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What Is a Case Study?

When you’re performing research as part of your job or for a school assignment, you’ll probably come across case studies that help you to learn more about the topic at hand. But what is a case study and why are they helpful? Read on to learn all about case studies.

Deep Dive into a Topic

At face value, a case study is a deep dive into a topic. Case studies can be found in many fields, particularly across the social sciences and medicine. When you conduct a case study, you create a body of research based on an inquiry and related data from analysis of a group, individual or controlled research environment.

As a researcher, you can benefit from the analysis of case studies similar to inquiries you’re currently studying. Researchers often rely on case studies to answer questions that basic information and standard diagnostics cannot address.

Study a Pattern

One of the main objectives of a case study is to find a pattern that answers whatever the initial inquiry seeks to find. This might be a question about why college students are prone to certain eating habits or what mental health problems afflict house fire survivors. The researcher then collects data, either through observation or data research, and starts connecting the dots to find underlying behaviors or impacts of the sample group’s behavior.

Gather Evidence

During the study period, the researcher gathers evidence to back the observed patterns and future claims that’ll be derived from the data. Since case studies are usually presented in the professional environment, it’s not enough to simply have a theory and observational notes to back up a claim. Instead, the researcher must provide evidence to support the body of study and the resulting conclusions.

Present Findings

As the study progresses, the researcher develops a solid case to present to peers or a governing body. Case study presentation is important because it legitimizes the body of research and opens the findings to a broader analysis that may end up drawing a conclusion that’s more true to the data than what one or two researchers might establish. The presentation might be formal or casual, depending on the case study itself.

Draw Conclusions

Once the body of research is established, it’s time to draw conclusions from the case study. As with all social sciences studies, conclusions from one researcher shouldn’t necessarily be taken as gospel, but they’re helpful for advancing the body of knowledge in a given field. For that purpose, they’re an invaluable way of gathering new material and presenting ideas that others in the field can learn from and expand upon.


case series study design definition

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  • v.46(4); Jul-Aug 2020

Case series: an essential study design to build knowledge and pose hypotheses for rare and new diseases

Série de casos: delineamento de estudo essencial para a construção de conhecimento e a proposição de hipóteses para doenças raras e novas, carlos arturo torres-duque.

1 . Methods in Epidemiologic, Clinical, and Operations Research-MECOR-program, American Thoracic Society/Asociación Latinoamericana del Tórax, Montevideo, Uruguay.

2 . CINEUMO Research Center, Fundación Neumológica Colombiana, Bogota, Colombia.

3 . Universidad de la Sabana, Bogota, Colombia.

Cecilia Maria Patino

4 . Department of Preventive Medicine, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA.

Juliana Carvalho Ferreira

5 . Divisão de Pneumologia, Instituto do Coração, Hospital das Clínicas, Faculdade de Medicina, Universidade de São Paulo, São Paulo (SP) Brasil.


At the end of December of 2019, a pneumonia outbreak of unknown origin appeared in China. Soon afterwards, the causative virus was identified-SARS coronavirus 2 (SARS-CoV-2), and the disease was named coronavirus disease 2019 (COVID-19). In January of 2020, Chinese investigators published a detailed case series describing the characteristics and outcomes of 41 adults with confirmed COVID-19. 1 The study showed that 15% of those patients died during the study period. That case series 1 was extremely important because it was the first published description of the impact of the new disease, helping clinicians around the world to face a new pandemic.


A case series includes a description of the characteristics and outcomes among a group of individuals with either a disease or an exposure (which can be an intervention) over a period of time and without a control group. Data are collected retrospectively or prospectively, and there is no randomization. The objective is to describe the population and outcomes, rather than compare risks across groups. Therefore, a case series differs from cohort studies because the latter compares the risk between two groups (exposed and unexposed) and allows for the estimation of an absolute risk for the occurrence of a given outcome in the exposed group and of a relative risk in comparison with the unexposed group.

