Community Medicine
Epidemiology
Community medicine epidemiology for MBBS: incidence versus prevalence, case-control and cohort studies, relative and attributable risk, screening validity and epidemic investigation, mapped to NMC codes CM7.1 to CM7.9.
MedNext Academy | 3 min read
Epidemiology
Community medicine epidemiology for MBBS: incidence versus prevalence, case-control and cohort studies, relative and attributable risk, screening validity and epidemic investigation, mapped to NMC codes CM7.1 to CM7.9.
This chapter defines epidemiology and its core measures of disease frequency, distinguishing incidence from prevalence and relative from attributable risk. It compares descriptive, case-control, cohort and experimental study designs, explains screening test validity, and outlines the criteria for causation and the steps of epidemic investigation.
High-yield: Epidemiology
- Epidemiology is the study of the distribution and determinants of health-related states in populations and the application of this to control health problems.
- Incidence measures new cases over a time period, while prevalence measures all existing cases at a point or over a period.
- Point prevalence equals incidence multiplied by average duration of disease, so long-lasting diseases accumulate high prevalence.
- Descriptive epidemiology examines disease by time, place and person to generate hypotheses.
- Analytical study designs include case-control and cohort studies, which test hypotheses about associations.
- Case-control studies start from disease and look back at exposure, giving an odds ratio, and suit rare diseases.
- Cohort studies start from exposure and follow forward for outcome, giving relative risk and incidence.
- Relative risk is the ratio of incidence in the exposed to the unexposed, and a value above one indicates increased risk.
- Attributable risk is the incidence difference between exposed and unexposed, showing the risk due to the exposure.
- Sensitivity is the ability of a screening test to correctly identify those with disease, and specificity to identify those without.
- Positive predictive value depends heavily on disease prevalence, falling sharply when the disease is rare.
- The randomised controlled trial is the gold standard experimental design for evaluating interventions.
- Bradford Hill criteria, including strength, consistency, temporality and biological plausibility, help judge causation.
- Investigating an epidemic follows systematic steps beginning with confirming the diagnosis and establishing the existence of an epidemic.
Analytical study designs compared
- **Case-control:** Starts from disease, looks back at exposure. Gives odds ratio. Best for rare diseases; cheaper and quicker.
- **Cohort:** Starts from exposure, follows forward to outcome. Gives relative risk and incidence. Best for rare exposures.
- **Relative risk:** Incidence in exposed divided by incidence in unexposed. Above one means increased risk.
- **Screening validity:** Sensitivity finds true cases; specificity finds true non-cases. Positive predictive value falls as prevalence falls.
NMC competencies in this chapter
- **CM7.1:** Define epidemiology and describe its principles, concepts and uses
- **CM7.3:** Sources of epidemiological data
- **CM7.4:** Calculate and interpret morbidity and mortality indicators
- **CM7.5:** Epidemiological study designs
- **CM7.6:** Need and evaluation of screening tests
- **CM7.7:** Steps in the investigation of an epidemic
- **CM7.8:** Association, causation and biases in epidemiological studies
Frequently Asked Questions
What is the difference between incidence and prevalence?
Incidence counts new cases occurring over a defined period, reflecting the risk of developing disease, while prevalence counts all existing cases at a point or over a period, reflecting the burden of disease. Prevalence roughly equals incidence times duration.
When is a case-control study preferred over a cohort study?
A case-control study is preferred for rare diseases and when quick, inexpensive results are needed, since it starts from cases and looks back at exposure, whereas a cohort study is better for rare exposures and gives direct incidence and relative risk.
Why does positive predictive value depend on prevalence?
Because when a disease is rare, even a test with high sensitivity and specificity produces many false positives relative to true positives, so the positive predictive value falls sharply as prevalence decreases.
What are the first steps in investigating an epidemic?
The investigation begins with confirming the diagnosis and verifying that an epidemic truly exists, then defining and counting cases, describing them by time, place and person, formulating and testing hypotheses, and implementing control measures.
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