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Research Methodology 
‘Case control study’ 
Dr. Rizwan S A, M.D., 
Assistant Professor, 
Department of Community Medicine, 
VMCH&RI, Madurai. 
17.11.2014 
II MBBS, Epidemiology series 1
Classification of research methods 
Research 
methods 
Observational 
Descriptive 
Case series, 
case reports, 
CS, cohort 
Analytical 
Ecological Cross-sectional 
Experimental 
Controlled 
Case control Cohort 
Uncontrolled, 
Non-random 
II MBBS, Epidemiology series 2
Procedures in descriptive epidemiology 
1. Define the population 
2. Define and describe the disease 
3. Measure the disease 
4. Compare 
5. Formulate hypothesis 
II MBBS, Epidemiology series 3
Association 
• Defined as the co-occurrence of two or more 
variables at a frequency which is more than 
that expected by chance 
• Association does not mean causation 
II MBBS, Epidemiology series 4
Causation - Hills criteria 
1. Temporality 
2. Strength 
3. Specificity 
4. Consistency 
5. Biological plausibility 
6. Coherence 
II MBBS, Epidemiology series 5
Dogma of cohort study 
Healthy people Exposure occurs Exposed & 
unexposed 
Disease 
occurs 
Diseased & 
non-diseased 
Time 
Direction of enquiry 
6 
II MBBS, Epidemiology series
CASE CONTROL STUDY 
II MBBS, Epidemiology series 7
Position in the evidence hierarchy 
II MBBS, Epidemiology series 8
Dogma of case control study 
Assemble 
cases – 
diseased 
Time 
Direction of enquiry 
Assemble 
controls – 
not having 
disease 
Measure 
exposure 
status 
Exposed 
and non-exposed 
II MBBS, Epidemiology series 9
II MBBS, Epidemiology series 10
Introduction 
• Synonyms – retrospective study 
• A study that compares two groups of people: those 
with the disease or condition under study (cases) and a 
very similar group of people who do not have the 
disease or condition (controls). 
• Essential elements 
– Both exposure and disease have occurred 
– Proceeds from effect to cause 
– Uses a comparison ‘control’ group 
II MBBS, Epidemiology series 11
2 by 2 table 
Diseased 
- Cases 
Non-diseased 
– Controls 
Total 
Exposed A B A+B 
Non-exposed C D C+D 
Total A+C B+D A+B+C+D 
II MBBS, Epidemiology series 12
Steps in case control study 
1. Selection of cases and controls 
2. Matching 
3. Measurement of exposure 
4. Analysis and interpretation 
II MBBS, Epidemiology series 13
1. Selection of cases and controls 
• Define case – diagnostic and eligibility criteria 
• Source of cases – hospital, general population 
• Define control – free from disease 
• Source of controls – hospital, relatives, 
neighbourhood controls, general population 
• Number of controls 
II MBBS, Epidemiology series 14
2. Matching 
• Ensure comparability between cases and controls 
• A confounding factor is defined as one which is 
associated both with exposure an disease, and is 
distributed unequally in study and control groups. 
• Examples: 
• Suspected aetiological factor should not be 
matched 
• Types of matching: pair, frequency 
II MBBS, Epidemiology series 15
2. Matching 
II MBBS, Epidemiology series 16
3. Measure the exposure 
• Define exposure 
• Measure in precisely the same manner both 
for cases and controls 
II MBBS, Epidemiology series 17
4. Analyse 
• Exposure rates among cases and controls 
• Odds Ratio (Cross-product ratio) 
– AD/BC 
II MBBS, Epidemiology series 18
4. Analysis – Example 
• E.g. 1. Depression and Vegetable eating 
Individuals With 
Depression 
(Cases) 
Individuals Without 
Depression 
(Controls) 
Total 
Eat 
Vegetables 
90 90 180 
Do Not Eat 
Vegetables 
130 130 260 
Total 220 220 440 
• Odds of exposure among cases: a/c = 90/130 = 0.6923 
• Odds of exposure among controls: b/d = 90/130 = 0.6923 
• Odds ratio = 0.6923/0.6923 = 1.0 
II MBBS, Epidemiology series 19
Biases 
• Bias due to confounding 
• Memory or recall bias 
• Selection bias 
• Berkesonian bias 
• Interviewer bias 
II MBBS, Epidemiology series 20
Pros & Cons 
Advantages Disadvantages 
Easy to carry out Subject to several biases 
Rapid results Selection of controls difficult 
Inexpensive Incidence can’t be measured 
Suitable for rare diseases Association doesn’t mean causation 
No risk to subjects Not practical for rare exposure 
Minimal attrition 
Multiple exposures can be 
studied 
II MBBS, Epidemiology series 21
Case controls studies Cohort studies 
Direction of inquiry from effect to 
cause 
Direction of inquiry from cause to 
effect 
Starts with the disease 
Starts with people exposed to the 
risk factor or suspected cause 
Usually the 1st approach to the 
testing of hypothesis, but also useful 
for exploratory studies 
Reserved for the testing of 
precisely formulated hypothesis 
Fewer study subjects Larger number of subjects 
Quick Long follow-up, delayed results 
Suitable for rare diseases 
Inappropriate when disease or 
exposure under investigation is rare 
Generally, yields only Odds ratio 
Yields incidence rates, relative risk, 
attributable risk 
Cannot yield information about 
disease other than that under study 
Can give information about more 
than one disease outcome 
Inexpensive Expensive 
II MBBS, Epidemiology series 22
Examples 
• Adenocarcinoma of vagina and DES 
• OCP and thrombosis 
• Thalidomide tragedy 
II MBBS, Epidemiology series 23
Email your doubts to: sarizwan1986@outlook.com 
You can download these slides at http://www.slideshare.net/RizwanSa 
