Case control studies are observational because no intervention is attempted and no attempt is made to alter the course of the disease. The goal is to retrospectively determine the exposure to the risk factor of interest from each of the two groups of individuals: cases and controls. These studies are designed to estimate odds. Care should be taken to avoid confounding, which arises when an exposure and an outcome are both strongly associated with a third variable. Controls should be subjects who might have been cases in the study but are selected independent of the exposure. Cases and controls should also not be "over-matched." A patient with the disease or outcome of interest. A study that compares patients who have a disease or outcome of interest (cases) with patients who do not have the disease or outcome (controls), and looks back retrospectively to compare how frequently the exposure to a risk factor is present in each group to determine the relationship between the risk factor and the disease. This study would be retrospective in that the former lifeguards would be asked to recall which type of sunscreen they used on their face and approximately how often. This could be either a matched or unmatched study, but efforts would need to be made to ensure that the former lifeguards are of the same average age, and lifeguarded for a similar number of seasons and amount of time per season. Teo, K. K. Ounpuu, S. Hawken, S. Pandey examples of five paragraph essay, M. Valentin, V. & Hunt, D. et al. (2006). Tobacco use and risk of myocardial infarction in 52 countries in the INTERHEART study: A case-control study. Lancet, 368(9536), 647-658. by Annette Gerritsen, Ph.D. Cohort studies begin with a group of people (a cohort) free of disease. The people in the cohort are grouped by whether or not they are exposed to a potential cause of disease. The whole cohort is followed over time to see if the development of new cases of the disease (or other outcome) differs between the groups with and without exposure. The article is clear. However administration cover letter no experience, I would like you to help me on the methodology of a study that I would like to do. Cohort studies work well for rare exposures –you can specifically select people exposed to a certain factor. But this design does not work for rare diseases –you would then need a large study group to find sufficient disease cases. This article is really well written .It helped me alot to solve my confusion.thanks You can then calculate how many diarrhea cases there were among those children using the suspected water source and those using other sources of water supply (cumulative incidence of diarrhea). How to compare the cumulative incidence rates of the two groups, in order to conclude whether the suspected water source is a risk factor for the disease or not, will be discussed in a future blog. An added advantage is that you can examine a range of outcomes/diseases caused by one exposure (e.g. heart disease, lung disease, renal disease caused by smoking). Previously in this series I have given an overview of the main types of study design and the techniques used to minimise biased results. In this article I describe more fully case control studies, their uses, advantages and limitations. Case-control studies are quick and cheap and are particularly suited to the study of rare diseases as the diseased are selected at the outset of the study. Exposure measurements are reliant either on memory where cases and controls are interviewed retrospectively, and/or medical records. Exposure estimates are therefore vulnerable to recall bias; commonly those with the disease are more likely to remember exposure than those without, interview or measurement bias; where the interviewer interviews or reports findings systematically differently between cases and controls and confounding factors. Interview and measurement bias can be overcome by including blinding in the design so that they do not know who is a case and who is a control at the time of interview. 1 Dental Health Services Research Unit social work essay writers, University of Dundee, Dundee, Scotland, UK Controls should come from the same population at risk of disease, should not have the disease and should be representative of the target population. Selecting controls often proves harder than cases and requires great care in the prevention of bias. A sampling frame of hospital patients is often used to select controls, however risk factors such as diet and smoking are commonly linked to many diseases. Selecting controls in this way might therefore over-estimate population exposure to such risk factors, resulting in an underestimation of association between disease and exposure. Using more than one control group helps to overcome this type of issue. Advantages and Disadvantages for Using a Matching Strategy 13. Lane-Claypon JE. A further Report on Cancer of the Breast, with Special Reference to its Associated Antecedent Conditions. 1926 24. Vandenbroucke JP. The making of STROBE. Epidemiology. 