Distinguishing case series from cohort studies.
Dekkers OM, Egger M, Altman DG, Vandenbroucke JP.
SourceLeiden University Medical Center, the Netherlands. [email protected]
Abstract
Case series are a commonly reported study design, but the label "case series" is used inconsistently and sometimes incorrectly. Mislabeling impairs the appropriate indexing and sorting of evidence. This article tries to clarify the concept of case series and proposes a way to distinguish them from cohort studies. In a cohort study, patients are sampled on the basis of exposure and are followed over time, and the occurrence of outcomes is assessed. A cohort study may include a comparison group, although this is not a necessary feature. A case series may be a study that samples patients with both a specific outcome and a specific exposure, or one that samples patients with a specific outcome and includes patients regardless of whether they have specific exposures. Whereas a cohort study, in principle, enables the calculation of an absolute risk or a rate for the outcome, such a calculation is not possible in a case series.
PMID: 22213493 [PubMed - indexed for MEDLINE]
Dear Fahmi Khan,
Studies called case series are generally small descriptive studies, tracking patients derived by a health care setting (i.e. register of cases) with a known exposure or receiving the same treatment and examin their outcome. This type of study can describe characteristics or outcomes in a particular group of people, but cannot determine how they compare with people who are treated differently or who do not have the condition. They aloud understanding for example the natural history of a disease. Case series may be consecutive or non-consecutive. They may be confounded by selection bias. They are valuable in epidemiology for studying symptoms and signs and creating case definitions. There are also population-based case series consisting of a population register of cases (serious diseases, deaths, health service utilization)..
Cohort studies (retrospective or prospective), identifies a group of people and follows them over a period of time to see how their exposures affect their outcomes. This type of study is normally used to look at the effect of suspected risk factors that cannot be controlled experimentally, for example the effect of smoking on lung cancer.
Several outcomes could be studied at the same time alouding for incidence estimation and relative risks calculation. Prospective cohort studies are generally expensive, time consuming and not suitable for rare diseases.
Best regards
HL
Absolutely , Jay
Also I found the comment by Hermine , very inclusive ,it looks perfect , Tnx BTW
But I'd like to add some my opinion:
All what has been mentioned indirectly , I think, was the different levels of these two (Case series and Cohort) . Because as far as I've conceived, Case series are not concerning of the "Reason" or neither the "Way" and "Why" it happens! not because of their nature but because it's not necessary on this step (Describing the situation only!) simply to see and collect some bulk of data , while they could be used in conjunction with a cross sectional view (so only will concern about frequencies and prevalences ,,,) or in a trila to see effect of an intervention and so on . although this pure report can be of importance in some cases , so will lead to "reports of case series"
but what is coming in an other level is whether the study is 'control'ed or not , if yes, how is the control process , what is direction of researcher's view to the poin, if 'from reason to outcome' (Cohort) or 'from outcomes to reasons' (Case control)
This way I don't think these two (Case series / Cohort) can be evaluated with each other , since they're different aspects of the matter.
Y.Y
A case series is usually composed of patients, while a cohort includes individuals belonging to a popolation group to be investigated in a (retrospective or perspective) longitudinal framework.
This is the link to the definition of
case series: http://www.cancer.gov/dictionary?CdrID=44006
cohort: http://www.cancer.gov/dictionary?CdrID=561393
adding to the above, the major difference is that in a case series, the individuals (patients) have been exposed to the intervention of interest whereas in a cohort, we are looking at the effect(s) of an agent of exposure (risk factor) in a population
in my opinion,
case series is descriptive study, the observational study , observes Experience of a group of patients with a similar diagnosis,Generally report on new/unique condition,May be useful for rare disorders.
on other hand cohort study is analytic study , useful design for comparing individuals with a known risk factor or exposure with others without the risk factor or exposure .Subjects selected before disease detection,
Can study several outcomes for each exposure. cohort is group of individuals with same characteristics.with the help of cohort study we can calculate relative risk as well as attributable risk.
Absolutely agree with Roya safari
The cohort study allows measure incidence of some desease in two or more grups exposed and not exposed to some riskfactor or protective factor, it sis important that cohort studies allows to understand what happend in the time line. . Case studies are only a description and review of some characteristics using differents approaches (cualicuantitative) fom this case studies emerge important research questions .
Pls refer essential epidemiology book,its is clearly described over there.
Very nice description from Roya Safari. They key thing I tell my students is that there is no comparison group. You cannot calculate incidence rates or relative risks (or odds ratios). Multiple case studies or a case series can provide valuable information. Unfortunately, it is not unusual for a case series to cause an unwarranted stir in the media that can have long-term adverse consequences, as the example of Wakefield et al. and the false vaccine-autism link demonstrated There are good case series and bad, however, and good and bad interpretations of the findings. This is a nice editorial on the subject: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1297135/pdf/jrsocmed00010-0007.pdf
Case series is cross sectional type of observational study where at a particular point of time you describe all of them with out followup. In cohort we follow them for a purticular event.
