Jehan, Fyezah

Kazi, Abdul Momin

eCommons@AKU

408 J Pak Med AssocBasic study designs in health researchFyezah Jehan, Abdul Momin KaziDepartment of Pediatrics and Child Health, Aga Khan University, Karachi, Pakistan.Learning ResearchMost studies related to clinical health research have oneor more of the following aims:1) To define and quantify occurrence of disease (incidence orprevalence) OR 2) To identify associated factors such as an environmentalexposure OR3) To study or compare efficacy of a medical intervention to thedisease.To address these questions various types of studydesigns may be employed. Each design not only represents adifferent way of answering a question but is also different in thetypes of questions it can answer. The type of study designselected depends on many factors for example the particularresearch question, validity, efficiency, practicality and ethicalconsiderations. Study designs may broadly be classified intodescriptive and analytical. The simplest are the descriptive(non analytical) studies. These studies are primarily"hypothesis generating", have no comparison group and are aprecursor to analytical studies where hypotheses are tested. Nodescription of studies to assess causal relationships would becomplete without mentioning experimental studies. In thehierarchy of research study designs, experimental studydesigns are the most superior in showing disease etiologyrelationships and for evaluating the effect of a drug or treatmenton disease. Figure represents the traditional hierarchy of thebasic study designs in use to assess causal relationships. Tableprovides their basic characteristics, strengths and weaknesses.We aim to give an overview of the basic architecture of variousstudy designs in health research, considerations for their use aswell as their strengths and weaknesses. Adherence to aparticular design will determine the way results are analyzedand conclusions are presented; eventually contributing toscientific quality and clinical relevance.Case Reports and SeriesAmong them case report and case series are done togive detailed description of the occurrence of a disease and areemployed mostly in the clinical setting. Case report for singlecase or case series for multiple cases provide context and detailof a new disease or problem which occurs out of routine. Forexample to describe an uncommon presentation of a paediatricrenal mass or to explain an unconventional or novel method ofevaluating laryngeal function in patients after intubation.1,2However case reports suffer from subjectivity and qualitativedata, hence can be generalized in a particular context only.Case studies are not designed for doing a detailed statisticalanalysis. For the purpose of description of a rare disease or toshow the unusual manifestation of a known disease it isjustified to perform case studies.3Cross Sectional Studies:Cross sectional studies are the most popular type ofdescriptive studies. They are conducted at one point in timeover a short period providing the disease and to identify thecharacteristics associated with it in a snapshot, at a particulartime.4 This kind of study is commonly employed when the aimis to describe a certain disease with respect to a set of riskfactors or to learn the prevalence of a disease or risk factor. Forexample, to assess the sero positive prevalence of hospitalized,pregnant women infected with Hepatitis B.5These studies havea major benefit that they are inexpensive and can be conductedover a short period of time. Their utility is however limited bythe fact that they do not show causality between risk factor anddisease. One important problem is that of "survival bias" that isseen in cases of diseases that have long term survivors where arisk factor associated with survivorship will be overrepresented and appear to be associated with disease.6 In spiteVol. 62, No. 4, April 2012 409Table: Study designs in health research, their basic characteristics and strengths and weaknesses.Name of design Type of research question Characteristics Strengths WeaknessesCase report orcase seriesCross sectionalstudyCase control studyCohort studyExperimentalstudyDescriptiveDescriptiveAnalyticalobservationalAnalyticalobservationalAnalytical,experimentalDetailed description of the occurrence of one ormore cases of a rare disease or rare manifestationof common disease; done by cliniciansSnapshot of a disease and its risk factorsmeasured at a point in timeCases(diseased)compared with controls(nondiseased) with regards to risk factor informationobtained retrospectivelyCohort of subjects with and without risk factorsand free of disease are followed for developmentof diseaseRandomized controlled trials are gold standard.Experimenting a new treatment in controlledenvironmentFast, cheap, exploratoryFast;cheap; generateshypothesis for futurework. MeasuresprevalenceEfficient for rare diseasesand more than oneexposure. Fast, relativelycheap. Usually smallernumbers requiredMeasures incidence. Canstudy multiple outcomes.Good for studying rareexposuresEstablishes causality andeffect of interventionContext is narrow, hence poorgeneralizability. Not designedfor establishing causality Limited potential forestablishing causality as riskfactor and disease aremeasures at same point intime. Survival biasCan only study one outcome.Some potential to establishcausality. Problems withrecall. Selection biasExpensive. Long and resourceexhaustive. Selection biasExpensive and resourceexhaustive. PoorgeneralizabilityFigure: Traditional hierarchy of basic study designs in health research.of these problems, cross sectional studies are highly beneficialin determining prevalence and enumerating risk factors forunderstanding disease etiology and generating hypotheses.Cohort Study:A cohort study is performed by examining one or morerisk factors at two or more different levels and observing forfactor related disease. In a classical cohort study varying levelsof risk factors are assessed in a non-diseased group. Thesegroups are then followed for development of disease and thenthe two groups are