But there may be other variables which bring about both heavy drinking and decreased. The role of randomization to address confounding variables in. Unlike cohort studies, casecontrol studies cannot calculate incidence rates, relative risks or attributable risks. Matching in case control studies matching addresses. Jan 01, 2012 matching is commonly used in case control studies for example, if age and sex are the matching variables, then a 45 year old male case is matched to a male control with same age. Therefore, in case control studies, matching for confounding may result in overadjustment and even introduce confounding. The table below shows the results for this casecontrol study. Randomization is a simple tool in experimental design that allows the confounding variables to have their effect across a sample. Matching cases and controls on these variables ensured they were equally distributed between groups. A confounding variable is associated with the exposure and it affects the outcome, but.
Assistant professor school of pharmacy, national taiwan university 30th annual meeting of the international society for pharmacoepidemiology. Extensions to the more realistic case where a set of variables are candidates for adjustment are outlined in section 2. Thus, confounding is a major threat to the validity of inferences made about cause and effect internal validity. Confounding should not be allowed to distort the estimation of effect. The stratification score for a case control study is the probability of disease modeled as a function of potential confounders. For example, if somebody wanted to study the cause of myocardial infarct and thinks that the age is a probable confounding variable, each 67yearold infarct patient will be matched with a healthy 67yearold control person. Table 1 example of confounding in a hypothetical cohort. Matching in case control studies matching addresses issues of confounding in the design stage of a study as opposed to the analysis phase a means of providing a more. A problem in the design and analysis of a casecontrol study is the identification of confounding factors. The present chapter covers the basic concepts of confounding and interaction and provides a brief overview of analytic approaches to these phenomena. A confounding factor is one that is associated with the disease and the exposure, but may not be of interest or observed. In this lesson, we will look at ecological and case control study designs. A confounding variable is a factor associated with both the exposure.
Example of confounding in a hypothetical cohort study of ischemic. Methods to identify and address confounding are discussed, as well as their strengths and limitations. Case control studies are therefore placed low in the hierarchy of evidence. Assessment and control for confounding by indication in. Any risk factor for a disease is a potential confounder. The implications of interaction effects for sample size requirements are more important. Request pdf confounding in casecontrol studies last weeks question described a population based casecontrol study that investigated whether infants sleeping environment was a risk. Instead odds ratio are the measure of association used when outcome is uncommon, e. Assume there is a rare congenital disease called congenia.
But all these methods mentioned above are applicable at the time of study design and before the process of data gathering. Alarge number of different sources and possible mechanisms of. This article discusses the importance, definition, and types of confounders in epidemiology. Imagine that in this case, there is in fact no relationship between drinking and longevity. For each case, a control matched for age and sex with no history of coronary heart disease was enrolled. Confounding and control of confounding in nonexperimental.
How to control confounding effects by statistical analysis article pdf available in gastroenterology and hepatology from bed to bench 52. Still, the choice of matching variables needs careful attention because, as will be described later, errors are frequently made. A potential confounding variable not measured in the study is called a lurking variable. Dealing with confounding in the analysis iarc publications. If restriction is carried to extremes the study may have a limited number of eligible participants. Role of chance, bias and confounding in epidemiological. A confounder is an extraneous variable whose presence affects the variables being studied so that the results do not reflect the actual relationship between the variables under study. Analysis of case control studies the odds ratio or is used in case control studies to estimate the strength of the association between exposure and outcome. This contrasts with case control studies as they assess only one outcome variable that is, whatever outcome the cases have entered the study with.
Restriction can be a powerful method to address a limited. Confounding in case control studies pubmed central pmc. This work is licensed under a creative commons attribution. Matched case control studies control for confounding by introducing stratification in the design phase of a study. Pdf how to control confounding effects by statistical. Confounding variables change with the independent variable as. Confounding and bias in casecontrol studies chinglan cheng, ph. In case control studies, matched variables most often are the age and sex.
Matching, an appealing method to avoid confounding. Controlling for confounding in casecontrol studies parodi s. Effectively, whereas in a cohort study matching is done to make exposure groups comparable on the matching variable, in a casecontrol study. An approach for controlling confounding after the completion of a study during data analysis is stratification. Confounding variables or confounders are often defined as the variables correlate positively or negatively with both the dependent variable and the independent variable. A confounding variable gives rise to situations in which the effects of two. Confounding variables and causation randomized experiments. One of the most significant triumphs of the case control study was the demonstration of the link between tobacco smoking and lung cancer, by richard doll and bradford hill. In total, 1122 survivors of suspected acute myocardial infarction at. In nonexperimental studies of patients with ckd or who are on chronic dialysis, confounding is a significant concern owing to the high burden of comorbid disease, extent of required clinical management.