2007; 18 :797–799. [PubMed ] 20. Wacholder S, McLaughlin JK, Silverman DT best medical school essays, et al. Selection of controls in case-control studies. I. principles. Am. J. Epidemiol. 1992; 135 :1019–1028. [PubMed ] This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. 21. Ury HK. Efficiency of case-control studies with multiple controls per case: Continuous or dichotomous data. Biometrics. 1975; 31 :643–649. [PubMed ] 8. Everitt BS, Palmer CR. Encyclopaedic Companion to Medical Statistics. Hodder Arnold; London: 2005. Because prospective cohort studies may require long follow-up periods application job letter example, it is important to minimize loss to follow-up. Loss to follow-up is a situation in which the investigator loses contact with the subject custom research papers review, resulting in missing data. If too many subjects are loss to follow-up, the internal validity of the study is reduced. A general rule of thumb requires that the loss to follow-up rate not exceed 20% of the sample. 6 Any systematic differences related to the outcome or exposure of risk factors between those who drop out and those who stay in the study must be examined, if possible, by comparing individuals who remain in the study and those who were loss to follow-up or dropped out. It is therefore important to select subjects who can be followed for the entire duration of the cohort study. Methods to minimize loss to follow-up are listed in Table 3 . Case-control studies identify subjects by outcome status at the outset of the investigation. Outcomes of interest may be whether the subject has undergone a specific type of surgery, experienced a complication, or is diagnosed with a disease ( Figure 3B ). Once outcome status is identified and subjects are categorized as cases, controls (subjects without the outcome but from the same source population) are selected. Data about exposure to a risk factor or several risk factors are then collected retrospectively, typically by interview, abstraction from records, or survey. Case-control studies are well suited to investigate rare outcomes or outcomes with a long latency period because subjects are selected from the outset by their outcome status. Thus in comparison to cohort studies, case-control studies are quick, relatively inexpensive to implement, require comparatively fewer subjects, and allow for multiple exposures or risk factors to be assessed for one outcome ( Table 4 ). 2. 9 1. Chung KC, Swanson JA, Schmitz D, et al. Introducing evidence-based medicine to plastic and reconstructive surgery. Plast. Reconstr. Surg. 2009; 123 :1385–1389. [PMC free article ] [PubMed ] In 2004, the first meeting of the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) group took place in Bristol, UK. 24 The aim of the group was to establish guidelines on reporting observational research to improve the transparency of the methods basic outline for a persuasive essay, thereby facilitating the critical appraisal of a study's findings. A well-designed but poorly reported study is disadvantaged in contributing to the literature because the results and generalizability of the findings may be difficult to assess. Thus a 22-item checklist was generated to enhance the reporting of observational studies across disciplines. 25. 26 This checklist is also located at the following website: www.strobe-statement.org. This statement is applicable to cohort studies, case-control studies, and cross-sectional studies. In fact, 18 of the checklist items are common to all three types of observational studies, and 4 items are specific to each of the 3 specific study designs. In an effort to provide specific guidance to go along with this checklist, an “explanation and elaboration” article was published for users to better appreciate each item on the checklist. 27 Plastic surgery investigators should peruse this checklist prior to designing their study and when they are writing up the report for publication. In fact, some journals now require authors to follow the STROBE Statement. A list of participating journals can be found on this website: http://www.strobe-statement.org./index.php?id=strobe-endorsement . 18. Chung KC, Kowalski CP, Kim HM, et al. Maternal cigarette smoking during pregnancy and the risk of having a child with cleft lip/palate. Plast. Reconstr. Surg. 2000; 105 :485–491. [PubMed ] None of the authors has a financial interest in any of the products, devices, or drugs mentioned in this manuscript. Due to the limitations in carrying out RCTs in surgical investigations, observational studies are becoming more popular to investigate the relationship between exposures, such as risk factors or surgical interventions, and outcomes, such as disease states or complications. Recognizing that well-designed observational studies can provide valid results is important among the plastic surgery community, so that investigators can both critically appraise and appropriately design observational studies to address important clinical research questions. The investigator planning an observational study can certainly use the STROBE statement