Proposal for classifying the different types of descriptive epidemiological studies

Merchán-Hamann, Edgar; Tauil, Pedro Luiz

doi:10.1590/s1679-49742021000100026

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Epidemiology applications • Epidemiol. Serv. Saúde 30 (1) • 2021 • https://doi.org/10.1590/s1679-49742021000100026 linkcopy

Proposal for classifying the different types of descriptive epidemiological studies

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Abstract

Descriptive epidemiological studies are of relevance, given that there are inconsistencies in the literature with regard to their nomenclature and classification. We reviewed 19 international and six national academic textbooks on epidemiology, where the main criterion was to have them available in order to undertake an in-depth review of chapters on descriptive epidemiology and study types. In 11 books, the authors prioritize analytical studies. Twelve foreign texts and two from Brazil include descriptive studies, although the majority did not specifically refer to a category with this name. We propose a classification based on the answers to research questions, including the following types of study: case report, case series, clinical cohort, prevalence study, incidence study (cohort) and descriptive ecological study. We discuss potential uses, implementation of novel data analysis methods and their relevance in health surveillance.

Keywords: Epdemiological Studies; Descriptive Studies; Classification

Authors	Year	Reference
MacMahon B, Pugh TE.	1970	Epidemiology, principles and methods. 1^st ed. Boston: Little, Brown and Company.
Mausner JS, Bahn AK.	1974	Epidemiology. An Introductory text. 1^st ed. Philadelphia: W. B. Saunders Company.
Goodman RA, Peavy JV.	1996^a	Describing epidemiologic data. In: Gregg MB. Field epidemiology. 1^st ed. Oxford: Oxford University Press; p. 60-80.
Buehler JW, Dicker RC.	1996^a	Designing studies in the field. In: Gregg MB. Field epidemiology. 1^st ed. Oxford: Oxford University Press; p. 81-91.
Beaglehole R, Bonita R, Kjellström T.	2001	Epidemiologia básica. 2ª ed. São Paulo: Organização Mundial da Saúde e Livraria Santos Editora.
Friis RH, Sellers TA.	2004	Epidemiology for public health practice. 3^rd ed. Mississauga, ON: Jones and Bartlett Publishers.
Hulley SB, Cummings SR, Browner WS, Grady D, & Newman TB.	2008	Delineando pesquisas clínicas: uma abordagem epidemiológica. 3^a ed. Porto Alegre: Artmed.
Rothman KJ, Greenland S, Lash TL.	2008	Modern epidemiology. 3^rd ed. Baltimore: Wolters Kluwer/Lippincott Williams & Wilkins; Chapter 6, Types of epidemiologic studies; p. 87-99.

