Classification of study designs

Transcription

1 CASE CONTROL STUDY Prof. Dr. Zaleha Md. Isa, BSc(Hons), PhD, Department of Community Health, Faculty of Medicine, Universiti Kebangsaan Malaysia Medical Centre, Cheras, Kuala Lumpur.

2 Classification of study designs 1. Observational Studies 2. Experimental Studies 3. Meta-Analysis

3 Observational Studies 1. Descriptive or case-series 2. Case-control studies (retrospective) 3. Cross-sectional studies, surveys (prevalence) 4. Cohort studies (prospective) 5. Historical cohort studies

4 Case-control studies (retrospective studies) Case-control studies begin with the absence or presence of an outcome and then look backward in time to try to detect possible causes or risk factors that may have been suggested in a case series report. The cases are individuals selected on the basis of some disease or outcome; the controls are individuals without the disease or outcome.

5 Schematic diagram of case-control study design Question: What happened? Exposed Cases Compare Unexposed Exposed Controls Unexposed Onset of study Time Direction of inquiry

6 Case-control studies (retrospective studies) The history or previous events of both cases and controls are analyzed in an attempt to identify a characteristic or risk factor present in the cases histories but not in the controls histories. Subjects in the study are chosen at the onset of the study after they are known to be either cases with the disease or controls without the disease.

7 Case-control studies (retrospective studies) The histories of cases and controls are examined over a previous period of time in order to detect the presence or absence of predisposing characteristics or risk factors, or if the disease is infectious, whether the subject has been exposed to the presumed infectious agent. In case control designs, the nature of the inquiry is backward in time to illustrate the retrospective nature of the research process. We can characterize case control studies as studies that ask What happened?. Case control studies are longitudinal as well, because the inquiry covers a period of time.

8 Case-control studies Advantages Suitable to study rare diseases or diseases that occur for a long period of time. Preferably used to investigate initial hypothesis. Results can be obtained faster. Less expensive. Disadvantages Too many biases (misclassification of cases and subjects did not remember the risk factors studied). It depends on the completeness of the existing records.

9 Case-control studies Matching is used to associate controls with cases on factors like age and sex; Example: Usually two groups are recommended: 1. Group almost similar to cases e.g. patients in hospital. 2. Group that is healthy.

10 Example 1 A case control study aimed at identifying the risk factors for childhood enuresis (Foxman, Valdez & Brook 1986). Both cases and controls were identified among children from approximately 2,700 families enrolled in a national health insurance experiment. The cases were children who wet their beds at least once during the preceding 3 months, and the controls were children who had not. From information on a questionnaire, the investigators found that children with enuresis were more likely to be younger, were more often boys than girls, and had higher levels of psychologic distress than children who did not wet their beds.

11 Example 2 Kaku and Lowenstein (1990) identified patients between the ages of 15 and 44 who had had a stroke (cases) as well as patients who had not (controls) to investigate the relationship between stroke and recreational drug abuse in young people. A clinical association between drug abuse and stroke was based upon the occurrence of stroke symptoms within 6 hours of drug administration, positive toxicologic testing in stroke patients admitted in coma, or infectious endocarditis related to intravenous drug abuse. Controls were selected from patients admitted to the hospital with acute medical or surgical conditions for which recreational drug abuse has not been shown to be a risk factor.

12 Example 2 The investigators used matching to associate controls with cases on age and sex characteristics. If an investigator feels that such characteristics are so important that an imbalance between the two groups of patients would affect any conclusions, the investigator should employ matching. This process ensures that both groups will be similar with respect to important characteristics that may otherwise cloud or confound the conclusions.

13 THANK YOU 1. Dawson-Saunders, B. & Trapp, R.G Basic and Clinical Biostatistics. Prentice-Hall International Inc. London. 2. Md. Idris Mohd. Nor Asas Statistik dan Penyelidikan, Dewan Bahasa dan Pustaka, Kuala Lumpur. 3. Elston, R.C. & Johnson, W.D Essential of Biostatistics, F.A. Davis Company, Philadelphia.