Hello there,
a short answer to your question would be: a cohort study is always from cause to effect study in regard to a particular risk factor or protective factor which is your study interest. Whereas, a case series is series of similar cases complied to know the smaller difference may be in treatment modalities or any particular causative agent, to say. Furthermore, cohort studies are analytical wherein incidence is obtained but you do not get incidence from case series since it is just a very basic type of descriptive study.
I hope this explanation might help you understand the subject
Case-series is a descriptive study design and as the name suggests, its just a series of cases of any particular disease or disease discrepancy that one might observe in one's clinical practice etc. These cases are described to suggest at best a hypothesis. However, like others have said before me, there is no comparison group so you can't draw too many conclusions about the disease or the disease process. Thus, in terms of generating evidence regarding various aspects of the disease process, a case-series is a very weak study design.
Cohort studies are analytical (as opposed to descriptive) observational studies (as opposed to experiments) and are mostly prospective. Its conducted in the population and mostly a random sample of the study population is classified according to the exposure status and then are followed up for a period of time for the development of a disease to compare the number of exposed people getting the disease vs. the number of unexposed people getting the disease. Through this study design one can calculate population level risks and study multiple disease outcomes for the same exposure. Cohort studies are robust studies and are pretty high on the hierarchy of evidence - just below experiments (or randomized controlled trials/RCTs).
All problems can be solved if you look at the site Supercourse єpidemiologii and public health http://www.pitt.edu/~super1/lecture/lec11821/index.htm Good luck in your search, there's my lectures on the epidemiology of radiation and nuclear energy
A case series is a descriptive type of study just describing few cases; and used to formulate a hypothesis regarding any association between a certain exposure and out come. There is no followup of cases or analysis conducted to see statistical association between exposure and the out come.
A cohort study is an analytic study conduced to test an already formulated hypothesis regarding an exposure and out come. There you select a group of people known as a cohort and follow them up for a certain period of time which would be enough to have the out come;assess the incidence of out come in the exposed and non exposed people to the risk factor of interest. There you can do statistical analysis to have relative risk and the attributable risk.
a cohort study is often undertaken to obtain evidence to try to refute the existence of a suspected association between cause and effect, The study groups follow a group of people who do not have the disease for a period of time and see who develops the disease (new incidence). The cohort cannot therefore be defined as a group of people who already have the disease.
Case studies typically examine the interplay of all variables in order to provide as complete an understanding of an event or situation as possible.refers to the collection and presentation of detailed information about a particular participant or small group, frequently including the accounts of subjects themselves
A case series is a 'report' on a series of cases with similarities - in dentistry often used to illustrate treatment for a particular condition, and is not a statistically analysed piece of research, whilst a cohort study is
in a cohort study, the investigator starts with a group of individuals apparently free of disease(s), divides them into those exposed to a possible factor and those not exposed, and then follows them forward through time in order to determine the incidence (or mortality) rate among the exposed and incidence (or mortality) rate among the unexposed.
A case series is Collections of individual case reports that often used as an early means to identify the beginning of an epidemic or a new disease, it may lead to the generation of a new hypothesis. HIV was first described through a case series study that described a series of pneumosystis carnii cases. Its chief limitation is the lack of an appropriate comparison group
I have not read all of the posts here, but there seems to be a fair degree of confusion between a case series and a case study. The descriptive, hypothesis generating study of a group of similar cases or individuals is repeatedly described above as a case series. It is not, it is a case study.
A case series is an analytical study containing cases only. Each case acts as its own control in non-risk periods, obviating the need for a comparison group of non-cases. Forthis approach to work there are a few conditions. Most notably, there must be a defined short term risk period related to exposure, so that cases occurring inside the risk period can be attributed to the exposure, and those outside are not atrributed to the exposure.
A useful reference for this topic is:
Farrington CP, Whitaker HJ. Semi-parametric analysis of case series data. J R Stat Soc [Ser A] 2006;55 (5): 553-594.
Case study is a qualitative approach (as Roya said you can describe the case using the narratives and/or some data, but the latter are not reported per se but as part of the description ). A case series study could concern different topics within a common issue (a veryh good example is "Case Studies in Food Safety and Environmental Health" Peter Ho and Margarida Vieira editors, Springer. Each case study refers to one aspect of food safety and all refers to the general theme).
A cohort study is a quantitative approach measuring variables on a group of people followed along time. You can consult references reported in http://en.wikipedia.org/wiki/Cohort_study to have a general idea
Childs Nerv Syst. 2014 Aug;30(8):1321-32. doi: 10.1007/s00381-014-2460-1.
Case series and descriptive cohort studies in neurosurgery: the confusion and solution.
Esene IN1, Ngu J, El Zoghby M, Solaroglu I, Sikod AM, Kotb A, Dechambenoit G, El Husseiny H.
Author information
Abstract
BACKGROUND:
Case series (CS) are well-known designs in contemporary use in neurosurgery but are sometimes used in contexts that are incompatible with their true meaning as defined by epidemiologists. This inconsistent, inappropriate and incorrect use, and mislabeling impairs the appropriate indexing and sorting of evidence.
