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Case control studies.ppt

  1. • There is a outbreak of endopthalmitis in patients undergone surgery in the camp held by District hospital • how will you investigate? • Which study design will you used investigate?
  2. Study designs Quantitative Observational Descriptive Case report Case series Cross sectional longitudinal Experimental Analytical Case control cohort Cross sectional Qualitative Field expt Clinical trial Community trial
  3. Case Control Study Dr Suvarna Joshi
  4. Case Control Study Dr Suvarna Joshi Associate Professor Microbiology Grant Govt Medical College & JJ group of hospitals Mumbai
  5. SLO • Indications • Layouts of typical case control study • Steps • Interpretation of odds ratio & other statistics emerging out of case control study • Possible biases and their resolution • Comparison with cohort studies
  6. Indications • Investigating outbreak • Evaluating effectiveness of new vaccine • Evaluation of treatment & program efficacy • Genetic epidemiology • Occupational health research • Evaluation of screening • Outbreak investigations
  7. Case Control study design
  8. General characteristics • Observational epidemiological study • Analytical type • Testing hype • Hypothesis – association type • Two groups-case , control • Retrospective – going back from outcome(disease) to exposure • Also called Flashback study
  9. Steps • Sample size estimation • Selection of cases • Control matching • Collection of data • Applying statistics
  10. Selection of cases Definition of cases • Individuals with the disease entity under investigation (as per standard definition) • Based on standard diagnostic criteria • These criteria should be objective, valid, reproducible
  11. Selection of Cases • Source- Hospital , workplace, community/general population Time (eligibility)- New cases- incident cases within specified time Old cases- prevalent cases -derived from cross sectional survey Selection usually done from the available cases
  12. Selection of controls • Control- One who does not have disease under investigation • Need not be healthy • Control and cases should be from same study base(source) if cases selected from hospital then controls also from same hospital
  13. Sources for Control • Hospital, relatives, neighbors, occupational associates, general population d) Neighbors (i) walk (door to door) (ii) Phone (random digit dialing) (iii) Letter carrier routes e) Friends or associates of cases f) Siblings, spouses or other relatives g) Other
  14. Matching • Done for confounding factors to ensure the comparability • Eg age, sex, occupation, education, social status • Individual/frequency matching • Overmatching: do not match for risk factor under study • Increase power of study; increasing no. of controls per case
  15. Matching… • Control >4 per case will just increase the cost • Disadvantage of matching • Time consuming • Costly • Exact match may not be found • Multiple controls per case • (one from hospital and one from general population per case)
  16. Bias • Recall bias Selection bias Information bias Non-response bias Analysis bias
  17. Recall bias • eg A case control study on adults which gathered information relating to hospitalization prior to the age of 16 was concerned about poor recall • Recall bias (cases may remember their exposure with a higher or lower accuracy than controls do)
  18. Overcoming recall bias • In subsample of both cases and controls, medical records were examined to determine evidence of for difference in recall between cases and control
  19. Selection bias • Case control study on duodenal ulcer and smoking • Cases- patients attending gastroenterology clinic (likely to be severe cases) • Any risk factor derived from those cases may represent risk factors for severity rather than ulcer susceptibility per se • Referal practice
  20. Information/interviewer’s bias • Case control study on possible link between febrile illness and subsequent early pregnancy loss • Greater care in introducing questions to the case mothers, the study coordinator was concerned that the interviewer might have probed more closely for history of fever in the cases • It was found that interviewer was taking 10 min more than normal pregnant controls
  21. Case Control Design Time Direction of Inquiry Population Cases with the Disease Controls without the disease Exposed Exposed Not Exposed Not Exposed
  22. Case-Control Studies: Methodology Then measure, post-exposure First Select the Cases and Controls Cases (with disease) Control (without disease) Were Exposed A B Not Exposed C D A+C B+D Population Exposed A/A+C B./B+D
  23. Figure 1 (continued) a c ad Odds Ratio = = b bc d Risk = a a + b = c c + d a b c d Case Control E+ E-
  24. Kidney stone present Kidney stone absent Total Drinking hard water 90 (a) 20 (b) 110 Not drinking hard water 10 (c) 80 (d) 90 total 100 100 200 Exposure rate in cases = a/a+c = 90/90+10= 90% Exposure rate in control = b/+d= 20/20+80= 20% Odds ratio= ad/bc = 90x80/20x10=7200/200=36
  25. • Odd ‘s ratio =1 no association • > 1 positive association • < 1 protective effect
  26. Strength of Association Relative Risk;(Prevalence); Odds Ratio Strength of Association 0.83-1.00 1.0-1.2 None 0.67-0.83 1.2-1.5 Weak 0.33-0.67 1.5-3.0 Moderate 0.10-0.33 3.0-10.00 Strong <0.01 >10.0 Approaching Infinity
  27. Nested Case-Control Studies Figure 3 Study Population TIME 1 YEARS TIME 2 Develop Disease Do Not Develop Disease CASES CONTROLS CASE-CONTROL STUDY Obtain interviews, bloods, urines, etc.