The case series design is not considered the strongest source of evidence due to the absence of a control group and the risk of bias, in particular selection bias, since typical or severe cases of the disease are more easily identified, and rare presentations or mild cases may not be included. In the Chinese report, 1 for example, patients with less severe COVID-19 were not hospitalized and therefore were not included in the case series. However, case series are particularly important when a new disease or treatment emerges, because it provides descriptive information and contributes to building knowledge and generating hypotheses. Case series is also an appropriate study design to describe new treatments, previously unknown medication adverse events, and rare diseases. 2


  • Inclusion criteria - A precise operational definition of a “case” is crucial for the reliability of the study.
  • Sampling - Two strategies are possible: 1) based on disease or exposure; 2) based on a specific outcome.
  • Selection of variables of interest - A detailed selection and a clear definition of predictive variables of interest are necessary, as well as test results, interventions, complications, adverse events, and outcomes.
  • Systematic collection of data and robust analysis - They assure the quality of a case series study.

Table 1 presents a tool for evaluating the methodological quality of case series. 2

Adapted from Murad et al. 2 Questions 4, 5 and 6 are more relevant for adverse drug events.

Quantitative study designs: Case Studies/ Case Report/ Case Series

Quantitative study designs.

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  • Randomised Controlled Trial
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Case Study / Case Report / Case Series

Some famous examples of case studies are John Martin Marlow’s case study on Phineas Gage (the man who had a railway spike through his head) and Sigmund Freud’s case studies, Little Hans and The Rat Man. Case studies are widely used in psychology to provide insight into unusual conditions.

A case study, also known as a case report, is an in depth or intensive study of a single individual or specific group, while a case series is a grouping of similar case studies / case reports together.

A case study / case report can be used in the following instances:

  • where there is atypical or abnormal behaviour or development
  • an unexplained outcome to treatment
  • an emerging disease or condition

The stages of a Case Study / Case Report / Case Series

case series study design definition

Which clinical questions does Case Study / Case Report / Case Series best answer?

Emerging conditions, adverse reactions to treatments, atypical / abnormal behaviour, new programs or methods of treatment – all of these can be answered with case studies /case reports / case series. They are generally descriptive studies based on qualitative data e.g. observations, interviews, questionnaires, diaries, personal notes or clinical notes.

What are the advantages and disadvantages to consider when using Case Studies/ Case Reports and Case Series ?

What are the pitfalls to look for.

One pitfall that has occurred in some case studies is where two common conditions/treatments have been linked together with no comprehensive data backing up the conclusion. A hypothetical example could be where high rates of the common cold were associated with suicide when the cohort also suffered from depression.

Critical appraisal tools 

To assist with critically appraising Case studies / Case reports / Case series there are some tools / checklists you can use.

JBI Critical Appraisal Checklist for Case Series

JBI Critical Appraisal Checklist for Case Reports

Real World Examples

Some Psychology case study / case report / case series examples

Capp, G. (2015). Our community, our schools : A case study of program design for school-based mental health services. Children & Schools, 37(4), 241–248. A pilot program to improve school based mental health services was instigated in one elementary school and one middle / high school. The case study followed the program from development through to implementation, documenting each step of the process.

Cowdrey, F. A. & Walz, L. (2015). Exposure therapy for fear of spiders in an adult with learning disabilities: A case report. British Journal of Learning Disabilities, 43(1), 75–82. One person was studied who had completed a pre- intervention and post- intervention questionnaire. From the results of this data the exposure therapy intervention was found to be effective in reducing the phobia. This case report highlighted a therapy that could be used to assist people with learning disabilities who also suffered from phobias.

Li, H. X., He, L., Zhang, C. C., Eisinger, R., Pan, Y. X., Wang, T., . . . Li, D. Y. (2019). Deep brain stimulation in post‐traumatic dystonia: A case series study. CNS Neuroscience & Therapeutics. 1-8. Five patients were included in the case series, all with the same condition. They all received deep brain stimulation but not in the same area of the brain. Baseline and last follow up visit were assessed with the same rating scale.

References and Further Reading  

Greenhalgh, T. (2014). How to read a paper: the basics of evidence-based medicine. (5th ed.). New York: Wiley.

Heale, R. & Twycross, A. (2018). What is a case study? Evidence Based Nursing, 21(1), 7-8.

Himmelfarb Health Sciences Library. (2019). Study design 101: case report. Retrieved from

Hoffmann T., Bennett S., Mar C. D. (2017). Evidence-based practice across the health professions. Chatswood, NSW: Elsevier.

Robinson, O. C., & McAdams, D. P. (2015). Four functional roles for case studies in emerging adulthood research. Emerging Adulthood, 3(6), 413-420.

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Asploro Open Access Publications

What is Case Series?

