Unintended outcomes evaluation approach: A plausible way to evaluate unintended outcomes of social development programmes

•Contributions to evaluation theory to understand unintended effect have been limited.•To advance the knowledge of evaluation theory, methods and practice, the article presents an evaluation approach to study unintended effects.•The article offers a comprehensive classification of unintended outcomes on the basis of knowability, value, distribution and temporality.•Corresponding to this classification, a three-step unintended outcome evaluation approach (UOEA) has been developed and tested.•UOEA provides the discipline of evaluation with an empirically-based reference point for further theoretical developments in the study of unintended outcomes.•Contributions to evaluation theory to understand unintended effect have been limited.•To advance the knowledge of evaluation theory, methods and practice, the article presents an evaluation approach to study unintended effects.•The article offers a comprehensive classification of unintended outcomes on the basis of knowability, value, distribution and temporality.•Corresponding to this classification, a three-step unintended outcome evaluation approach (UOEA) has been developed and tested.•UOEA provides the discipline of evaluation with an empirically-based reference point for further theoretical developments in the study of unintended outcomes. Social development programmes are deliberate attempts to bring about change and unintended outcomes can be considered as inherent to any such intervention. There is now a solid consensus among the international evaluation community regarding the need to consider unintended outcomes as a key aspect in any evaluative study. However, this concern often equates to nothing more than false piety. Exiting evaluation theory suffers from overlap of terminology, inadequate categorisation of unintended outcomes and lack of guidance on how to study them. To advance the knowledge of evaluation theory, methods and practice, the author has developed an evaluation approach to study unintended effects using a theory building, testing and refinement process. A comprehensive classification of unintended outcomes on the basis of knowability, value, distribution and temporality helped specify various type of unintended outcomes for programme evaluation. Corresponding to this classification, a three-step evaluation process was proposed including a) outlining programme intentions b) forecasting likely unintended effects c) mapping the anticipated and understanding unanticipated unintended outcom