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Case studies are in-depth accounts that may expand some understandings of a phenomenon and limit others, and authors are responsible for how knowledge is constructedIn a linked BMJ Medicine article (doi:10.1136/bmjmed-2025-001623), Trisha Greenhalgh provides a first class, comprehensive guide to case study design and different case study types.1 The paper comes at an important time: researchers are being increasingly called on to design research impact case studies to demonstrate the real world difference their research makes, such as in the Research Excellence Framework process in the UK.2 Greenhalgh's aim is to improve the quality, usefulness, and credibility of the claims made by all types of case studies, whether they are used to secure funding or not.Research case studies are in-depth, detailed, qualitative accounts of a phenomenon, and while they may draw on quantitative data, the main focus is a descriptive narrative account or explanation. A case study may describe how an organisation has changed, how a new practice was implemented, why a particular ward is an exceptionally high or low performer, or how a replicated intervention may work differently in a different contexts. In contrast, a case report is narrower: it describes a patient or a series of patients (a case series).Case studies seek to generate new knowledge, tracking a range of explanations for observed results. They seek in-depth, detailed understanding of a microcosm or instance, and are therefore ambitious and holistic. In particular, the researcher takes responsibility for the construction of how we understand something, describing what is included, what is excluded, the justification for this, and the difference it might make. Greenhalgh provides a guide to critical decisions about case selection, including the data sources, the voices heard, the analysis, and reporting, in addition to the way the case is bounded by time and place.These design decisions have major consequences, as can be seen in a case study cited by Greenhalgh in her paper: the Broad Street pump cholera outbreak. “The truth” of this celebrated case study in public health has been produced according to when the story starts, when it ends, who is in the story, and the underlying message.The conventional telling of this story focuses on John Snow's analysis of the higher number of deaths around the Broad Street pump in London in 1854. From his analysis, he concluded that the causative agent was waterborne and that water from the pump was contaminated. Noting that the number of deaths was already beginning to decline, he persuaded the Committee for Scientific Enquiries to remove the pump handle, ending the epidemic.3 The story is described as an early example of science-led policy because Parliament passed legislation overhauling London's sewage systems “as a result of this episode”.3 John Snow is thus depicted as a hero and one of the founders of epidemiology.Stated like this, it is an impressive piece of action research, but to be more true to the case study form, we should look further. Fortunately, the data are available. If we take a longer time frame into account, we learn that Snow's waterborne hypothesis formed more slowly; Snow had started tabulating death rates related to cholera in different parts of London some five years earlier.4 He presented multiple sources of evidence to the committee,3 4 and his own account shows that he did not think the pump evidence alone was convincing.4 He expanded his investigations of possible waterborne transmission by comparing cholera deaths in different areas of London that used different water supply systems,3 4 and he collaborated with others in the observation and deduction that led to the identification of the source of the contamination.5 The famous map was used as a means of communication, rather than an analytical tool, to investigate the Broad Street pump.6 Far from being an example of policy making as a result of his actions, the committee was not convinced7; the pump handle was removed, but the committee thought that another explanation, specifically so-called atmospheric infection, could also account for Snow's data.7 The grand reforms of London's sewage system occurred more than a decade later, as part of a more generic sanitation movement that had already started when Snow began his work.7In other words, the choice to end the case story when the epidemic subsides, as the pump handle is removed, is deliberately dramatic. Its narrative form matches a memoir written on Snow's death just four years after the pump handle event by Snow's friend, Benjamin Ward Richardson, who was a doctor and a writer of romantic novels.8To epidemiologists nowadays, the John Snow story ending is premature and misleading. Even Sir Austin Bradford-Hill, a renowned medical statistician of the 20th century who wrote “an appreciation” of John Snow 70 years ago, took pains to correct the “facts” while giving full credit to the detailed and pioneering set of investigations Snow carried out,4 as have others since.5 9So, if the John Snow account was reported now as a case study with a longer time frame, multiple protagonists, and more data sources, the epidemiology and policy action would be more accurately portrayed. But would it be as memorable?Whether we are designing or reading a case study, Greenhalgh reminds us to ask: what is this a case of? Confined to the Broad Street pump story alone, the case is an incomplete example of evidence in action.5 However, Margaret Pelling, a medical historian, argues that the perpetuation of the John Snow pump handle story is a good example of myth making in medicine.8 Pelling argues that the myth has served the function of individualising and simplifying public health (generating the idea of a preventive silver bullet), at times when public health action might otherwise seem too complex and politically threatening.8 In this context, the persistence of the story is illuminating.For good reason therefore, Greenhalgh invites readers to interrogate the way we construct and generate knowledge in case studies and to read them critically. Greenhalgh's article will improve the design, reporting, and use of case studies because of its insight, breadth, accessibility, and clarity. It also provides a much needed navigation guide to the scholarship in this field, which currently ranges across a range of disciplines. The article is set to become a citation classic.