  • Asploro Journal of Biomedical and Clinical Case Reports
  • ISSN : 2582-0370
  • Article Type : Editorial
  • Asp Biomed Clin Case Rep. 2018 Aug 17;1(1):10-15

El-Gilany AH 1* 1 Professor of Public Health, Faculty of Medicine,  Mansoura University , Egypt

Corresponding Author:  Abdel-Hady El-Gilany MD  ORCID ID Address:  Professor of Public Health & Preventive Medicine, Faculty of Medicine,  Mansoura University  Egypt. Received date : 06 August 2018;  Accepted date : 16 August 2018;  Published date : 17 August 2018

Citation:  El-Gilany AH. What is case series?. Asp Biomed Clin Case Rep. 2018 Aug 17;1(1):10-15.

Copyright  © 2018 El-Gilany AH. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Keywords:  Case Series, Exposure Series, Definition, Types, Importance, Limitations

This is the second part of a series on case reports and case series studies. It will help junior researchers to comprehend what is the case series/exposure series as a type of research. It highlights the definition, types, importance, limitations, and differences between case series and exposure series. Examples of important historical case series that were instrumental in the early identification of health problems will be mentioned. A special editorial will be devoted to consecutive controlled case series (CCCS)/ self-controlled case series (SCCS) as an important research method.


Case series (also known as clinical series) represents one of the most basic types of study designs, in which researchers describe the experience of a small group of people. It presents a detailed account of the clinical experience of individual study subjects and can evaluate large numbers of individuals and summarize the data using descriptive statistical measures [1]. A case series is a variation of a single case report in which the author describes several cases and their relation to one another and to the existing body of literature.

The Dictionary of Epidemiology defined a case series as – a collection of patients with common characteristics used to describe some clinical, pathophysiological or operational aspects of a disease, treatment or diagnostic procedures [2]. Case series is an observational, descriptive research design. It is most useful for describing the potential effectiveness of new interventions, for describing the effectiveness of interventions on unusual diagnoses, and for describing unusual responses (either good or bad) to interventions. Case series can be conducted retrospectively or prospectively. The primary distinction between case reports/series and the single-subject experiment is that the researcher does not manipulate the intervention in a case report/series but merely describes/documents what happened during the normal course of the intervention. Despite limitations, case series can often have a significant impact on the current practice of medicine and they are often used to put together case definitions of new diseases and to define future areas of clinical study [3]. However, no causal inferences should be made from case series regarding the efficacy of the investigated treatment [4]. A case series samples patients with both a specific outcome and a specific exposure, or samples patients with a specific outcome and includes patients regardless of whether they have specific exposures [5].

Types of Case Series

There are many classifications for case series.

Informal vs. Formal Case Series : [6,7]

  • Informal case series : Cases are selected for specific reasons: best case, worse case, significant variations. The format of this kind of case series is: introduction; case 1, case 2, case 3, etc. (each case is presented as a short case description); discussion (cases will be compared to one another, related cases to the current literature, implications of the findings, teaching points and what changes in clinical practice this might engender).
  • Formal case series : Include all cases of a specific type, or with specific selection criteria, presented more like a cohort study than a single case report and its format is introduction methods, results, and discussion/conclusions.

Consecutive vs. Non-Consecutive Case Series : [5,8]

  • Consecutive case series : Includes all eligible patients identified by the researchers during the study period. The patients are treated in the order in which they are identified. Consecutiveness increases the quality of the case series.
  • Non-consecutive case series : Includes some, but not all, of the eligible patients identified by the researchers during the study period.

Exposure or Outcome-Based Sampling : [5,9]

  • Exposure-based sampling : Include all patients treated and have specific outcomes or adverse events. Sampling is based on both a specific outcome and presence of a specific exposure.
  • Outcome-based  sampling: Includes patients with the specific outcome regardless of exposure. Thus neither absolute risk nor relative risk can be calculated. Selection is based only on a specific outcome, and data are collected on previous exposures.

Clinical vs. Population-Based Series : [10-12]

  • Clinical case series : Usually a coherent and consecutive set of cases of a disease recruited from one or more center by one or more researcher. It is a clinic-based register of cases that are analyzed together to learn about the disease. They are of value in epidemiology for studying symptoms and signs, creating case definitions as well as clinical education, audit, and research.
  • Population-based case series : When a clinical case-series is limited and complete for a defined geographical area for which the population is known, it is a population-based case-series consisting of a population register of cases. It is usually compiled for administrative and legal reasons.