METHOD:
Using PubMed, we systematically identified published articles that had "case series" in the "title" in 15 top-ranked neurosurgical journals from January 2008 to December 2012. The abstracts and/or full articles were scanned to identify those with descriptions of the principal method as being "case series" and then classified as "true case series" or "non-case series" by two independent investigators with 100 % inter-rater agreement.
RESULTS:
Sixty-four articles had the label "case series" in their "titles." Based on the definition of "case series" and our appraisal of the articles using Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines, 18 articles (28.13 %) were true case series, while 46 (71.87 %) were mislabeled. Thirty-five articles (54.69 %) mistook retrospective (descriptive) cohorts for CS. CS are descriptive with an outcome-based sampling, while "descriptive cohorts" have an exposure-based sampling of patients, followed over time to assess outcome(s). A comparison group is not a defining feature of a cohort study and distinguishes descriptive from analytic cohorts.
CONCLUSION:
A distinction between a case report, case series, and descriptive cohorts is absolutely necessary to enable the appropriate indexing, sorting, and application of evidence. Researchers need better training in methods and terminology, and editors and reviewers should scrutinize more carefully manuscripts claiming to be "case series" studies.
http://www.ncbi.nlm.nih.gov/pubmed/24938735
Z. Liu....read this paper
https://www.researchgate.net/deref/http%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpubmed%2F24938735
https://www.researchgate.net/deref/http%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpubmed%2F24938735
A case series include patients with both a specific outcome and a specific exposure, or include patients with a specific outcome regardless of exposures. Absolute risk or rates for the outcome calculations are not possible.
If I have two groups in a case series study, What are the best statistics method to try to compare the outcomes
Michel Hovsepian: http://www.ncbi.nlm.nih.gov/pubmed/24938735
http://www.ncbi.nlm.nih.gov/pubmed/24938735
Dear Fahmi,
I think cohort study, you are able to determine the relative risk (ratio of probability of an event occuring. For example, smokers are 10 times as likely as non-smokers to develop lung cancer. Case series you are unable to determine this risk.
Every study design has its own importance, I think case series and cohort design with some differences are important for a public health expert, so that they understand the study designs in totality, Thanks for all, providing detail information about the question in a simple way so that all can understand this topic well.
Case series and descriptive cohort studies in neurosurgery: the confusion and solution.
Link: Article Case series and descriptive cohort studies in neurosurgery: ...
Esene IN, Ngu J, El Zoghby M, Solaroglu I, Sikod AM, Kotb A, Dechambenoit G, El Husseiny H. Case series and descriptive cohort studies in neurosurgery: the confusion and solution. Childs Nerv Syst. 2014 Aug;30(8):1321-32. doi: 10.1007/s00381-014-2460-1. PubMed PMID: 24938735.
In case series , investigator select patient already with disease and follow and examine then while in cohort studies it always start with individual who are only exposed to a factor under study but not yet developed disease and then follows these patients to see if the outcome develop or not.
Case series study is descriptive only (no comparison group). It includes group of patients with certain disease or with abnormal sign and symptom. while cohort study include healthy people but they exposed to certain exposure and follow them for certain period to see if the outcome develop or not (incidence study).
Hi everyone! what about single arm cohort study?.... is really a comparison group a main characteristic for cohort? . My principal doubt is: how can i differenciate a case series and a single arm retrospective cohort study.Thanks a lot!
I think the issue here is not about a comparison group, and there could be descriptive cohorts. In my opinion, cohort is always about an exposure and following it for one or more outcomes, whereas case series is a descriptive synthesis of characteristics of disease or its outcome in different patients, not necessarily related to a single exposure or a clearly defined outcome. Case series flows by its own characteristics whereas for descriptive cohorts, there are fixed/defined exposures and outcomes.
What is the number of cases in case report and case series?
Reference required.
Thanks.
Dear all, I hope this article may aid in answering your question.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2998589/
hey Guys, can anybody tell me that how we can differentiate/ what are the main criterias to so say this study is cohort or Case-control?
I agree with Dheeraj Shah that the main difference between case series and cohort study is not about the presence of a comparison group. This is the most useful definition that I found so far:
"Case series are descriptive with an outcome-based sampling, while "descriptive cohorts" have an exposure-based sampling of patients, followed over time to assess outcome(s). A comparison group is not a defining feature of a cohort study and distinguishes descriptive from analytic cohorts."
The term "descriptive cohorts" here refers to retrospective cohort studies. "Analytic cohort studies" include a control group.
Source: 10.1007/s00381-014-2460-1
@Ashenafi Gebregiorgis
Case control moves from disease to exposure while cohort studies move from exposure to disease
I am also struck by this same question as a pathologist and as a systematic reviewer. My quick answer would be : if the case series provides with the outcome information and follow-up time, it is a cohort study. I am not talking about strength of the designs etc. It's just a concept. The same group of patients without outcome information (with only co-variates) would be case series (descriptive stat study) by design. There might be some deviations from this, but the idea is the same. Case control design would include clearly defined groups for comparison (cases and controls) at fixed point (and does not include a flow of outcomes over time).
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