A comparative case study (CCS) is defined as ‘the systematic comparison of two or more data points (“cases”) obtained through use of the case study method’ (Kaarbo and Beasley 1999, p. 372). A case may be a participant, an intervention site, a programme or a policy. Case studies have a long history in the social sciences, yet for a long time, they were treated with scepticism (Harrison et al. 2017). The advent of grounded theory in the 1960s led to a revival in the use of case-based approaches. From the early 1980s, the increase in case study research in the field of political sciences led to the integration of formal, statistical and narrative methods, as well as the use of empirical case selection and causal inference (George and Bennett 2005), which contributed to its methodological advancement. Now, as Harrison and colleagues (2017) note, CCS:

“Has grown in sophistication and is viewed as a valid form of inquiry to explore a broad scope of complex issues, particularly when human behavior and social interactions are central to understanding topics of interest.”

It is claimed that CCS can be applied to detect causal attribution and contribution when the use of a comparison or control group is not feasible (or not preferred). Comparing cases enables evaluators to tackle causal inference through assessing regularity (patterns) and/or by excluding other plausible explanations. In practical terms, CCS involves proposing, analysing and synthesising patterns (similarities and differences) across cases that share common objectives.

What is involved?

Goodrick (2014) outlines the steps to be taken in undertaking CCS.

Key evaluation questions and the purpose of the evaluation: The evaluator should explicitly articulate the adequacy and purpose of using CCS (guided by the evaluation questions) and define the primary interests. Formulating key evaluation questions allows the selection of appropriate cases to be used in the analysis.

Propositions based on the Theory of Change: Theories and hypotheses that are to be explored should be derived from the Theory of Change (or, alternatively, from previous research around the initiative, existing policy or programme documentation).

Case selection: Advocates for CCS approaches claim an important distinction between case-oriented small n studies and (most typically large n) statistical/variable-focused approaches in terms of the process of selecting cases: in case-based methods, selection is iterative and cannot rely on convenience and accessibility. ‘Initial’ cases should be identified in advance, but case selection may continue as evidence is gathered. Various case-selection criteria can be identified depending on the analytic purpose (Vogt et al., 2011). These may include:

  • Very similar cases
  • Very different cases
  • Typical or representative cases
  • Extreme or unusual cases
  • Deviant or unexpected cases
  • Influential or emblematic cases

Identify how evidence will be collected, analysed and synthesised: CCS often applies mixed methods.

Test alternative explanations for outcomes: Following the identification of patterns and relationships, the evaluator may wish to test the established propositions in a follow-up exploratory phase. Approaches applied here may involve triangulation, selecting contradicting cases or using an analytical approach such as Qualitative Comparative Analysis (QCA).
Download a Comparative Case Study here
Download a longer briefing on Comparative Case Studies here

Useful resources

A webinar shared by Better Evaluation with an overview of using CCS for evaluation.

A short overview describing how to apply CCS for evaluation:

Goodrick, D. (2014). Comparative Case Studies, Methodological Briefs: Impact Evaluation 9, UNICEF Office of Research, Florence.

An extensively used book that provides a comprehensive critical examination of case-based methods:

Byrne, D. and Ragin, C. C. (2009). The Sage handbook of case-based methods. Sage Publications.