Note that it is not possible to estimate the incidence of disease from a casecontrol study unless the study is population based and all cases in a defined population are obtained. We conduct a casecontrol study recruiting mothers of 200 cases of congenia and mothers of 400 control children. Infants usual sleeping environment in relation to their parents was classified as usual room sleeper shared room but not. Confounding, effect modification, and stratification. Comparison group is chosen to be similar to the case group with respect to one or more potential confounding variables. Briefly, a commonly used procedure for making statistical adjustments for confounding variables in case control and crosssectional studies when a disease is either present or absent e. The difference in power between matched and unmatched studies diminishes rapidly as the control to case ratio is increased. In many studies, the possible bias of the researchers is one of the most important confounding variables. This paper will deal both with the issues of confounding and of control, as the definition of a confounding factor is far from universal and there exist. Evaluation of confounding in epidemiologic studies.
Control of confounding in study design restriction one of the conditions necessary for confounding to occur is that the confounding factor must be distributed unequally among the groups being compared. Linear regression, adjusted for confounding variables, was used to. Matching in case control studies matching addresses issues. The study was conducted with the aim to compare the performance on a battery of six. A confounding factor is one that is related to both the exposure and the outcome. Observational studies are particularly susceptible to the effects of chance, bias and confounding, and these need to be considered at both the design and analysis stage of an epidemiological study so that their effects can be minimized. A confounding variable gives rise to situations in which the effects of two processes are not separated, or the contribution of causal factors cannot be separated, or the measure of the effect of exposure or risk is distorted because of its association with other factors influencing the outcome of the study. Matched casecontrol studies control for confounding by introducing stratification in the design phase of a study. Positive confounding when the observed association is biased away from the null and negative confounding when the observed association is biased toward the null both occur. Confounding occurs when a confounding variable, c, is associated with the exposure, e, and also influences the disease outcome, d. In casecontrol studies, however, matching is frequently used. Certain issues will not be addressed, notably methods of controlling confounding, andthe statistical analysis ofcase control studies.
A situation in which the effect or association between an exposure and outcome is distorted by the presence of another variable. Dealing with confounding in drug studies loreto carmona. Sometimes control of confounding only possible by matchingi. Sometimes theres a lot of variation in confounding variables that you cant control. Pdf confounding variables in epidemiologic studies. Finding a statistically significant result is almost always more interesting than not finding a difference, so you need to constantly be on guard to control the effects of this bias. However, these studies tend to enroll people from broader backgrounds, possibly strengthening the generalizabilty of their results. For example, cohort studies of smokers can simultaneously look at deaths from lung,cardiovascular, and cerebrovascular disease. Role of chance, bias and confounding in epidemiological studies. In all these cases, the recommended remedy is to control for the confounding factor, e. It is a variable that interferes with the results of the study. What can go wrong sampled from the target population. Among those not exposed to e let exposure to c increase the risk of disease by a factor rc.
The interpretation of study findings or surveys is subject to debate, due to the possible errors in measurement which might influence the results. If a pairmatched design is used, then matching must be taken. Wholly or partially accounts for apparent effect of exposure on disease either direction. In particular, the stratification score is a retrospective balancing score defined below for a casecontrol study. However, the absence of a formal statement of the problem has led to disagreements, confusion, and occasionally to erroneous conclu sions. A multiple logistic regression model for screening diabetes tabaei and herman. Like restriction, this approach is straightforward, and it can be effective. Restriction by the indication for a drug can be used to address confounding byindication in observational studies of medication use. Confounding is an important source of bias in nonexperimental studies, arising when the effect of an exposure on the occurrence of an outcome is distorted by the effect of some other factor. Experimental studies next topic uses randomization as a tool to fight the occurrence of confounding. A case control study examined the association between chronic helicobacter pylori infection and coronary heart disease at young ages. More details in the confounding and bias in case control studies. Confounding and bias in cohort studies chichuan emma wang, ph. If the confounder that is used as a matching variable is strongly related to the risk factor causing the outcome, matching will lead to an underestimation of the effect of that risk factor.
Confounding in case control studies can be controlled for during the design stage. In research that draws on a quantitative research design, especially experimental research designs also called intervention studies, we try and control these. Any variable that you are not intentionally studying in your dissertation is an extraneous variable that could threaten the internal validity of your results see the article. In total, 1122 survivors of suspected acute myocardial infarction at age 3049 years were recruited. Confounding variable an overview sciencedirect topics. Confounding is a distortion of the true relationship between exposure and disease by the in. The article also describes the difference among confounders, mediators, and effect modifiers. Matching may be done on an individual basis pairmatching or on a group basis frequency matching. This paper considers quantitatively the extent to which the interaction or confounding effects of covariates may influence the design of case control studies with particular reference to sample size requirements and the role of matching. Epi info user guide chapter 12 statcalc 124 stratified analysis of 2by2 tables if confounding is present, associations between disease and exposure can be missed or falsely detected. This reduces potential for confounding by generating groups that are fairly comparable with respect to known and unknown confounding variables. Control for confounding in casecontrol studies using the.
