However, the extension to continuous outcome measures does require further discussion. Programme Grant from the Health Research Council of New Zealand (The Centre for Public Health Research). The main designs of epidemiological studies are observational study, descriptive study, analytical (cross-sectional, cohort, and study of "case-control") study, hospital control design, communal (public) study, and experimental research. Epidemiology. In the presentation of prevalence studies above, the health outcome under study was a ‘state’ (e.g. Analytic epidemiology incorporates a comparison group in its study designs. Study design can be mainly classified into two types, Observational study design and Experimental study design. the ‘survivors’ (those who did not develop the outcome at any time during the follow-up period). The fundamentals of epidemiologic study design have remained a matter of confusion. is the study of the distribution, determinants, and deterrents… upon. Epidemiological studies of exposures to disinfected drinking-water and to specific DBPs are evaluated in sec- … Epidemiologic study types have their roots in the concepts of scientific experimenta- tion. epi. demos. Search for other works by this author on: Classification schemes for epidemiologic research designs, Principles of study design in environmental epidemiology, Occupational and Environmental Respiratory Disease, Research Methods in Occupational Epidemiology. This classification system has previously been proposed by Greenland and Morgenstern (1988)1 and Morgenstern and Thomas (1993),2 all of whom followed previous authors3,4 in rejecting directionality (i.e. logos. Descriptive studies are used to describe exposure and disease in a population, and can be used to generate hypotheses, but they are not designed to test hypotheses. The extension to continuous exposure measures requires minor changes to the data analysis, but it does not alter the 4-fold categorization of study design options presented above. In this course, you will learn about the main epidemiological study designs, including cross-sectional and ecological studies, case-control and cohort studies, as well as the more complex nested case-control and case-cohort designs. Epidemiologic Study Designs •Descriptive studies –Seeks to measure the frequency of disease and/or collect descriptive data on risk factors •Analytic studies –Tests a causal hypothesis about the etiology of disease •Experimental studies –Compares, for example, treatments . Epidemiologists must have a sound understanding of the principles of study design. 2. Corresponding to these three measures of disease occurrence, the three ratio measures of effect used in incidence studies are the ‘rate ratio’, ‘risk ratio’ and ‘odds ratio’. The scheme presented here involves ‘ideal types’ that are not always followed in practice and mixes can occur along both axes. historical records on past asbestos exposure levels, birthweight recorded in hospital records), or integrated over time (e.g. upon. Also note that some prevalence studies may involve sampling on exposure status, just as some incidence studies may involve such sampling. Cross-sectional Case-Control Cohort TIME EXPOSURE OUTCOME EXPOSURE OUTCOME EXPOSURE OUTCOME . Determining individual trajectories of joint space loss: improved statistical methods for monitoring knee osteoarthritis disease progression. Findings from a hypothetical prevalence case–control study based on the population represented in Table 3. because it measures the population burden of disease. Types of Epidemiological Studies The study of disease distributions in the populations and the factors that influence this distribution is called as Epidemiology. The method of calculation of the OR is the same as for any other case–control study, but special formulas must be used to compute confidence intervals and P-values.15, The third approach is to select controls longitudinally throughout the course of the study, an approach now usually referred to as ‘density sampling’7 (or concurrent sampling11); the resulting OR will estimate the rate ratio in the source population (which is 2.00 in Table 3). What does the odds ratio estimate in a case–control study? Finally, it should be noted that there are other possible axes of classification or extension of the above classification scheme. Observational studies – we do not interfere in the process of the disease, but simply observe the disease and the associated factors. 2007).As compared with experimental studies, in which exposures are randomly assigned to study participants, in observational designs, … óÀ._ÓY²LW/Ü°Õé|ÀNI¡ë¸jW÷¸¨ÊÈ5¨(tBÐh¶ÙÀïc¬%=kEi4)ö^ÄÆ0J²,ÍæÅw:;6YË!W!ógÃÆëVVjõ¾ ¥wiQHÍi'A ÀNVe9«/Ì.6U£²:#0Ø *K«Ü¶ >-ZçiR¬hÓ)GÅÖ. A well designed randomised control trial provides the strongest evidence of any epidemiological study design that a given intervention has a postulated effectiveness and is safe. Analytic epidemiology studies are conducted to obtain a relationship between different exposures to the disease condition and to obtain its outcome in a measurable manner. Basic Epidemiology Study Designs in Epidemiologic Research Thomas Songer, PhD Modified by Supercourse team * Descriptive study designs include case reports, case series, incidence studies, and ecologic studies. ‘descriptive’ studies of national death rates. Finally, it clarifies the range of possibilities and problems of different study designs, particularly by emphasizing that the issues of the timing of data collection are not unique to case–control studies and are not crucial in terms of classification of epidemiological study designs. non-hypertensive, mild hypertension, moderate hypertension and severe hypertension) or may be represented by a continuous measurement (e.g. Table 4 shows data from a prevalence study of 20 000 people (this example has been designed to correspond to the incidence study examples given above, assuming that the exposure has no effect on disease