Case Control Study
DEFINITION:
- A case control study involves two populations – cases and controls and has three distinct features :
- Both exposure and outcome have occurred before the start of the study.
- The study proceeds backwards from effect to cause(Retrospective)
- It uses a control or comparison group to support or refute an inference.
DESGIN:
- The investigator selects cases with the disease and appropriate controls without the disease and obtains data regarding past exposure to possible etiologic factors in both groups.
- The investigator then compares the frequency of exposure of the two groups.
- Hallmark of Case Control Study: Starts from cases and controls and searches for exposure.
|
CASES
(With disease) |
CONTROL
(Without disease) |
|
| Were Exposed | a | b |
| Were Not Exposed | c | d |
| Totals | a+c | b+d |
| Proportions Exposed | a/a+c | b/b+d |
SELECTION OF CASE & CONTROL:
Selection of CASES:
- Representativeness:
- Random sample of all cases of interest in the source population (e.g. from vital data, registry data).
- Selection of available cases from a medical care facility. (e.g. from hospitals, clinics)
- Information: Collected from cases themselves, or from a respondent by proxy (relative/ friend), from records or a combination of the above.
- Method of Selection:
- From incidence or prevalence case:
- Incident cases:Ongoing ascertainment of cases over time.
- Prevalent cases : Crosssectional survey.
- Selection of INCIDENT CASES is OPTIMAL.
- All newly diagnosed cases over a given period of time in a defined population.
- Excludes patients who died before diagnosis.
- Prevalent cases
- So patients who recovered early and those who died will not be included.
- Additional protection against bias by including deceased cases as well as those alive
- Diagnostic criteria for case studies
- Specificity
- Diagnostic bias
- Validation
Selection of CONTROLS:
- Comparability vs Representativeness:
- Similar to the cases in all respects other than having the disease i.e. COMPARABLE
- Representative of all non-diseased people in the population from which the cases are selected i.e. REPRESENTATIVE
- Representative of the general population in terms of probability of exposure to the risk factor
- They should also have had the same opportunity to be exposed as the cases have.
- Criteria:
- Comparability is more important than representativeness in the selection of controls
- The control should be at risk of the disease
- The control should resemble the case in all respects except for the presence of disease
- Sources:
- Hospital based
- Population based (registry cases)
- Neighbourhood controls/ Telephone exchange random dialing
- Best friend control/ Sibling control
- Number:
- Large study: Cases: Control :: 1:1
- Small study: Cases: Control :: 1:2, 1:3, 1:4.
- Use of multiple controls
- Types:
- Controls of same type:Cases: Control :: 1:1
- Multiple controls of different types: controls- 1 hospital, 1 neighborhood
- Objectives:
- Elimination of selection bias – Selection
- Minimization of information bias – Blinding
- Minimization of confounding – Matching
BIAS IN CASE CONTROL STUDY:
- Selection bias
- Information bias:
- Recall bias
- Interviewer bias
- Confounding bias
OUTCOMES OF CASE CONTROL STUDY:
- Exposure rates:
- The frequency of exposure to suspected risk factor in cases and in controls
- Exposure rates:
- Cases = a/ (a + c)
- Controls = b/ (b + d)
- Estimation of disease risk associated with exposure: (Odds ratio)
- Odds Ratio / Relative odds (estimate of relative risk)
- Odds ratio=Odds that case was exposed/Odds that control was exposed= (a/c)/ (b/d)
- Odds ratio ( = cross products ratio) can also be viewed =Product of the two cells that support the hypothesis of an association/product of the two cells which negate the hypothesis of an association= ad/bc
ADVANTAGES:
- Uncover etiology in rare diseases
- Understanding new diseases
- Commonly used in outbreaks investigation
- Useful if inducing period is long
- Relatively inexpensive
- Causal association factors can be revealed
DISADVANTAGES:
- Susceptible to bias
- Exposure misclassification
- Single outcome
- Incidence rates not usually calculate
- Cannot assess effects of matching variables
NESTED CASE CONTROL STUDY:
- Also k/as Prospective case-control study
- Cases and controls are drawn from the population in a fully enumerated cohort.
- Used when exposure of interest is difficult or expensive to obtain and when the outcome is rare.
- Cases and controls are developed during follow up.
- Exposure frequencies are compared by the data collected at the start of study.
- Advantages:
- Time and cost of beginning a new case–control study is avoided.
- Recall bias eliminated
- Exposure data are more likely to represent the preillness state
- Avoids problems (in study design) related to Temporal association
- By only measuring the covariate in as many participants as necessary, the cost and effort of exposure assessment is reduced.
Exam Important
- Recall bias is most commonly associated with Case control study
- Nested case control study is a type of Prospective study
- Odds ratio can be calculated in case control study
- Case control study are Relatively cheap
- Case control study Used for rare diseases
- Case control study proceeds from effect to cause
- Exposure already occur in case control study
- Risk factor can be identified using case control study
- There is no risk to subjects in case control study
- In a case control study, the most characteristic feature is Odds ratio estimation
- In case control study, the risk is calculated by Cross product ratio
- Possible to study many diseases in case control study
- Direction of study is reverse but time is forward in case control study
- As compared to a routine case control study, nested case control study avoids problems (in study design) related to Temporal association
- Causal association factors can be revealed by Case control study
- Matching is method to eliminate confounding bias used in Case control study
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