L11 | Epidemiology

Epidemiology

• Epidemiology studies health and illness patterns, aiming to understand the factors influencing these patterns in human populations.
• The term is derived from the Greek words "epi" (upon) and "demos" (people).

Epidemiological Research Applications

• Epidemiological studies help investigate the causes and natural history of diseases, aiming at prevention and health promotion.
• They are used to measure healthcare needs and evaluate clinical management, enhancing healthcare efficacy and efficiency.
• Research contributes to advancements in risk screening tools and diagnostic instruments.

Epidemiological Study Designs

• Epidemiological studies are divided into Experimental and Non-Experimental designs.
• Experimental studies include RCT/Clinical trials, Community interventions, Other experimental designs, Randomised crossover, and Laboratory studies.
• Non-Experimental studies are further divided into Descriptive and Analytical.
• Descriptive studies are Cross-Sectional, Case reports, and Case series.
• Analytical studies are Cohort, Case Control, Cross-sectional (analytic), and Ecological.

Sampling Techniques

• Sampling methods are categorized into Probability sampling and Non-Probability sampling.
• Probability sampling includes Random, Systematic, and Stratified methods.
• Non-Probability sampling includes Convenience, Judgment, Quota, and Snowball methods.

Data Analysis

• Cohort studies allow estimating incidence and mortality rates as descriptive measures of frequency, alongside relative risk (RR) or hazard ratio (HR) as comparative effect measures.
• Case-control studies use the odds ratio (OR) as a comparative effect measure.
• Cross-sectional studies determine the prevalence of diseases and risk factors, also allowing OR calculation.

Epidemiological Measures

• Incidence is the rate at which new cases of a disease or condition develop within a specific time period.
• Prevalence is the proportion of a population with a given disease or condition at a specified point in time.
• Relative risk is the ratio of the incidence of a disease in exposed individuals to that in unexposed individuals, calculated in cohort or prospective studies.
• Odds ratio is an estimate of relative risk in case-control studies, representing the ratio of the odds of exposure in cases to the odds of exposure in controls.
• Morbidity rate is the number of patients in a defined population who develop a morbid condition over a specific period of time.
• Mortality rate is the number of deaths in a defined population over a specific period.
• Hazard ratio is the ratio of the risk of an outcome (e.g., death) occurring in one group compared to another.

Data Calculation

• Incidence rate is calculated = (Number of new cases in the time period / Number of person-years) x 100,000.
• Cumulative incidence per period = (Number of new cases in the time period / Number of persons in the cohort) x 100,000.
• Mortality rate = (Number of deaths in the time period / Number of person-years) x 100,000.
• Cumulative mortality per period = (Number of deaths in the time period / Number of persons in the cohort) x 100,000.
• Case-fatality = (Number of deaths from the defined disease in the period / Number of new cases of the defined disease in the population) x 100.
• Odds ratio is calculated =(Number of exposed diseased patients / Number of non-exposed diseased patients) / (Number of exposed controls / Number of non-exposed controls).
• Prevalence = (Number of diseased patients in the study population / Number of persons in the study population) x 100.
• Relative risk = (Number of exposed diseased patients / Number of exposed persons) / (Number of non-exposed diseased patients / Number of non-exposed persons).
• Risk difference = (Number of exposed diseased patients / Number of exposed persons) - (Number of non-exposed diseased patients / Number of non-exposed persons).
• Standardized incidence ratio (SIR) = Observed number of cases / Expected number of cases.
• Standardized mortality ratio (SMR) = Observed number of deaths / Expected number of deaths.

Strengths and Limitations of Epidemiologic Studies

• Cohort studies can calculate disease rates in exposed and unexposed subjects over time (incidence, relative risk), gather data on event sequence to assess causality, examine multiple outcomes for a given exposure, and are good for investigating rare exposures.
• Cohort studies require large subject numbers for rare exposures, may be expensive, require long follow-up durations, and are susceptible to selection bias, loss to follow-up, recall bias and have less control over variables.
• Case-control studies are useful for studying rare diseases, identifying current cases, and evaluating historical factors and potential links. They need fewer subjects than cohort studies.
• Case-control studies cannot measure incidence or relative risk, have complicated bias minimization in control selection, struggle to establish causation due to their retrospective nature.
• Cross-sectional studies are quick and inexpensive to conduct, have no ethical difficulty, and have multiple outcomes and exposures studied.
• Cross-sectional studies cannot measure incidence, are difficult to infer causation, are unable to investigate temporal relations between outcomes and risk factors, and are no good for studying rare diseases.

Conclusions

• Epidemiological findings play a major role in clinical decision making by assessing, diagnosing, and identifying at-risk people and populations. They aid planning, implementing, and evaluating health services, while developing healthcare policies.