Case Control Study | Medical Science Optional Notes for UPSC PDF Download

Steps of Case Control Study 

Case and Control Selection in Case-Control Studies:

• Controls in a case-control study are typically individuals free of the disease.
• In smaller studies, the number of controls can be up to four times that of cases.
• Larger studies may aim for a 1:1 ratio of cases to controls.
• Sources of controls may include the general population, hospitals, relatives, neighbors, or colleagues.

Matching:

• Matching refers to the process of selecting controls in a way that makes them similar to cases concerning specific relevant variables (e.g., age) known to influence the disease outcome. This matching is essential to prevent potential distortion or confounding of results.
• A "confounding factor" is a variable associated with both exposure and disease, distributed unevenly in study and control groups. More precisely, a "confounding factor" is one that, while linked to the investigated "exposure," is independently a "risk factor" for the disease.

Measurement of Exposure:

• The investigation progresses from the effect to the cause, conducted after both the exposure and outcome have taken place.

Case control study-Measurement

Analysis in terms of exposure rates, risk ratio, and odds ratio:

• The odds ratio serves as a gauge of the association's strength between a risk factor and outcome in case-control studies.
• Case-control studies are instrumental in deriving the odds ratio, also referred to as the cross-product ratio.
• A visual representation of the odds ratio (OR) is provided in the image below.
• The calculation of the odds ratio (OR) is expressed as (a/b)/(c/d) = ad/bc, where: [variables not provided].
  

When the occurrence of exposure is more prevalent among cases, the odds ratio (OR) will surpass 1, signifying an elevated risk. Conversely, if the frequency is lower, the OR will be <1, suggesting potential protective effects. An OR of 1 indicates no association between risk and exposure or an equal likelihood of the event in both groups.

Case control study-BIAS

• Confounding bias can be mitigated through matching.
• Memory or recall bias may arise when cases and controls struggle to recollect specific details about their history.
• Selection bias can occur when cases and controls are not truly representative of the population, leading to systematic differences.
• Berkesonian bias, a form of selection bias, emerges in hospital cases and controls due to differing admission rates for individuals with various diseases.
• Interviewer's bias may arise if the interviewer is aware of the hypothesis and the identity of cases, potentially leading to more thorough interviews with cases than controls. Double blinding can eliminate this bias.
• Neyman's bias or prevalence-incidence bias is a consequence of selective survival in prevalent cases, often observed in fatal diseases.

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