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Case Control Studies
Case Control Studies
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Case control studies

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a community medicine topic on one of the study designs

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Case control studies

  1. 1. CASE CONTROL STUDIES KESHAVA PAVAN K 1
  2. 2. INTRODUCTION  Unit of observation & analysis – individual, but conclusion is generalized for the population.  Most frequently undertaken  To identify risk factors for chronic & rare diseases  Best suited to study diseases for which medical care is sought, like cancers, cirrhosis, SLE & CCF. 2
  3. 3. FEATURES  Both exposure & outcome have occurred before start of study  RETROSPECTIVE study – from effect to cause. CAUSE EFFECT STUDY  Uses comparison group to support or refute an inference. 3
  4. 4. STEPS 1. Selection of cases and controls 2. Matching – making two groups comparable 3. Measurement of exposure 4. Analysis and interpretation 4
  5. 5. DISEASED (CASES) EXPOSED NOT EXPOSED TARGET POPULATION EXPOSED NOT EXPOSED NOT DISEASED (CONTROL) 5
  6. 6. SELECTION DIAGNOSTIC CRITERIA  Establish diagnostic criteria and definition of disease & stage of disease.  Once established, diagnostic criteria should not be changed.  Study cases should be representative of all cases.  Sources of cases can be hospitals or general population. 6
  7. 7. SELECTION ELIGIBILITY CRITERIA  Incident cases are preferred to prevalent cases to reduce-  Recall bias  Over-representation of cases of long duration.  Most desirable way is to include all incident cases in the population in a specific time period. 7
  8. 8. SELECTION SELECTING CONTROLS  From same population at risk for the disease as the cases.  Should be representative of population  Help to estimate exposure rate to be exposed.  Sources can be hospitals, relatives, neighbours or general population.  Best possible ratio for number of cases to controls is 1 : 1  Maximum permissible option is 1 : 4 (for rare diseases) 8
  9. 9. MATCHING  Can be group matching or pair matching  Example: If there are 10 people in cases of age group 15-20, even controls are matched such that they too contain 10 people of age group 15-20 (Group matching). If each person is matched with respect to the factors, then it is pair matching (i.e., there is one person in control for each person in case with similar features except for the disease under study.)  Confounders should be avoided. 9
  10. 10. MEASUREMENT  Is an estimate unless past measurements are available.  It has to be assumed that exposure incurred at the time the disease process began (this may not be valid)  Subjected to recall & interviewer bias  Potential confounders need to be assessed. 10
  11. 11. ANALYSIS ENTERING DATA CASES CONTROLS EXPOSED a b NOT EXPOSED c d TOTAL a+c b+d PROPORTION 𝑎 𝑏 EXPOSED 𝑎+ 𝑐 𝑏+ 𝑑 11
  12. 12. ANALYSIS ODDS RATIO  Odds of exposure for cases compared to controls 𝑎𝑑  OR = 𝑏𝑐  INTERPRETATION OF OR If OR = n, then the cases are n times more likely to be exposed than the controls. 12
  13. 13. ADVANTAGES  Quick and easy to complete  Cost effective  Efficient for rare diseases  Small sample size sufficient 13
  14. 14. DISADVANTAGES  Temporality of association  Inability to provide a direct estimate of risk  Not efficient for studying rare exposures  Subject to biases. 14
  15. 15. THANK YOU  THANK YOU for reading my presentation.  If you have any doubts or interesting cases in any subject of medicine, I will be delighted if you share at keshavapavan533@gmail.com 15

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