  28. A. Advantages of Nested Case-Control Studies 1. Possibility of recall bias is eliminated, since data on exposure are obtained before disease develops. 2. Exposure data are more likely to represent the pre-illness state since they are obtained years before clinical illness is diagnosed. 3. Costs are reduced compared to those of a prospective study, since laboratory tests need to be done only on specimens from subjects who are later chosen as cases or as controls.
  29. • A major limitation of cross-sectional surveys and case-control studies is difficulty in determining if exposure or risk factor preceded the disease or outcome. to overcome this problem ?
  30. Cohort study
  31. WHAT IS COHORT • Ancient Roman military unit, A band of warriors. • Persons banded together. • Group of persons with a common statistical characteristic. [Latin] • E.g. age, birth date,
  32. Indication • When a good evidence of association between exposure and disease • When exposure is rare but incidence of disease in exposed is high • When follow up is easy and cohort is relatively stable • Ample funds available
  33. Elements of cohort study • Selection of study subjects • Obtaining data on exposure • Selection of comparison group • Follow up • Analysis
  34. Selection of subject • Group of people exposed to suspected cause • Eg cohort group who drink hard water (study cohort) • And cohort who don’t drink hard water(control cohort) • Comparison group are in built based on gradation of exposure
  35. • Cohort must be free from disease under study • Both cohort should be equally susceptible to the disease under study • Both cohort should be comparable • Diagnostic & eligibility criteria must be defined before study commences
  36. Characteristic of Cohort study  Longitudinal  Prospective studies  Forward looking study  Incidence study  starts with people free of disease  assesses exposure at “baseline”  assesses disease status at “follow-up”
  37. b+d Frame work of Cohort studies c c+d a a+b Total Yes Disease Status Yes No Exposure Status b d a+c N No Study cohort Comparison cohort
  38. Selection of study subjects • General population – Whole population in an area – A representative sample • Special group of population – Select group • occupation group / professional group (Dolls study ) – Exposure groups • Person having exposure to some physical, chemical or biological agent – e.g. X-ray exposure to radiologists
  39. Obtaining data on exposure • Personal interviews / mailed questionnaire • Reviews of records – Dose of drug, radiation, type of surgery etc • Medical examination or special test – Blood pressure, serum cholesterol • Environmental survey • By obtaining the data of exposure we can classify cohorts as – Exposed and non exposed and – By degree exposure we can sub classify cohorts
  40. Selection of comparison group • Internal comparison – Only one cohort involved in study – Sub classified and internal comparison done • External comparison – More than one cohort in the study for the purpose of comparison – e.g. Cohort of radiologist compared with ophthalmologists • Comparison with general population rates – If no comparison group is available we can compare the rates of study cohort with general population. – Cancer rate of uranium miners with cancer in general population
  41. Follow-up • To obtain data about outcome to be determined (morbidity or death) – Mailed questionnaire, telephone calls, personal interviews – Periodic medical examination – Reviewing records – Surveillance of death records – Follow up is the most critical part of the study • Some loss to follow up is inevitable due to death change of address, migration, change of occupation. • Loss to follow-up is one of the draw-back of the cohort study.
  42. ANALYSIS • Calculation of incidence rates among exposed and non exposed groups • Estimation of risk
  43. Incidence rates of outcome N d c b a Yes No Disease Status Yes No Exposure Status a+b c+d b+d a+c Total Study cohort Comparison cohort
  44. Incidence rate • Incidence among exposed = a a+b • Incidence among non-exposed = c c+d
  45. Estimation of risk • Relative Risk incidence of disease among exposed RR = ______________________________ Incidence of disease among non-exposed a/a+b = _________ c/c+d
  46. Estimation of Risk • Attributable Risk Incidence of disease among exposed – incidence of disease among non exposed AR = _______________________________ Incidence of disease among exposed a/a+b – c/c+d AR = _______________ a/a+b
  47. Smoking Lung cancer Total YES NO YES 70 6930 7000 NO 3 2997 3000 73 9927 10000 Find out RR and AR for above data
  48. • Incidence of lung cancer among smokers 70/7000 = 10 per 1000 • Incidence of lung cancer among non-smokers 3/3000 = 1 per thousand RR = 10 / 1 = 10 (lung cancer is 10 times more common among smokers than non smokers) AR = 10 – 1 / 10 X 100 = 90 % (90% of the cases of lung cancer among smokers are attributed to their habit of smoking)
  49. Types of Cohort Study • Prospective cohort study • Retrospective (historical) cohort study • Combination of Retrospective and Prospective cohort study.
  50. Cohort studies Strengths • We can find out incidence rate and risk • More than one disease related to single exposure • can establish cause - effect • good when exposure is rare • minimizes selection and information bias Weaknesses • losses to follow-up • often requires large sample • ineffective for rare diseases • long time to complete • expensive • Ethical issues
  51. Comparison between Case control & Cohort Case control • Proceed from effect to cause • Involves few subjects ,quick ,less expensive • Yields only estimate of relative risk by virtue of odds ratio • Sampling bias • Measuremnet bias Cohort • Proceed from cause to effect • Involves large no. prolong duration, expensive • Yield incidence rate,relative risk, attributable risk • No sampling bias • No information bias
  52. Thank U