Case-Control Studies PDF

Case-Control Studies Dr.Sijal Fadhil Farhood Al Joborae F.I.C.M.S Community Medicine. M.Sc. Community Medicine. M.B.Ch.B. Objectives:  Objectives Description of: 1-Major principles 2-Differences and similarities with other designs. 3-Suitability for a particular epidemiological qu...

Case-Control Studies Dr.Sijal Fadhil Farhood Al Joborae F.I.C.M.S Community Medicine. M.Sc. Community Medicine. M.B.Ch.B. Objectives:  Objectives Description of: 1-Major principles 2-Differences and similarities with other designs. 3-Suitability for a particular epidemiological question. 4- limitations. 5-Estimation of the strength of an association 6-calculation and interpretation of the error factor and 95% CI. Classification of Epidemiological Studies 1- Descriptive studies: A-Case reports and case series. B-Correlation. C-Cross sectional. Analytic Studies A-Observational 1-Case control 2-Cohort B-Interventional (experimental) How is a case- control study conducted? 1. Identification of cases 2. Identification of non-diseased comparison group (“controls”) 3. Documentation of exposures among both 4. Calculation of OR,95%CI 5. Interpretation of results  Examples Study to determine an association between autism and vaccination Study to determine an association between lung cancer and radon exposure  Sources of Selection of the cases: 1. Hospital-based case control studies common, easy, and inexpensive. 2- Population based case-control studies.  Selection of cases strict diagnosis case definition diagnostic criteria Sources of selection of controls: Hospital General population Specific How many controls will be taken for a case? Although the number of available cases is often limited, it is usually quite easy to increase the number of controls in a case- control study. The reason this is done is to increase the precision of the odds ratio, i.e. to reduce the error factor and produce a narrower 95% confidence interval How many controls will be taken for a case? In large studies generally 1 (1:1) In small studies (below 50) up to 4 (1:4)  Criteria of control 1. Not having the disease being studied. 2. Represents population from which cases arise. 3. Represents persons who, if they develop disease, would have been a case in the study. 4. To be selected independently of exposure.  Types of bias 1.selection 2-observational a- recall b- misclassification  Matching Selections of controls in a such a way that they are similar to cases in important variables (age, occupation, race, sex etc.) We can neutralize any confounding factor.  MEASURES OF ASSOCIATION Odds ratio: is the ratio of the number of the ways the event can occur to the number of ways the events cannot occur. It measures the association between exposure and outcome.  a c ad Odds Ratio = = b bc d Case Control E+ a b E- c d  Interpretation OR =1 (risk factor not related to the disease) OR1 (risk factor positively associated with the disease  95%confidence interval OR is subject to random variation. We can estimate the range within which the underlying (true)OR lies using the error factor (e.f.): e.f.= exp{2x√( + + + )} and the 95%CI = OR/e.f. to ORxe.f.  example Cigarette smoking Cases Control (lung cancer) (no lung cancer) Exposed 647(a) 622 (b) Unexposed 2 (c) 27 (d) Total 649 649 Odds of being exposed in case: 647/2 = 323.5 Odds of being exposed in control: 622/27 = 23.04 323.5 OR = = = 14.0 ( 14) times cigarette smokers are more 23.04 prone to lung cancer than non smokers.  e.f.= exp{2x√( + + + )} =4.35 95%CI = OR/e.f. to ORxe.f =14/4.35 to 14 x4.35 = (3.23 - 61.1 ) (interpret?)  IF Pass through the null value (1) Not sig. Not pass through the null value (1) Sig.  Strengths: 1-Quick and inexpensive 2-Suitable for disease with long latent period. 3-Is optimal for rare diseases. 4-Can examine multiple etiological factors for a single disease.  Limitations 1-Is inefficient for rare exposures 2-Can estimate the relative risk (odds ratio) 3-In some situations the temporal relationship between exposure and disease may be difficult to establish. 4- prone to bias (selection and recall).  Case control Cohort Suitable for rare disease suitable for rare exposure Cheap Expensive Quick Time consuming many exposures can be Many outcomes can be Measured measured Estimate measurement of Absolute measurement of incidence rate (OR ) Incidence rates (RR) exposure precede disease cause precede disease More prone Less prone to selection and recall bias Less Prone to losses during follow-up Q:Which of the following is true regarding case control study: A. It is descriptive study B. Can use for rare disease C. Can use for rare exposure D. It is a gold standard method in epidemiological study design E. The participants is exposed and non exposed group References: #Park’s text book of preventive and social disease 2021 #Textbook in research methodology 2023  ‫‪Thank you‬‬ ‫ﺳﻠﺴﻠﺔ ‪1‬‬ ‫ﺳﻠﺴﻠﺔ ‪2‬‬ ‫﮳‬ ‫د‪.‬ﺳﺤﺎل‬