Program evaluation is described by Michael Scriven (2003) as a transdiscipline that is characterized as a discipline that supplies “essential tools for other disciplines, while retaining an autonomous structure and research effort of [its] own” (p. 19). Evaluation is not merely the application of social science methods to solve social problems; rather, evaluators use social science methods to examine the merit, worth and significance of a program or project or policy for the purposes of describing values associated with different stakeholder groups, as well as reaching evaluative conclusions “about good and bad solutions to social problems” (p. 21). The transdisciplinary nature of evaluation allows its application in diverse contexts, with diverse stakeholder groups, to address diverse social problems, through the use of diverse methodologies. With these multiple dimensions of diversity, it should come as no surprise that there are also diverse sets of philosophical assumptions that underlie the choices that evaluators make regarding their methodologies.

“Evaluation is situated in a broad landscape in terms of its diverse meanings in different disciplines, sectors, nations, and venues. The hallmarks of the evaluation field are its interdisciplinary roots and the ways in which the resultant conversations around the meaning of evaluation have benefited from this diversity of perspectives” (Mertens & Wilson 2012, p. 1). The evaluation field has experienced many decades of differences of opinions about which methodologies are best; at times these differences have been acrimonious. However, Shadish (1998) claims that differences about methodologies are not based on arguments about methods choices, but they are reflective of the different philosophical assumptions that guide methodological choices. He wrote that most debates in the evaluation field are “about epistemology and ontology, about what assumptions we make when we construct knowledge, about the nature of many fundamental concepts that we use in our work like causation, generalization and truth” (p. 3).

Mertens (2009; 2015) and Mertens and Wilson (2012) built on the work of Guba and Lincoln’s (1989; 2005) concept of paradigms in research and evaluation; they described four sets of philosophical assumptions that constitute a paradigm: axiology, ontology, epistemology and methodology. Mertens and Wilson identified four major paradigms that are operating in the world of evaluation: postpositivist, constructivist, transformative, and pragmatic. These paradigms are associated with four branches of evaluation that reflect the diverse perspectives in the field. Christie and Alkin (2013) identified three branches of evaluation: Methods, Use and Values. Mertens and Wilson (2014) added the fourth branch of Social Justice. The paradigms and branches of evaluation align in the following way: The Methods Branch maps onto the postpositivist paradigm, the Use Branch onto the pragmatic paradigm, the Values Branch onto the constructivist paradigm, and the Social Justice Branch onto the transformative paradigm. The following figure depicts the primary focus of each paradigm and its associated evaluation branch.