According to Webster's Eleventh New Collegiate Dictionary, to diagnose is "to analyze the cause or nature of a problem" and diagnostic is defined as "the art or practice of diagnosis." Diagnostic testing in education (e.g., testing for accountability, ranking, placement, monitoring), then, is a means to identify the nature or cause of an educational problem. According to experts Dylan Wiliam and Marnie Thompson (2006, p. 62), "an assessment monitors learning to the extent that it provides information about whether the student, class, school or system is learning or not; it is diagnostic to the extent that it provides information about what is going wrong; and it is formative to the extent that it provides information about what to do about it." They also caution that "not all diagnoses are instructionally tractable - an assessment can accurately diagnose what needs attention without indicating what needs to be done to address the issue." Diagnostic testing can operate at all levels of the education system: a state that is interested in diagnosing the strengths and weaknesses of student performance; a district seeking to "diagnose" which schools may need the most help with particular subgroups; a school looking to "diagnose" why some grade level math scores are low; a teacher wanting to "diagnose" why a particular student is having trouble with reading comprehension are all examples of diagnostic testing. Educators should use this section of the data use website to find important criteria for selecting diagnostic tests for student learning. If you would like to develop further capacity for using diagnostic assessments, please visit the professional development section of our website.

Wiliam, D., & Thompson, M. (2007). Integrating assessment with instruction: What will it take to make it work? In C.A. Dwyer (Ed.), The future of assessment: Shaping teaching and learning. Mahway, NJ: Lawrence Erlbaum Associates.