compared with regards to the risk factorsand disease. A famous example is the Framingham heart studywhich looked into the risk factors of different patterns ofcoronary heart diseases.7 Cohort studies provide incidence andnatural history of disease. Since the temporal sequence of riskfactor to outcome is clearly evident such studies are appealingto clinicians doing health research. These studies are mostsuitable where the factor is rare. However, these studies mayrequire large numbers of subjects especially for uncommondisease outcomes and may be extremely time and resourceintensive. If the period of follow-up is long there can beproblems of drop outs (lost to follow up). Given the above,cohort designs are most powerful in showing causalassociations between risk factor and disease if appropriateextraneous factors are taken care of.Case Control Studies:Case control studies are also done to identify riskfactors that may contribute to an outcome. Subjects are selectedon the basis of those who have disease (cases) and those whodo not have the disease (controls); and risk factors areidentified looking back in time. The numbers of subjects thatneed to be studied are smaller than those in cohort studies. Theassociation of lung cancer with smoking was demonstratedmostly from case control studies.8 Thus this design is moreefficient especially for studying rare outcomes.9 The benefitsare somewhat offset by the fact that like cross sectional studies,case control studies also lack temporal flow. Since risk factorinformation is obtained in the past it may be incomplete and notin the context of the study. A researcher might not get all theinformation that is needed. They also have the problem ofsurvivor bias that was described with cross sectional studies.Special care needs to be taken to obtain controls from the samepopulation as the cases to avoid "selection bias". Having saidthis case control studies continue to be a design favored byclinicians who want to show etiological relationships.Randomized Control Trial (RCT):The gold standard of experimental studies is the RCT.They are used to assess the efficacy of therapies andinterventions. In a RCT every study subject is randomlyallocated to either an intervention group or a control. Throughthis randomization the assignment to an intervention group ismade purely on the basis of chance. Every subject is thenfollowed for effect or disease. This achieves groups that arefairly balanced with regards to known and unknownconfounders. If executed well, RCTs can deal with thecommon problem of extraneous factors (confounders) andselection bias seen in observational studies. However, RCTsneed strict monitoring with investigator accountability foradverse events related to the intervention. It results inresource intensive studies. Also, because of ethical issuesrelated to performing experiment on humans RCTS can bedifficult to design. While experimental research is the a superior methodfor establishing risk factor to disease associations, their designand implementation is hindered by important practical andethical issues. If implemented properly, observation studydesigns are a good alternative. Cohort designs are mostefficient to establish causal associations of risk factor withdisease outcome particularly if the risk factor is rare andseveral outcomes need to be studied. However carefulconsideration needs to be made if the period of follow up islong to minimize "loss to follow up". Also added informationis required for confounder control. Case control design ispreferred to study risk factors if the disease has alreadyoccurred. This is in particular applicable to the disease with along latent time and for studying many risk factors. Howeveras risk is assessed after disease has occurred there may beinherent biases such as recall bias, misclassification of riskfactor or incomplete information (information bias). Also ifnon-diseased (controls) are not selected from a population thatis similar to cases invalid association can be produced(selection bias). Cross sectional studies are efficient toquantify disease burden and exploration of new risk factorsand generating avenues for further analytical studies. Casereports are reserved for reporting new or new manifestationsof old disease or innovative therapies. As medical researchadvances there is an increasing recognition of the individualstrengths of each design and their uniqueness in answering aparticular research question.References1. Smith EA, Styn N, Wan J, McHugh J, Dillman JR. Xanthogranulomatouspyelonephritis: an uncommon pediatric renal mass. Pediatr Radiol Aug; 40: 1421-5.2. Van der Meer G, Ferreira Y, Loock JW. The S/Z ratio: a simple and reliable clinicalmethod of evaluating laryngeal function in patients after intubation. J Crit CareSep; 25: 489-92.3. Vandenbroucke JP. Case reports in an evidence-based world. J R Soc Med 1999;92: 159-63.4. Levin KA. Study design III: Cross-sectional studies. Evid-based Dent 7: 24-5.5. Maclean B, Hess RF, Bonvillain E, Kamate J, Dao D, Cosimano A, et al.Seroprevalence of hepatitis B surface antigen among pregnant women attendingthe Hospital for Women & Children in Koutiala, Mali. S Afr Med J Jan; 102: 47.6. Neyman J. Statistics; servant of all sciences. Science 1955; 122: 401-6.7. Lerner DJ, Kannel WB. Patterns of coronary heart disease morbidity and mortality410 J Pak Med Associn the sexes: a 26-year follow-up of the Framingham population. Am Heart J1986; 111: 383-90.8. Wynder EL, Graham EA. Landmark article May 27, 1950: Tobacco Smoking asa possible etiologic factor in bronchiogenic carcinoma. A study of six hundred andeighty-four proved cases. By Ernest L. Wynder and Evarts A. Graham. JAMA1985; 253: 2986-94.9. Rothman KJ, Greenland S, Lash TL. Case-Control Studies. Encyclopedia ofQuantitative Risk Analysis and Assessment: John Wiley & Sons, Ltd; 2008.Vol. 62, No. 4, April 2012 411

Basic study designs in health research

http://www.jpma.org.pk/PdfDownload/3373.pdf

Basic study designs in health research

Abstract

Similar works

Full text

Available Versions

eCommons@AKU