By knowing the past history of these patients, including examination of past medical records, and by continuing to observe them to death, health professionals can build up a picture of the natural history of a disease in clinical case series. Population case-series is a systematic extension of this series but which includes additional cases, e.g. those dying without being seen by the clinicians. It adds breadth to the understanding of the spectrum and natural history of the disease. Information on the population permits calculation of rates, understanding the distribution of disease in populations and to the study of variations over time, between places and by population characteristics. Epidemiologically the most important case-series are registers of serious diseases or deaths, and of health service utilization, e.g. hospital admissions.

Design of Case Series

Case series research is a descriptive study to present patients in their natural clinical setting. The case series can be retrospective or prospective and may be consecutive or nonconsecutive depending on whether all cases presenting to the reporting authors over a period were included, or only a selection [13].

The study question should be focused and appropriate. The question should not be whether the investigated treatment is more effective or safer than another treatment. It should list the study population, the intervention and the primary outcome [4].

The setting : Select a suitable observation period and identify cases with events in this period. It may be tempting to include patients seen over a large period of time to increase sample size. However, the use of a short inclusion period minimizes known and unknown changes over time in co-interventions, prognosis, and even in the intervention under study [4,14].

Number of cases : there is no thumb rule or a magic number. The general number of cases reported in a case series range from 20 to 50, but may vary from as few as 2 or 3 to as many as more than 100 [10] or even thousands [6].

N.B. Case-only analyses are sometimes performed in genetic epidemiology to investigate the association between an exposure and a genotype [15].

What to look for : Look at when the events arose in relation to the exposures. Reports of case series usually contain detailed information about the individual patients. This includes demographic information (for example, age, gender, ethnic origin) and information on diagnosis, treatment, response to treatment, and follow-up after treatment [4,16,17].

  • What : The diagnosis or case definition should be clear and applied equally to all individuals in the series. The case definition should mention the inclusion and exclusion criteria, which should be based on widely used validated definitions. If authors use their own criteria, definition and justification are necessary to enable readers to compare the studied population with their own patients.
  • When : The date when the disease or death occurred (time).
  • Where : The place where the person lived, worked etc (place).
  • Who : The characteristics of the population (person). Noting the socio-demographic characteristics of a series of cases, as well as the temporal and spatial distributions can sometimes provide a clue to risk factors and hence help generate a hypothesis. This can be tested subsequently with more elaborate analytic studies.
  • The opportunity to collect additional data from medical records (possibly by electronic Data linkage) or the person directly.

A detailed description of the intervention and the co-intervention should be stated. This will ensure repeatability of the study by other investigators. It is very important to thoroughly describe co-interventions. Additionally, indications for the studied treatment should be explained. This will primarily determine the consistency of the patient group [4].

The blinding of outcome assessors is ideal in every kind of research design and can be implemented quite usefully in case-series studies (e.g., by having some investigators collect data only on an outcome and others collect data only on patient characteristics). This prevents the investigators’ measurements from being influenced (intentionally or unintentionally) by their personal treatment preference [4].

The method of data acquisition (telephone interview, clinical measurement, or chart review) should be addressed in the study report for the sake of repeatability and the appraisal of measurement bias [4].

Analysis : As the design of a case series is descriptive, only descriptive statistics should be used. Case-series data are analyzed using rates. In three circumstances (spatial clustering, stable population and when there is no suitable denominator) use proportional ratios, not rates.

Findings can be presented as proportions (%) of the study populations with the outcome, confidence intervals; means, standard deviations for continuous variables and consider subgroups that need data presented separately [18]. No comparative tests yielding p values should be done. By describing summary statistics, the author errs on the conservative side of speculation and avoids misleading with fancy probability statistics [19].

Reporting:  A statement of the external validity of the obtained data should be given. This includes (1) patient characteristics and (2) completeness of follow-up. The presence of chance and the presence, direction, and magnitude of bias should be acknowledged [4].

Patients may differ according to prognostic variables, such as age, etiology, and disease severity among geographical regions. This may complicate comparisons with other reports or explain discrepancies.

The follow-up rates and reasons for loss to follow-up should be stated. Completeness of follow-up varies considerably among similar case series, making it difficult for readers to compare them. Therefore, authors should be cautious when interpreting their own results in relation to results of apparently similar case series [20].