duration and that there is no immigration into or emigration from the prevalence pool, so that no one leaves the pool except by disease onset, death or recovery7). is the study of the distribution, determinants, and deterrents… epi. One special type of longitudinal study is that of ‘time series’ comparisons in which variations in exposure levels and symptom levels are assessed over time with each individual serving as their own comparison. Outcome. In this instance, the controls will estimate the exposure odds in the source population at the start of follow-up, and the OR obtained in the case–control study will therefore estimate the risk ratio in the source population (which is 1.90 in Table 3). In this course, you will learn about the main epidemiological study designs, including cross-sectional and ecological studies, case-control and cohort studies, as well as the more complex nested case-control and case-cohort designs. Cross-sectional studies provide a snapshot of a population by determining both exposures and outcomes at one time point. View this table: View inline; View popup; Table 1 . Introduction to epidemiology Study designs. The latter may have been measured at the time of data collection [e.g. Cohort. population or individual). Experimental studies – deliberate intervention is made and the effect of such intervention is observed. Educational inequalities and regional variation in colorectal cancer survival in Finland. prospective/retrospective or from exposure to outcome vs from outcome to exposure) as a key feature for distinguishing study designs. Just as an incidence case–control study can be used to obtain the same findings as a full cohort study, a prevalence case–control study can be used to obtain the same findings as a full prevalence study in a more efficient manner. Care transitions in the first 6 months following traumatic brain injury: Lessons from the CENTER-TBI study. They differ in whether their denominators represent person–time at risk, persons at risk or survivors. It should first be emphasized that all epidemiological studies are (or should be) based on a particular population (the ‘source population’) followed over a particular period of time (the ‘risk period’). asthma and diabetes), incidence may be difficult to measure without very intensive follow-up. Studies could involve observing the incidence of the ‘event’ of acquiring the disease state (e.g. This will enable us to estimate the exposure odds of the non-cases, and the OR obtained in the prevalence case–control study will therefore estimate the POR in the source population (2.00).17 Alternatively, if the PR is the effect measure of interest, controls can be sampled from the entire source population (i.e. The studies often include epidemiological research. Epidemiological study designs, sources of systematic and random error (bias), and guidelines for assessing the causality of associations are discussed in section 5.1. Experimental studies – deliberate intervention is made and the effect of such intervention is observed. Nonetheless, exposure information may include factors that do not change over time (e.g. Findings from a hypothetical incidence case–control study based on the cohort in Table 1, In incidence case–control studies, the relative risk measure is the ‘odds ratio’. The majority of life course studies are observational studies (Pickles et al. Three measures of disease occurrence are commonly used in incidence studies.9 Perhaps the most common measure is the person–time ‘incidence rate’; a second measure is the ‘incidence proportion’ (average risk), which is the proportion of study subjects who experience the outcome of interest at any time during the follow-up period. One option is to select controls at random from those who do not experience the outcome during the follow-up period, i.e. There is no restriction on when the exposure information is collected or whether it relates to current and/or historical exposures. The basic study designs presented above can be extended by the inclusion of continuous exposure data and continuous outcome measures. a series of linked cross-sectional studies in the same population). Experimental epidemiological study can also be referred to as clinical trials. These three measures of disease occurrence all involve the same numerator: the number of incident cases of disease. The case report is the most elementary study design in the literature. Table 2 shows the findings of a hypothetical incidence study involving 10 000 people who are exposed to a particular risk factor and 10 000 people who are not exposed. For example, rather than comparing the incidence of hypertension (as in an incidence study) or the prevalence at a particular time (as in a prevalence study), or the mean blood pressure at a particular point in time (as in a cross-sectional study), a longitudinal study might involve measuring baseline blood pressure in exposed and non-exposed persons and then comparing changes in blood pressure (i.e. A RCT provides the best type of epidemiological study from which to draw conclusions on causality. 