THANKS FOR LISTENING 
II MBBS, Epidemiology series 24

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Research Methodology - Case control study

  • 1. Research Methodology ‘Case control study’ Dr. Rizwan S A, M.D., Assistant Professor, Department of Community Medicine, VMCH&RI, Madurai. 17.11.2014 II MBBS, Epidemiology series 1
  • 2. Classification of research methods Research methods Observational Descriptive Case series, case reports, CS, cohort Analytical Ecological Cross-sectional Experimental Controlled Case control Cohort Uncontrolled, Non-random II MBBS, Epidemiology series 2
  • 3. Procedures in descriptive epidemiology 1. Define the population 2. Define and describe the disease 3. Measure the disease 4. Compare 5. Formulate hypothesis II MBBS, Epidemiology series 3
  • 4. Association • Defined as the co-occurrence of two or more variables at a frequency which is more than that expected by chance • Association does not mean causation II MBBS, Epidemiology series 4
  • 5. Causation - Hills criteria 1. Temporality 2. Strength 3. Specificity 4. Consistency 5. Biological plausibility 6. Coherence II MBBS, Epidemiology series 5
  • 6. Dogma of cohort study Healthy people Exposure occurs Exposed & unexposed Disease occurs Diseased & non-diseased Time Direction of enquiry 6 II MBBS, Epidemiology series
  • 7. CASE CONTROL STUDY II MBBS, Epidemiology series 7
  • 8. Position in the evidence hierarchy II MBBS, Epidemiology series 8
  • 9. Dogma of case control study Assemble cases – diseased Time Direction of enquiry Assemble controls – not having disease Measure exposure status Exposed and non-exposed II MBBS, Epidemiology series 9
  • 11. Introduction • Synonyms – retrospective study • A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls). • Essential elements – Both exposure and disease have occurred – Proceeds from effect to cause – Uses a comparison ‘control’ group II MBBS, Epidemiology series 11
  • 12. 2 by 2 table Diseased - Cases Non-diseased – Controls Total Exposed A B A+B Non-exposed C D C+D Total A+C B+D A+B+C+D II MBBS, Epidemiology series 12
  • 13. Steps in case control study 1. Selection of cases and controls 2. Matching 3. Measurement of exposure 4. Analysis and interpretation II MBBS, Epidemiology series 13
  • 14. 1. Selection of cases and controls • Define case – diagnostic and eligibility criteria • Source of cases – hospital, general population • Define control – free from disease • Source of controls – hospital, relatives, neighbourhood controls, general population • Number of controls II MBBS, Epidemiology series 14
  • 15. 2. Matching • Ensure comparability between cases and controls • A confounding factor is defined as one which is associated both with exposure an disease, and is distributed unequally in study and control groups. • Examples: • Suspected aetiological factor should not be matched • Types of matching: pair, frequency II MBBS, Epidemiology series 15
  • 16. 2. Matching II MBBS, Epidemiology series 16
  • 17. 3. Measure the exposure • Define exposure • Measure in precisely the same manner both for cases and controls II MBBS, Epidemiology series 17
  • 18. 4. Analyse • Exposure rates among cases and controls • Odds Ratio (Cross-product ratio) – AD/BC II MBBS, Epidemiology series 18
  • 19. 4. Analysis – Example • E.g. 1. Depression and Vegetable eating Individuals With Depression (Cases) Individuals Without Depression (Controls) Total Eat Vegetables 90 90 180 Do Not Eat Vegetables 130 130 260 Total 220 220 440 • Odds of exposure among cases: a/c = 90/130 = 0.6923 • Odds of exposure among controls: b/d = 90/130 = 0.6923 • Odds ratio = 0.6923/0.6923 = 1.0 II MBBS, Epidemiology series 19
  • 20. Biases • Bias due to confounding • Memory or recall bias • Selection bias • Berkesonian bias • Interviewer bias II MBBS, Epidemiology series 20
  • 21. Pros & Cons Advantages Disadvantages Easy to carry out Subject to several biases Rapid results Selection of controls difficult Inexpensive Incidence can’t be measured Suitable for rare diseases Association doesn’t mean causation No risk to subjects Not practical for rare exposure Minimal attrition Multiple exposures can be studied II MBBS, Epidemiology series 21
  • 22. Case controls studies Cohort studies Direction of inquiry from effect to cause Direction of inquiry from cause to effect Starts with the disease Starts with people exposed to the risk factor or suspected cause Usually the 1st approach to the testing of hypothesis, but also useful for exploratory studies Reserved for the testing of precisely formulated hypothesis Fewer study subjects Larger number of subjects Quick Long follow-up, delayed results Suitable for rare diseases Inappropriate when disease or exposure under investigation is rare Generally, yields only Odds ratio Yields incidence rates, relative risk, attributable risk Cannot yield information about disease other than that under study Can give information about more than one disease outcome Inexpensive Expensive II MBBS, Epidemiology series 22
  • 23. Examples • Adenocarcinoma of vagina and DES • OCP and thrombosis • Thalidomide tragedy II MBBS, Epidemiology series 23
  • 24. Email your doubts to: sarizwan1986@outlook.com You can download these slides at http://www.slideshare.net/RizwanSa THANKS FOR LISTENING II MBBS, Epidemiology series 24