Analysis of casecontrol studies the odds ratio or is used in casecontrol studies to estimate the strength of the association between exposure and outcome. In a hypothetical casecontrol study to examine the rela. Note that it is not possible to estimate the incidence of disease from a case control study unless the study is population based and all cases in a defined population are obtained. Prediction studies interest centers on being able to accurately estimate or predict the response for a given combination of predictors focus is not much about which predictor variable allow to do this or what their coef. Both confounding and interaction can be assessed by stratification on these other factors i.
For the most part, attention is confined to a dichotomous exposure variable, and a single dichotomous covariate. Observer or subject bias as a confounding variable. In this case age the confounder is associated with. Results a total of 9706 papers were retrieved, and 14 observational epidemiological studies 9 case control studies and 5 prospective cohort studies comprising 10 764 cases of depression were finally included in this metaanalysis. Design and analysis of casecontrol studies uc davis health. The authors show that the stratification score is a retrospective balancing score and thus plays a similar role in case control studies as the propensity score plays in prospective studies. Assistant professor school of pharmacy, national taiwan university 30th annual meeting of the international society for. Confounding a variable that a is causally related to the disease under study or is a proxy for an unknown or unmeasured cause and b is associated with the exposure under study kesley. Confounding and interaction biometry 755 spring 2009 confounding and interaction p. This method actually requires the investigators to control confounding in both the design and analysis phases of the study, because the analysis of matched study groups differs from that of unmatched studies.
Confounding and selection bias in case control studies. A casecontrol study examined the association between chronic helicobacter pylori infection and coronary heart disease at young ages. Confounding is essentially an intrinsic limitation of observational studies e. How to control confounding effects by statistical analysis. Something we try to overcome in observational studies by using matched pairs of casecontrol, but almost always still part of the argument. Thus, conditional on the stratification score and absent residual confounding, the distributions of exposures among case and control participants can be directly compared. To summarize, confounding reflects the causal association between variables in the population under study, and will manifest itself similarly in both cohort and. Assistant professor institute of clinical pharmacy and pharmaceutical sciences, national cheng kung university 30 th annual meeting of the international society for pharmacoepidemiology taipei, taiwan october 23, 2014 1. Confounding in epidemiological studies health knowledge. Overall, it is unclear whether the lack of consistency across articles reflects the fact that there is little consensus about which variables are potential confounders or the beliefs that the protective effect of alcohol consumption on the risk for cardiovascular disease is independent of how well studies control for confounding, 29, 31, 32. Abstract in case control studies, the role of adjustments for bias, and in particular the role of matching, has been extensively debated. Myfilesarticlesconfoundimpact of a confounding variable. This paper explains that to be a potential confounder, a variable needs to. Confounding and bias in casecontrol studies, chinglan cheng.
How to control confounding effects by statistical analysis, 2012. Confounding effect of a third variable that accounts for all or some of the association between exposure and. Sep 22, 2010 last weeks question described a population based casecontrol study that investigated whether infants sleeping environment was a risk factor for sudden infant death syndrome. Restriction also limits the interpretation of the study. Confounding may lead to errors in the conclusion of a study, but, when confounding variables are known, the effect may be fixed. Case control and cohort studies compare outcomes between groups with different exposures, and confounding arises when the groups under comparison differ in other ways than the exposure alone. For epidemiologic matched case control study designs, sas programs were developed to match cases persons with diseaseevent of interest to controls comparison group of persons without diseaseevent of interest. Both to control confounding and to detect interaction, the effect of matching may sometimes be to reduce the power of a study.
Therefore, an appreciation of potential sources of bias has becomea critical issue in epidemiology. Numbers of controls matched to each case may also vary within a study dependent on the available number of controls possessing the necessary matching criteria. You will learn how to understand and differentiate commonly used terminologies in epidemiology, such as chance, bias and confounding, and suggest measures to mitigate them. Age, locality, and time of sleep were potential confounders. Situation in which c may confound the affect of the e to d. Introduction for simplicity we first consider measurement of the relationship between a disease 0 and an agent under study e in the presence of a single confounding variable z. Matching in case control studies matching addresses issues of. For a given disease, a casecontrol study can investigate multiple exposures. Observational studies can not control for all the things that people do or happen to them, so there is possibility of uncontrolled confounding. The weakness in the experimental design was that they failed to take into account confounding variables, and did not try to eliminate or control any other factors.
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