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Types of Study Designs DesignStudy Type Case reportObservational - Descriptive Case seriesObservational - Descriptive Cross sectionalObservational - Descriptive/Analytic Case controlObservational - Analytic CohortObservational - Analytic Clinical trialExperimental - Analytic 3
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Case Control Studies Case Control Studies
Dr Amna Rehana SiddiquiDepartment of Family and Community Medicine
October 17, 2010
Objectives
1. To understand the basic design features of case control study
2. To list the advantages and disadvantages of case control studies
3. To calculate the odds ratio as a measure of association in case control studies
Types of Study Designs
Design Study Type
Case report Observational - Descriptive
Case series Observational - Descriptive
Cross sectional Observational - Descriptive/Analytic
Case control Observational - Analytic
Cohort Observational - Analytic
Clinical trial Experimental - Analytic
3
Basic Question in Analytic Epidemiology
Are exposure and disease linked?
Direction of inquiry in case control studyDirection of inquiry in case control study
ExposureRisks e.gsmoking
Diseasee.g. Myocardial Infarction (MI)
Design of Case Control Study
The investigator selects cases with the diseasecases with the disease (MI) (MI)
and appropriate controls without the diseasecontrols without the disease
and obtains data regarding past exposure to possible etiologic factors (smoking & other ) in both groups then compares the frequency of exposure of the two groups.
Proportion of smoking in cases and controls
controlscases
Arrows show the extent of smoking among cases and controls
Smoking
Smoking
Proportion of exposure in cases and controls
controlscases
Arrows show the extent of factor among cases and controls
Saturated fatintake
Saturated fat intake
Study Design: Case Control Study Design: Case Control
-Exposure data collected retrospectively-Feasible where disease outcomes are rare -Case ascertainment system is present -More than one exposure can be studied -Control selection from the source population-Multiple controls possible; e.g. 2 controls/case) -Efficient in Cost and time
Study DesignStudy Design
Cases ControlsCases Controls
Myocardial infarction OTHER ILLNESS/ Free of any illness
EXPOSED
SMOKED
NOT EXPOSED
NEVER SMOKED
EXPOSED
SMOKED
NOT EXPOSED
NEVER SMOKED
Proportions of exposed and unexposed in cases and controls
Example Example MI Cases Controls
Smokers 110 150Never smokers 90 250
Total 200 400
% smokers 55% 37.5%If Cancer is related to smoking then more cases than
controls would be smokers
Selection of Cases Selection of Cases
• Source of cases / hospital based / other • Incident or prevalent • Representative• Homogenous criteria for case definition• Certainty of diagnosis
Selection of casesSelection of cases
Source population • Hospital• Physicians clinic• Registries • Community
Risk factors could be unique to any one particular source especially from a hospital/clinic
Selection of CasesSelection of Cases
• Incident cases; could take long time to wait to identify cases
• Early deaths before or after diagnosis may prevent determination of risk factors
• Prevalent cases; large pool of cases survival bias, change in exposures
Selection of ControlsSelection of Controls
Highly important for valid results Whether;• Controls be similar to cases in all aspects
except the occurrence of disease OR• Representative of all persons without the
disease in population from which the cases have arisen
Selection of ControlsSelection of Controls
Sources of Controls• Probability sample of a total population • Neighborhood controls • Sibling/ best friend control• Hospitalized controls
Measures of association• Odds ratio: A ratio of one odds to another
Odds of exposure in a case Odds of exposure in a case = probability of exposure (smoking)
probability of no exposure (not smoking)
Odds of exposure in a control Odds of exposure in a control = probability for exposure (smoking) probability for no exposure (not smoking)
Odds RatioOdds RatioIf any event takes place with a probability ‘p’ thenthe odds in the favor of event are ‘p/1-p’ to 1
If p=2/3 then the odds of event are 2/3 / 1/3 = 2 to1
=Prob (exposure|cases)/[1-prob(exposure|cases)] Prob(expsoure|controls)/[1-prob(exposure|controls)]
Odds Ratio is equivalent to Relative Risk (RR), onlyOdds Ratio is equivalent to Relative Risk (RR), onlywhen the disease is rare.when the disease is rare.
As incidence is not derived in case control studies then As incidence is not derived in case control studies then RR can not be derived from these studies.RR can not be derived from these studies.
Calculating Odds RatioCalculating Odds Ratio MI Cases Controls
Smokers 110 150Never smokers 90 250
Total 200 400
% smokers 55% 37.5%
Odds of exposure in cases=0.55/1-0.55 = 1.22Odds of smoking in controls=0.38/ 1-0.38 = 0.61
Odds Ratio= 1.22/0.61 = 2
Interpretation: Odds of smoking in cases are two times more than the odds of smoking in controls
Calculating Odds RatioCalculating Odds Ratio MI Cases Controls
Smokers 110 150Never smokers 90 250
Total 200 400
% smokers 55% 37.5%
Odds of exposure in cases=0.55/1-0.55 = 1.22Odds of smoking in controls=0.38/ 1-0.38 = 0.61
Odds Ratio= 1.22/0.61 = 2
Interpretation: Odds of smoking in cases are two times more than the odds of smoking in controls
ODDS RATIOODDS RATIO• Odds ratio ~ cross product ratio
Cases ControlsExposed 110 150
Not exposed 90 250
OR= a/b = 110/90 = ad/bc = [(110*250)/(150*90)] = 27500 = 2 c/d 150/250 13500
Odds of exposure in cases : two times compared to controls
Interpretation of (OR) odds ratioInterpretation of (OR) odds ratio> 1 means the exposure is a risk factor.
= 1 means the exposure is not associated with the disease.
< 1 means the exposure is protective
Factors considered in Design Factors considered in Design • Selection bias in selection of controls if selection is related to
exposure in any way; bias will occur • Recall bias; cases tend to recall more than controls; a high OR
could be a biased estimate
• Interviewer bias; interviewers would keenly interview cases to obtain information compared to controls
• Matching of cases and controls can be done on certain variables; e,g. age, gender, location, ethnicity
• Multiple controls increase the power of study as usually done on rare diseases and few cases are available during a specified time e.g. any cancer.
Advantages and Disadvantages
Advantages• Efficient in time & cost • Etiology of long latency
period can be studied e.g. chronic diseases • Multiple exposures could
be studied
Disadvantages• Errors in Case definitions
and in characterization of exposure
• Disease and exposure have already occurred and potential for bias remains
- selection - recall - interviewer
Example of Case Control Study Example of Case Control Study Toxic Shock SyndromeToxic Shock Syndrome
• Thirty-five patients were matched for age and menstruation to 105 controls:
• 34 of 35 cases (versus 80 of 105 controls) used tampons during every menstrual period;
• nine of 35 cases (versus 64 of 105 controls) practiced contraception.
• Make a two by two tables and• Calculate Odds Ratios
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