Inclusion of descriptive studies	Authors	Years	References	Studies cited	Notes
No separate chapter on descriptive studies; specific descriptive studies are cited in some chapters, or descriptive studies are cited using other names for them.	Gordis L. 2014.	2014	Epidemiology. 5^th ed. Philadelphia, PA: Elsevier / Saunders.	The author addresses clinical research without comparison groups which involve patient follow-up (patient cohort), case studies and “case series”.	Two chapters, “Natural history of disease: ways of expressing prognosis” and “Assessing preventive and therapeutic measures”, address in detail, respectively, clinical studies and their outcomes (cure, control and death), and measurement methods (life table and survival analysis).
	Kleinbaum DG, Kupper LL, Morgenstern H.	1982	Epidemiologic research: Principles and quantitative methods. 1^st ed. New York: Van Nostrand Reinhold; Chapter 3, Types of epidemiologic research; p. 40-50.	Mention of descriptive studies is included in “observational studies”. In this context, a descriptive study is defined as one which is conducted “when little is known about the occurrence, natural history or determinants of the disease”. Its objectives would therefore be to determine the frequency or temporal trend of a disease in a specific population and to formulate etiological hypotheses.	In their conceptualization of epidemiological studies, the authors note that their characterization depends on the level on which they are implemented, whereby this “level” is related to the period of the disease’s natural history, adopting Leavell & Clark’s model of levels of prevention. The authors recall that epidemiological research has the following objectives: describe, explain, predict and control.
	Abramson JH, Abramson ZH.	2008	Research methods in community medicine. Surveys, Epidemiological research, Programme evaluation, Clinical trials. 6^th ed. West Sussex: John Wiley & Sons; Chapter 2, Types of investigation; p. 13-34.	Mention is made of two types of studies in the item entitled Descriptive surveys: longitudinal studies, which examine changes, such as “child growth and development, changes in the suicide rate, natural history of a disease or a study of the occurrence of new cases of a disease or deaths in the population”, and cross-sectional studies. The authors include a “clinical studies” category to refer to the study of “characteristics or evolution of a series of patients”.	According to these authors, descriptive studies describe the “status of a disease” in the population and its distribution in relation to sex, age, religion, among other variables. The term survey is mentioned, to which the authors attribute the following meanings, among others: “descriptive study of population characteristics”, “household survey” or “field survey”.
	Jekel JF, Katz DL, Elmore JG.	2005	Epidemiologia, Bioestatística e medicina Preventiva. 2ª ed. Porto Alegre: Artmed; Chapter 5, Delineamentos comuns de pesquisa usados em epidemiologia; p.88-99.	Cross-sectional surveys that collect data on frequency of disease risk factors and disease prevalence; interview surveys; mass screening programs.	The authors indicate the existence of “observational designs for generating hypotheses” without using the term “descriptive”.
	Lilienfeld DE, Stolley PD.	1994	Foundations of epidemiology. 3^rd ed. Oxford: Oxford University Press.	The authors differentiate between “demographic” studies (of mortality or morbidity) and “epidemiological” studies. The former detect trends over time and different distribution patterns, according to the people and the place in which deaths or cases of a disease occur.	Studies which the authors call “epidemiological” studies, correspond, roughly speaking, to analytical studies (whether they be observational or experimental).
No separate chapter on descriptive studies; specific descriptive studies are cited in some chapters, or descriptive studies are cited using other names for them.	Szklo M, Nieto FJ.	2007	Epidemiology. Beyond the basics. 2^nd ed. Boston: Jones and Bartlett Publishers; Chapter 1, Basic study designs in analytical epidemiology; p. 3-43.	The authors refer to both descriptive epidemiology and analytical epidemiology. Descriptive epidemiology uses available data to examine the way in which rates (e.g. mortality rates) behave in relation to demographic variables (variables obtained via census).	Based on detection of non-uniform distributions, according to the authors, epidemiologists define “high-risk groups” for prevention purposes and also for generating causal hypotheses.
	Fletcher RH, Fletcher SW, Wagner EH.	1996	Epidemiologia clínica: elementos essenciais. 3ª ed. Porto Alegre: Artes Médicas.	There is no section on descriptive studies. However, in the chapter “Estudando casos (Studying cases)”, the authors mention case reports, case series and case-control studies.	These authors suggest that 10 patients is the number that is the criterion for separating case reports from case series. When describing case series they say that: “it is a study of a larger group of patients (e.g. 10 or more) with a particular disease”.
	Olsen J, Basso O.	2015	Study Design. In: Olsen J, Greene N, Saracci R, Trichopoulos D. Teaching epidemiology. A guide for teachers in epidemiology, public health and clinical medicine. 4^th ed. Oxford: Oxford University Press; 37-55.	There is no section on descriptive studies. However, in the chapter “Study design”, the authors mention surveys, defining them as prevalence studies in a defined population.	Mention is also made of “case-only studies” and “case-cross over studies”, without going into their characterization as descriptive studies.
Contains a section on descriptive studies.	Hennekens CH, Buring JE.	1987	Descriptive studies. In: Hennekens CH, Buring JE, Mayrent SL. Epidemiology in medicine. 1^st ed. Boston: Little, Brown and Company; p. 101-131.	Correlation studies (equivalent to analytical ecological studies); case reports; case series; and cross-sectional surveys, which can only describe exposures and/or outcomes, or move on to the category of analytical studies in order to test, even with limitations, hypotheses of association.	The publication has a chapter dedicated to descriptive studies, which they define as studies that describe patterns of disease occurrence in relation to people, time and place variables.
	Williams CFM, Nelson KE.	2007	Chapter 3. Study designs. In: Nelson KE, Williams CFM. Infectious epidemiology: Theory and practice. 2^nd ed. Toronto: Jones and Barlett Publishers; p. 63-117.	Case reports; case series; and ecological studies (analytical).	The authors differentiate between two dimensions, i.e. clinical and population-based.
	Koepsell TD, Weiss NS.	2003	Epidemiologic methods. Studying the occurrence of illness. 1^st ed. New York: Oxford University Press; Chapter 5, Overview of study designs; p. 93-115.	Case reports; case series; “rate-based descriptive studies” (studies in which cases in a given population are combined with denominators of the same population), based on data collected from continuous records (health surveillance and its systems), or from periodical health surveys.	The authors admit the existence of descriptive studies, the main characteristics of which, according to them, is the fact of their being conducted without a specific hypothesis. The authors differentiate between two dimensions, i.e. clinical and population-based.
	Colimon K-M.	1990	Fundamentos de epidemiología. 1ª ed. Madrid: Ediciones Díaz de Santos; Capítulo 6, Estudios descriptivos; p. 87-112.	Morbidity surveys (encuestas de morbilidad); prevalence surveys (encuestas de prevalencia); population sampling studies; studies of a segment or category not representative of a population; and institutional studies.	No mention is made of studies in the clinical sphere.

