes of best practice dealing with combined assurance but not with the coverage of the assurance models or the assurance methodologies being applied. The results may be indicative of the fact that internal controls and systems required to support integrated thinking and reporting are still in a developmental stage, making it difficult for them to be the subject of any formal verification by internal or external assurance providers.