No absolute conclusions on the studied treatment should be stated, the lack of a comparison group prohibits any hypothesis from being tested. Valid conclusions basically repeat the descriptive study findings e.g. our patients treated by treatment X showed good outcome Y after Z months of follow-up [21].

Advantages [4,14,22-26]

  • High external validity: the study results are closer to those obtained in routine clinical practice and may, therefore, be considered more relevant.
  • It could be useful when a randomized controlled trial is not appropriate or possible.
  • No interference in the treatment decision process
  • A wide range of patients
  • Study conduct takes little time
  • Easy to write and can be useful in new observations or disease.
  • Useful for hypothesis generation, but conclusions about etiology cannot be made.
  • Informative for very rare disease with few established risk factors.
  • Informs patients and physicians about natural history and prognostic factors.
  • Easy and inexpensive to do in hospital settings.
  • Can help identify potential health problems such as the acute outbreaks of the severe acute respiratory syndrome (SARS).
  • Stimulate interest in an area, leading to more detailed studies, and advancing knowledge.
  • An important link between clinical medicine and epidemiology
  • One of the first steps in the outbreak investigation
  • Can provide the key to sound case-control and cohort studies and trials. Many case series are followed by clinical trials.
  • Provides ―anecdotal‖ evidence about a treatment or adverse reaction.
  • The case series method is self-matched: estimation is within-individuals.
  • As a result, all fixed confounders are automatically controlled: for example,  socioeconomic level, genetic factors, location, diet, state of health.
  • Only uses information on cases: no controls are required.
  • Studying predictive symptoms, signs, and tests.
  • Creating case definitions
  • Clinical education, audit, and research
  • Health services research
  • Establishing safety profiles

Disadvantages [ 1,4,10,16,20,22,26-28 ]

  • Lack of a control (or comparison) group this raises the question ―compared to what?
  • Data collection often incomplete.
  • Generally short-term
  • Lack of a denominator to calculate rates of disease.
  • Strong publication bias favoring positive results.
  • Cannot study cause and effect relationships.
  • Cannot assess disease frequency
  • Lacks external validity because cases may not be representative.
  • An outcome may be a chance finding, not characteristic of the disease.
  • Cannot easily examine disease etiology
  • Not planned before and lacks hypothesis and are often not considered by many authorities to be research studies.
  • Selection bias (investigators self-select): The selective nature and the limited amount of information provide little evidence of causality and cannot say much about patterns of disease occurs due to the seasonal nature of some diseases. A series of patients with a certain illness and/or a suspected linked exposure draw their patients from a particular population (such as a hospital or clinic) which may not appropriately represent the wider population.
  • Very low internal validity due to the lack of a comparator group exposed to the same array of intervening variables. The effects seen may be wholly or partly due to intervening effects such as the placebo effect, Hawthorne effect, time effects, practice effects or the natural history effect.
  • Information bias:  Cases can be retrieved retrospectively or collected prospectively. Information on cases retrieved retrospectively is generally more objective, as it is collected routinely in relevant medical records. Missing or incomplete information could be an issue. For prospectively collected information, it is desirable to have standard protocols and forms to collect the necessary information, to avoid missing data for some patients. In a purely descriptive case series, confounding is not a concern, as the association between a certain factor and an outcome is not being studied.
  • Sampling variation : A precise estimate of the rate of a disease, independent from chance, can be obtained only by increasing the number of diseased subjects.

Case Series that was Instrumental in the Early Identification of Health Problems

– Congenital Rubella Syndrome : The classic description of a series of infants born with congenital cataracts, some with additional cardiac abnormalities, in Australia in 1941. This led Gregg in Sydney to postulate a causal link between a severe epidemic of rubella that had occurred six to nine months before the children were born and the subsequent abnormalities. It is now well known that if a woman develops rubella during pregnancy it may affect her unborn baby.

– A case report published in the UK in 1961 described the development of a pulmonary embolism in a 40-year-old pre-menopausal woman, five weeks after she had started using an oral contraceptive (OC) to treat endometriosis. Because pulmonary embolism is rare in women of that age, the author suggested that it might have been caused by the OC, particularly since it was a novel exposure at that time. More detailed studies have consistently shown that there is an association between the use of OCs and the risk of pulmonary embolism.