Authors	Years	References
Forattini O.	1970	Epidemiologia geral. São Paulo: Edgard Blucher, Editora da Universidade de São Paulo.
Benseñor IM, Lotufo PA.	2005	Epidemiologia. Abordagem prática. 1ª ed. São Paulo: Sarvier Editora de Livros Médicos. Capítulo 5, Principais desenhos de estudo - conceitos gerais; p. 63-89.
Bloch KV, Coutinho ESF.	2009^a	Fundamentos da pesquisa epidemiológica. In: Medronho RA, Bloch KV, Luiz RR, Werneck GL. Epidemiologia. 2ª ed. São Paulo: Atheneu. p.173-179.
Medronho RA.	2009^a	Estudos ecológicos. In: Medronho RA, Bloch KV, Luiz RR, Werneck GL. Epidemiologia. 2ª ed. São Paulo: Atheneu. p.265-274.
Almeida-Filho N, Barreto ML.	2011^a	Desenhos de pesquisa em epidemiologia. In: Almeida-Filho N, Barreto ML. Epidemiologia & Saúde. Fundamentos, métodos, aplicações. 1ª ed. Rio de Janeiro: Guanabara Koogan. Capítulo 14, Desenhos de pesquisa em epidemiologia; p. 165-174.
Santana VS, Dourado I, Ximenes R, Barreto S.	2011^a	Modelos básicos de análise epidemiológica. In: Almeida-Filho N, Barreto ML. Epidemiologia & Saúde. Fundamentos, Métodos, Aplicações. 1ª ed. Rio de Janeiro: Guanabara Koogan. Capítulo 21, Modelos básicos de análise epidemiológica: p. 232-251.

Authors	Years	References	Studies mentioned	Notes
Pereira MG.	1995	Epidemiologia: Teoria e Prática. 1ª ed. Rio de Janeiro: Guanabara-Koogan. Capítulo 12, p. 269-288.	The author lists nine types of epidemiological studies, the first four of which are descriptive: case study, case series, incidence study and prevalence study (descriptive cross-sectional).	According to the author, the first two types are not truly epidemiological studies, because there would only be “cases” in the study population. In turn, the next two types would relate “cases” to the respective population.
Zanetta DMT.	2004	Delineamento de estudos em medicina. In: Massad E, Menezes RX, Silveira PSP, Ortega NRS. Métodos Quantitativos em Medicina. 1ª ed. Barueri, SP: Editora Manole. p. 389-421.	When listing “designs of studies in medicine”, the author includes case report, case series and cross-sectional or prevalence studies, cited as descriptive studies. The remainder are analytical studies.	Zanetta recognizes the existence of descriptive studies. For this author, descriptive studies “report prevalence rates or describe a situation which appears to be abnormal”. They are useful for health service planning and for generating hypotheses “in the experimental field”.

Study scope		Type of study	Epidemiological measurements
1	Clinical
	1.1	Case report	-
	1.2	Case series	Proportion of cases
	1.3	Descriptive clinical cohort	Incidence of events, case fatality
2	Population/community-based
	2.1	Descriptive observational prevalence studies	Prevalence
	2.2	Descriptive observational incidence or descriptive cohort studies	Incidence, mortality
	2.3	Descriptive ecological studies	Incidence coefficients or mortality coefficients in the population

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[TFN3] a) chapters in the same book.