Purpose In recent decades, managerial desire to use internal audit services to promote well-controlled organizational processes has significantly increased in the public sector. Yet, there is rare information on how internal audits may effectively be implemented in the public sector. The present research aims to design a comprehensive internal audit effectiveness model for the public sector. Design/methodology/approach This research is an applied mixed-methods (qualitative–quantitative) and descriptive survey study. The research population in both qualitative and quantitative sections included all internal and external audit managers and staff of Iraqi public organizations. The study sample of the qualitative section included 28 audit managers and staff selected using purposive sampling, and the study sample of the quantitative section included 399 individuals selected using convenience sampling. The data collection instruments in the qualitative and quantitative sections included semi-structured interviews and questionnaires, respectively. Data analysis was carried out using thematic analysis in Atlas.ti software in the qualitative section. Quantitative analysis and model validation were also performed using path analysis and structural equation modeling in Smart-PLS software in the quantitative section. Findings The results of the qualitative section led to the identification of 38 effectiveness factors among seven main categories, which were finally presented in the form of a comprehensive model. The main categories included the communication between internal and external auditors, granting independence and authority to internal auditors, providing specialized human resources, providing technological resources, developing management support, strengthening organizational culture and developing audit plans. In the qualitative section, the model was validated and confirmed. Quantitative results further showed that the compilation of audit plans and the development of management support were the most important factors. Originality/value The present study contributes to the literature by providing a comprehensive framework on internal audit with an emphasis on resource-based theory and reducing the research gap in the field of agents influencing internal audit efficacy in the public sector of developing countries. Further, the study provides insights into an under-studied developing country, Iraq.

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