– A report of a series of five cases of Pneumocystis carinii pneumonia that occurred in young, previously healthy, homosexual men in three Los Angeles hospitals in a six-month period during 1980–81. Until then, this disease had been seen almost exclusively in immunosuppressed e.g. the elderly, the severely malnourished and those on anti-cancer chemotherapy. This cluster of cases in young men suggested that the men were suffering from a previously unknown disease, possibly related to sexual behavior which was proved to be HIV/AIDS.

[1].  Kestenbaum B, “An introduction to clinical research”.  Epidemiology and biostatistics : 25, 2009.

[2].  Porta M (ed). A dictionary of epidemiology /edited for the International Epidemiological Association, 5th edition.  Oxford University Press , UK: pp: 33, 2008 .

[3].  Suresh K, Suresh G, Thomas SV, “Design and data analysis 1 study design”.  Ann Indian Acad Neurol , vol.15, no.2: 76–80, 2012 .

[4].  Kooistra B, Dijkman B, Einhorn TA, et al., “How to design a good case series”.  J Bone Joint Surg Am , vol.91, (Suppl 3): 21 -26, 2009.

[5].  Dekkers OM, Egger M, Altman DG, et al., “Distinguishing case series from cohort studies”.  Ann Intern Med,  vol.156, (1 Pt 1): 37-40, 2012.

[6].  Carey TS, Boden SD, “A Critical Guide to Case Series Reports”.  Spine , vol.28, no.15: 1631–34, 2003 .

[7]. Lutheran HealthCare, “Guidelines for Writing Case Reports & Case Series”. Accessed: August 1, 2018

[8].  National Cancer Institute Dictionary of Cancer Terms. Accessed: August 12, 2018

[9].  Crandall M, Eastman A, Violano P, et al., “Prevention of firearm-related injuries with restrictive licensing and concealed carry laws: an eastern Association for the Surgery of trauma systematic review”.  J Trauma Acute Care Surg,  vol.81, no.5: 952–60, 2016.

[10].  Omair A, “Selecting the appropriate study design for your research: Descriptive study designs”.  J Health Spec , vol.3, no.3: 153-56, 2015.

[11].  Mandil A, “Study Designs in Epidemiology. Lecture presented at High Institute of Public Health University of Alexandria”.  Accessed: August 6, 2018

[12].  Study design Bhopal R, Bruce and Usher J Professor of Public Health, Public Health Sciences Section, Division of Community Health Sciences, University of Edinburgh, Edinburgh EH89AG. Accessed: August 6, 2018

[13].  Esene IN, Ngu J, El Zoghby M, et al., “Case series and descriptive cohort studies in neurosurgery: the confusion and solution”.  Childs Nerv Syst,  vol.30, no.8: 1321–32, 2014 .

[14].  Whitaker H. Case series studies:  rationale, design and analysis. Open University, UK. Accessed: August 11, 2018

[15].  Khoury MJ, Flanders WD, “Nontraditional epidemiologic approaches in the analysis of gene-environment interaction: case-control studies with no controls”. Am J Epidemiol, vol.144, no.3: 207–13, 1996.

[16].  Yu I, Tse S, “Workshop 3 — Sources of bias in case series, patient cohorts, and randomized controlled trials”.  Hong Kong Med J,  vol.17, no.6: 478-79, 2011.

[17].  Howick J. Introduction to study design. Accessed August 13, 2018

[18].  Ford DE, introduction to clinical research, study design, case series and cross-sectional.  July 14, 2010. Accessed: August 13, 2018

[19].  Griffin D, Audige L, “Common statistical methods in orthopaedic clinical studies”.  Clin Orthop Relat Res,  vol.413: 70-79, 2003 .

[20].  Dalziel K, Round A, Stein K, et al., “Do the findings of case series studies vary significantly according to methodological characteristics?”,  Health Technol Assess,  vol.9, no.2: iii-iv, 1-146, 2005.

[21].  von Elm E, Altman DG, Egger M, et al., “The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement: guidelines for reporting observational studies”.  Lancet , vol.370, no. 9596: 1453-57, 2007 .

[22].  Cummings P, Weiss NS, “Case series and exposure series: the role of studies without controls in providing information about the etiology of injury or disease”.  Inj Prev , vol.4, no.1: 54–57, 1998 .

[23].  Hartz A, Marsh JL, “Methodologic issues in observational studies”.  Clin Orthop Relat Res , vol.413: 33-42, 2003.