as a tool to outline key features of a study as well as coming back to it again at the end to enhance transparency in methodology reporting. Cohort and Case-Control Study Designs Case-control studies were historically borne out of interest in disease etiology. The conceptual basis of the case-control study is similar to taking a history and physical; the diseased patient is questioned and examined professional essay writer service, and elements from this history taking are knitted together to reveal characteristics or factors that predisposed the patient to the disease. In fact, the practice of interviewing patients about behaviors and conditions preceding illness dates back to the Hippocratic writings of the 4 th century B.C. 7 Temporal Design of Observational Studies: Cross-sectional studies are known as prevalence studies and do not have an inherent temporal dimension. These studies evaluate subjects at one point in time, the present time. By contrast, cohort studies can be . Levels of Subject Selection. Adapted from Ref 9. 16. Zhang QG, Zhang J, Yu P, et al. Environmental and genetic factors associated with congenital microtia: A case-control study in Jiangsu, China, 2004 to 2007. Plast. Reconstr. Surg. 2009; 124 :1157–1164. [PubMed ] A case-control study design that exemplifies this methodological feature is by Chung and colleagues, who examined maternal cigarette smoking during pregnancy and the risk of newborns developing cleft lip/palate. 18 A salient feature of this study is the use of the 1996 U.S. Natality database, a population database, from which both cases and controls were selected. This database provides a large sample size to assess newborn development of cleft lip/palate (outcome), which has a reported incidence of 1 in 1000 live births, 19 and also enabled the investigators to choose controls (i.e. healthy newborns) that were generalizable to the general population to strengthen the study's external validity. A significant relationship with maternal cigarette smoking and cleft lip/palate in the newborn was reported in this study (adjusted OR 1.34, 95% CI 1.36-1.76). 18 The selection of a proper control group may pose problems. A frequent source of controls is patients from the same hospital who do not have the outcome. However, hospitalised patients often do not represent the general population; they are likely to suffer health problems and they have access to the health care system. An alternative may be to enroll community controls, people from the same neighborhoods as the cases. Care must be taken with sampling to ensure that the controls represent a ‘normal’ risk profile. Sometimes researchers enroll multiple control groups. These could include a set of community controls and a set of hospital controls. Case-control studies are sometimes less valued for being retrospective. However, they can be a very efficient way of identifying an association between an exposure and an outcome. Sometimes they are the only ethical way to investigate an association. If care is taken with definitions, selection of controls, and reducing the potential for bias, case-control studies can generate valuable information. There are not necessarily any ‘right’ answers to these questions but they must be answered before the study begins. At the end of the study, the conclusions will be valid only for patients who have the same sort of ‘endophthalmitis’ as in the case definition. Case-control studies may prove an association but they do not demonstrate causation. Consider a case-control study intended to establish an association between the use of traditional eye medicines (TEM) and corneal ulcers. TEM might cause corneal ulcers but it is also possible that the presence of a corneal ulcer leads some people to use TEM. The temporal relationship between the supposed cause and effect cannot be determined by a case-control study. Will endophthalmitis have to be proven microbiologically, or will a clinical diagnosis be acceptable? Another use for case-control studies is investigating risk factors for a rare disease good scientific essay topics, such as uveal melanoma. In this example, cases might be recruited by using hospital records. Patients who present to hospital, however how to write a speech, may not be representative of the population who get melanoma. If, for example space essays, women present less commonly at hospital, bias might occur in the selection of cases. 5. For information about Epi Info (Version 6), a word processing, database, and statistics program for epidemiology on microcomputers, please contact Centers for Disease Control and Prevention, Atlanta, GA 30333. [contact The Division of Surveillance & Epidemiology, Epidemiology Program Office] Consider a situation in which a large number of cases of post-operative endophthalmitis have occurred in a few weeks. The case group would consist of all those patients at the hospital who developed post-operative endophthalmitis during a pre-defined period. Case-Control Studies: Advantages and Disadvantages
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