42 Terms. the change from the baseline measure) over time in the two groups. In epidemiology, the study mainly deals only with the distribution of diseases/conditions in humans and with the factors influencing the distribution and the frequency of diseases. Ethical considerations naturally prevent us from allocating potentially harmful exposures on an experimental basis in human populations. having or not having hypertension). a group of workers exposed to a particular chemical), then the study may be termed a ‘cohort study’ or ‘follow-up study’ and the former terminology will be used here. However, they are often very expensive in terms of time and resources, and the equivalent results may be achieved more efficiently by using an incidence case–control study design. Most case–control studies involve density sampling (often with matching on a time variable such as calendar time or age), and therefore estimate the incidence rate ratio without the need for any rare disease assumption.16, Incidence studies are usually the preferred approach to studying the causes of disease, but they often involve lengthy periods of follow-up and large resources.17 Also, for some diseases (e.g. 2. Study design. Early descriptions of the case–control approach were usually of this type.12 These descriptions emphasized that the OR was approximately equal to the risk ratio when the disease was rare (in Table 3; this OR = 2.11). In the epidemiological literature, studies are typically grouped into observational and experimental studies (Ahern and Leslie 2014; Pickles et al. Such a study would on an average achieve the same findings as the full cohort study (Table 2), but would be considerably more efficient, since it would involve ascertaining the exposure histories of 5530 people (2765 cases and 2765 controls) rather than 20 000 people. 80. Cohort studies identify the study groups based on the exposure and, then, the researchers follow up study participants to measure outcomes. Analytic Epidemiological studies are mainly categorized as experimental and observational studies. The prevalence is 0.0909 in the exposed group and 0.0476 in the non-exposed group, and the prevalence ratio (PR) and prevalence odds ratio (POR) are 1.91 and 2.00, respectively. all workers at a particular factory or all persons living in a particular geographical area) and incidence studies involving sampling on the basis of exposure, since the latter procedure merely redefines the study population (cohort).4, Findings from a hypothetical cohort study of 20 000 persons followed for 10 years. In particular, if obtaining exposure information is difficult or costly, then it may be more efficient to conduct a prevalence case–control study by obtaining exposure information on some or all of the prevalent cases and a sample of controls selected from the non-cases. Once this two-dimensional classification system has been adopted, then there are only four basic study designs (Table 1):2,5,6 (i) incidence studies; (ii) incidence case–control studies; (iii) prevalence studies; and (iv) prevalence case–control studies (Rothman et al.7 use the terms ‘incident case–control study’ and ‘prevalent case–control study’ where the adjective refers to the incident or prevalent cases2). It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide, This PDF is available to Subscribers Only. A systematic review. For full access to this pdf, sign in to an existing account, or purchase an annual subscription. Description of epidemiological study designs (adapted from Detels 8) The more appropriate the study design, the more convincing the evidence that will be produced. gender) or change in a predictable manner (e.g. STUDY DESIGNS - An Overview Dr Lipilekha Patnaik professor, Community Medicine Institute of Medical Sciences & SUM Hospital, Siksha ‘O’ Anusandhan deemed to be University Bhubaneswar, ODISHA, INDIA E mail– drlipilekha@yahoo.co.in 1 in a manner analogous to case–cohort sampling) and the resulting prevalence case–control ‘OR’ will estimate the PR in the source population. In this article, I will briefly illustrate these four different study designs for dichotomous outcomes; I then briefly consider the extension of this classification to include studies with continuous exposure or outcome measures and I briefly mention other possible axes of classification. 6003 Epidemiology: Epidemiological Study Design. The basic epidemiological study designs are cross-sectional, case-control, and cohort studies. Epidemiologists employ a range of study designs from the observational to experimental and generally categorized as descriptive, analytic (aiming to further examine known associations or hypothesized relationships), and experimental (a term often equated with clinical or community trials of treatments and other interventions). Neil Pearce, Classification of epidemiological study designs, International Journal of Epidemiology, Volume 41, Issue 2, April 2012, Pages 393–397, https://doi.org/10.1093/ije/dys049. Longitudinal studies (cohort studies) involve repeated observation of study participants over time. In many prevalence studies, information on exposure will be physically collected by the investigator and at the same time information on disease prevalence is collected. Findings from a hypothetical prevalence study of 20 000 persons. There is no definitive approach to classifying types of epidemiological studies, and different classification schemes may be useful for different purposes. Confounders & effect modifiers (covariates) 3 Exposures & Outcomes A major goal of epi research is to explain patterns of disease occurrence and causation (etiology) Epi measurements are aimed at quantifying 3 things: exposures, … The effect measure that the odds ratio (OR) obtained from this case–control study will estimate depends on the manner in which controls are selected. Case–control designs in the study of common diseases: updates on the demise of the rare disease assumption and the choice of sampling scheme for controls, A method of estimating comparative rates from clinical data: applications to cancer of the lung, breast and cervix, Relationship of oral contraceptives to cervical carcinogenesis, A case–cohort design for epidemiologic cohort studies and disease prevention trials, Adjustment of risk ratios in case-base studies (hybrid epidemiologic designs), On the need for the rare disease assumption in case–control studies. Population has been formally defined and enumerated ( e.g outcome to exposure ) as a key feature for distinguishing designs... Types ’ that are not always followed in practice and mixes can occur along both axes of cross-sectional. Outcome measures outcome exposure outcome exposure outcome, it is often more practical to study the ‘ prevalence ’ prevalence... 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