[24].   Audigé L, Hanson B, Kopjar B, “Issues in the planning and conduct of nonrandomized studies”.  Injury,  vol.37, no.4: 340-48, 2006.

[25].  Degu G, Yigzaw T, “Lecture notes. For health science students. Research methodology”.  Ethiopia Public Health Training Initiative:  P.25, 2006.

[26]. Webb P, Bain C, “Essential epidemiology. An introduction for students and health professionals. Second Edition”: P.72, 2011.

[27].  Cole P, “Introduction. In: Breslow NE, Day NE, editors. Statistical Methods in Cancer Research: The Analysis of Case-control Studies. IARC Publication No. 32. Vol. 1. Lyon: International Agency for Research on Cancer; 1980”. Accessed: August 13, 2018

[28].  Polgar S, Thomas SA, “Introduction to research in the health Sciences”.  Churchill Livingstone . 2013

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6. Types of Descriptive Studies

case series study design definition

1. Types of Descriptive Studies

1.1. case reports and case series.

These are descriptive studies of one (case study or case report) or a series of patients (case series) defined by eligibility criteria, and where the unfolding course of events (disease progression, therapies, outcomes, etc.) is described in detail and where interventions are not manipulated.  This study design is used to provide a detailed description of an uncommon disease or condition, a specific situation, or unique therapeutic approaches. Reports provide retrospective safety and efficacy details, as well as other clinical parameters (e.g., quality of life [QoL]) derived from cases of interest in their natural clinical setting.

case series study design definition

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  • Case series
  • Cross-sectional studies
  • Cross-sectional surveys
  • Potential biases
  • Analysis of cross-sectional studies
  • Strengths and Weaknesses

Case series (also known as clinical series)

Case series typically involve a much smaller number of patients than the more powerful case-control study or Randomised controlled trial (RCT). In this design patients with a known exposure to a treatment are tracked prospectively to identify new forms of disease or adverse effects. Case series can also be retrospective - here medical records are examined to identify a link between exposure and disease.

You can see how case series designs may be confounded by selection bias because physicians choose to study patients from their hospital or clinic who they suspect have an illness that may be linked to an exposure.

Case series can be consecutive or non-consecutive. The results of case series can generate hypotheses that are useful in designing further studies including RCTs. No causal inferences should be made from case series regarding the efficacy of an investigated treatment. Case series have no control group, so they have no statistical validity. The benefit of case series is that they are easy to understand and can be written up in a very short period of time.

A case series is simply a series of cases of disease which can raise awareness initially of a new disease or disease variant. Acquired immunodeficiency disease syndrome (AIDS) and bovine spongiform encephalopathy (BSE), were initially described from the excess occurrence of cases of related rare diseases, which eventually led to new disease definitions.

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Study Design 101

  • Helpful formulas
  • Finding specific study types

Case Report

  • Meta- Analysis
  • Systematic Review
  • Practice Guideline
  • Randomized Controlled Trial
  • Cohort Study
  • Case Control Study
  • Case Reports

An article that describes and interprets an individual case, often written in the form of a detailed story. Case reports often describe:

  • Unique cases that cannot be explained by known diseases or syndromes
  • Cases that show an important variation of a disease or condition
  • Cases that show unexpected events that may yield new or useful information
  • Cases in which one patient has two or more unexpected diseases or disorders

Case reports are considered the lowest level of evidence, but they are also the first line of evidence, because they are where new issues and ideas emerge. This is why they form the base of our pyramid. A good case report will be clear about the importance of the observation being reported.

If multiple case reports show something similar, the next step might be a case-control study to determine if there is a relationship between the relevant variables.

  • Can help in the identification of new trends or diseases
  • Can help detect new drug side effects and potential uses (adverse or beneficial)
  • Educational – a way of sharing lessons learned
  • Identifies rare manifestations of a disease


  • Cases may not be generalizable
  • Not based on systematic studies
  • Causes or associations may have other explanations
  • Can be seen as emphasizing the bizarre or focusing on misleading elements

Design pitfalls to look out for

The patient should be described in detail, allowing others to identify patients with similar characteristics.

Does the case report provide information about the patient's age, sex, ethnicity, race, employment status, social situation, medical history, diagnosis, prognosis, previous treatments, past and current diagnostic test results, medications, psychological tests, clinical and functional assessments, and current intervention?

Case reports should include carefully recorded, unbiased observations.

Does the case report include measurements and/or recorded observations of the case? Does it show a bias?

Case reports should explore and infer, not confirm, deduce, or prove. They cannot demonstrate causality or argue for the adoption of a new treatment approach.

Does the case report present a hypothesis that can be confirmed by another type of study?

Fictitious Example

A physician treated a young and otherwise healthy patient who came to her office reporting numbness all over her body. The physician could not determine any reason for this numbness and had never seen anything like it. After taking an extensive history the physician discovered that the patient had recently been to the beach for a vacation and had used a very new type of spray sunscreen. The patient had stored the sunscreen in her cooler at the beach because she liked the feel of the cool spray in the hot sun. The physician suspected that the spray sunscreen had undergone a chemical reaction from the coldness which caused the numbness. She also suspected that because this is a new type of sunscreen other physicians may soon be seeing patients with this numbness.

The physician wrote up a case report describing how the numbness presented, how and why she concluded it was the spray sunscreen, and how she treated the patient. Later, when other doctors began seeing patients with this numbness, they found this case report helpful as a starting point in treating their patients.

Real-life Examples

Hymes KB. Cheung T. Greene JB. Prose NS. Marcus A. Ballard H. William DC. Laubenstein LJ. (1981). Kaposi's sarcoma in homosexual men-a report of eight cases. Lancet. 2(8247), 598-600.

This case report was published by eight physicians in New York city who had unexpectedly seen eight male patients with Kaposi’s sarcoma (KS). Prior to this, KS was very rare in the U.S. and occurred primarily in the lower extremities of older patients. These cases were decades younger, had generalized KS, and a much lower rate of survival. This was before the discovery of HIV or the use of the term AIDS and this case report was one of the first published items about AIDS patients.

Wu, E. B., & Sung, J. J. Y. (2003). Haemorrhagic-fever-like changes and normal chest radiograph in a doctor with SARS. Lancet, 361(9368), 1520-1521.

This case report is written by the patient, a physician who contracted SARS, and his colleague who treated him, during the 2003 outbreak of SARS in Hong Kong. They describe how the disease progressed in Dr. Wu and based on Dr. Wu’s case, advised that a chest CT showed hidden pneumonic changes and facilitate a rapid diagnosis.

Related Terms

Case Series

A report about a small group of similar cases.

Preplanned Case-Observation

A case in which symptoms are elicited to study disease mechanisms. (Ex. Having a patient sleep in a lab to do brain imaging for a sleep disorder).

Now test yourself!

1. Case studies are not considered evidence-based even though the authors have studied the case in great depth.

a) True b) False

2. When are Case reports most useful?

a) When you encounter common cases and need more information b) When new symptoms or outcomes are unidentified c) When developing practice guidelines d) When the population being studied is very large

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A case report is a detailed report of the diagnosis, treatment, response to treatment, and follow-up after treatment of an individual patient. A case series is group of case reports involving patients who were given similar treatment. Case reports and case series usually contain demographic information about the patient(s), for example, age, gender, ethnic origin.

When information on more than three patients is included, the case series is considered to be a systematic investigation designed to contribute to generalizable knowledge (i.e., research ), and therefore submission is required to the IRB.

For all case reports and case series, a signed HIPAA authorization should be obtained from the patients or their legally authorized representatives for the use and disclosure of their Protected Health Information. The only exception to the requirement for obtaining authorization is if the author of a case report or case series believes that the information is not identifiable; in this case, the author must consult with the Privacy Officer at Boston Medical Center ( [email protected] ) or the HIPAA Privacy Officer of Boston University ( [email protected] ) to seek an expert opinion about the magnitude of the risk of identifying an individual.

For case reports or case series containing more than three patients, the HIPAA authorization should be part of the consent form that is reviewed by the IRB.

For case reports or case series containing three or fewer patients, authors should prepare an authorization form using the following templates and arrange for review as indicated below. The red text in the template should be customized for the specific case report or case series. Please note that for deceased patients, authorization must be obtained from the personal representative, who is the administrator or executor of the patient’s estate.

  • Boston Medical Center ( BMC Case Report HIPAA Authorization Template ) – review by the Privacy Officer at Boston Medical Center ( [email protected] ); a copy of the authorization must be filed in each patient’s medical record.
  • Goldman School of Dental Medicine ( GSDM Case Report HIPAA Authorization Template ) – review by the HIPAA Privacy Officer of Boston University ( [email protected] )


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