Epidemiology Calculations and Concepts

Epidemiology Calculations

• Incidence and prevalence are calculated
• Case fatality rate is calculated
• Proportionate mortality is determined
• Sensitivity and specificity are measured
• Incremental cost-effectiveness ratio is calculated
• Direct and indirect vaccine efficacy can be calculated
• Attributable risk can be calculated

Epidemiology Definition

• Epidemiology comprises the study of the distribution and determinants of health-related occurrences in populations, and applying this study to addressing health problems.
• MacMahon and Pugh (1970) defined it as the study of the distribution and determinants of disease frequency in man.
• Last (1983) defined it as the study of the distribution and determinants of health-related occurrences and events in populations.

Types of Epidemiological Questions

• Intervention questions address intervention studies. They involve a population, intervention, comparator, and outcome(s) within a particular time and setting.
• Exposure questions concern exposure studies. They involve a population, exposure, comparator, and outcome(s) within a particular time and setting.

Incidence

• Incidence refers to the number of affected individuals in a population at a specific time divided by the total population at that time.
• Point Prevalence is the prevalence based on a single point in time.
• Period Prevalence is the prevalence over a specific time period.

Incidence Proportion (Example)

• The example computes the incidence proportion of type 2 diabetes within a nursing home from January 1, 2022, to December 31, 2022, with 800 residents.
• 50 residents had type 2 diabetes initially, leaving 750 residents at risk.
• 25 new cases of type 2 diabetes arose during the year
• The incidence proportion is 25/750 = 0.033, or 3.3%.

Quantifying Death

• Mortality rate is the number of deaths in a population during a particular period of time.
• Proportionate mortality is the percentage of deaths occurring in a population during a specific time period from a specific cause
• Case fatality rate is the percentage of people who contract a particular condition who die from it

Case Fatality Rate Example

• As of September 12, 2023, Canada had reported 4,716,000 COVID-19 cases.
• As of September 12, 2023, Canada reported 53,541 COVID-19 deaths.
• The case fatality rate for COVID-19 is calculated as 53,541/4,716,000 = 1.14%

Proportionate Mortality Example

• Total mortality in 2016 was 740 per 100,000 people per year.
• Stroke mortality in 2016 was 39.2 per 100,000 people per year.
• Proportionate mortality from stroke was 39.2/740 = 5.3%.

Observational Study Designs

• Observational studies are considered "natural experiments"
• In observational studies, the researcher does not manipulate the exposure of the subjects to disease, but rather observes the natural variation.

Case Reports

• A case report describes the symptoms and course of a single patient.
• Case reports can include speculation about an exposure however the findings are not generalized to the general population.

Case Series

• A case series examines multiple patients with similar conditions.
• Details of patient presentation, diagnostics, treatment and outcomes are generally described.

Relationship Between Exposure and Outcome

• Exposure can be related to doing something (exercise), or being something (ethnicity).
• Outcome can be something that happens to the exposed individuals (disease, a disease diagnosis, disease event).
• Case-control studies are structured to examine the link between exposure and outcome

Measures of Association

• Case-control studies test hypotheses, exploring whether an exposure is associated with a disease.
• Measures of association are used to determine the nature of that association (positive or negative).

Risk Calculation

• Risk is the likelihood that an event will occur in the future.
• Risk Calculation uses the notation Risk = (D + )/ (D+ + D-)

Odds Calculation

• Odds represent the probability of an event occurring divided by the probability of it not occurring.
• Odds Calculation uses the notation Odds =(D +)/ (D-)

Risk in Case-Control Studies

• Risk cannot be directly calculated in case-control study situations from already occurred events.
• Odds are an appropriate estimation in most epidemiological scenarios.

Prospective Cohort Studies

• Prospective cohort studies follow a population over time and track exposures and outcomes.
• The structure is to follow the population forward in time.

Issue with Risk Ratio

• Risk ratio assumptions for each subject in a study, including the same amount of time and the whole study is complete.

Person-Time Calculation

• Person-time is a way to correct problems in incidence density calculation.
• The method tracks the time each person participates in a study until their removal or reaching a pre-defined end point.

Incidence Density Ratio

• Incidence density ratio (IDR) is a method of comparing incident rates between those exposed and those not exposed.
• The formula takes into account the person-time of each group.

Attributable Risk

• Calculates how much of the risk in an exposed population can be attributed to a specific exposure.
• Formula considers the incidence of disease in the exposed versus the unexposed populations.

Bias in Studies

• Bias can result from various sources.
• Selection Bias, Information Bias, and Confounding Bias are common sources.

Selection Bias

• Selection bias occurs when the characteristics of study participants are different from the participants in the target population.

Information Bias

• Information bias occurs when there is a difference in the accuracy and quality of information collected between comparative groups in the study.

Confounding

• A confounding variable is correlated with the exposure and outcome but isn't part of the direct causal pathway (distorting the effect).

Random Error

• Random error reduces the precision of the measurements.
• Sample size increase reduces random error.

Systematic Error

• Systematic error reduces the accuracy of the measurements.
• Sample size increase does not reduce systematic error.
• Systemic error must be reduced through design improvements.

Differential Misclassification

• Differential misclassification refers to differing inaccuracies of outcome measurements between groups in a study.
• For example, subjects with a characteristic like high BMI might encounter a more sensitive test for a disease compared to subjects with low BMI.

Recall Bias

• Recall bias is a type of information bias where participants might inaccurately remember their past experiences or exposures, influenced by their current state.
• In case-control studies, cases tend to remember past exposures more clearly compared to controls
• This is especially true in nutritional epidemiology studies that use food frequency questionnaires (FFQ).

Nutritional Epidemiology Considerations

• Nutritional epidemiology studies often use food frequency questionnaires (FFQ) which have limitations associated with recall bias.
• Adjusting for total energy intake is vital in studies since energy intake can often influence various nutrient intakes, so including it as a variable in the analysis can help to account for this.

WHO Screening Criteria

• WHO 10 Screening Criteria for screening programs should consider factors like an important health issue, treatment for that issue, diagnosis and treatment facilities, stages of the disease easily detected, suitable tests or examinations, understanding of natural history, testing acceptability to the population, cost-effectiveness, and routine evaluation.

Lead Time Bias

• Lead time bias is an effect caused by earlier diagnosis of, for example, a specific disease through a screening process, thereby extending perceived survival time.
• The extra lead time doesn't affect true disease survival.

Primary, Secondary, and Tertiary Prevention

• Primary prevention involves measures to prevent disease onset .
• Secondary prevention finds disease early and before noticeable symptoms, aiming to reduce the severity.
• Tertiary prevention focuses on treating and managing long-term diseases, aimed at improving quality of life.

Types of Health Technology Evaluations

• Cost-minimization analysis (CMA) finds the least expensive equivalent intervention when the interventions have the same outcomes.
• Cost-effectiveness analysis (CEA) compares interventions based on outcomes (monetary and non-monetary) in which costs and benefits are measured.
• Cost-utility analysis (CUA) uses quality-adjusted life years (QALYs) as a measure of relative benefit.
• Cost-benefit analysis (CBA) express both outcomes and costs in monetary values or units

Incremental Ratios

• Incremental ratios help compare the relative costs and benefits of different health interventions to determine the most effective options when two or more are comparable.

Cost-Effectiveness Plane

• The cost-effectiveness plane is a graphical tool that illustrates the cost-effectiveness of different interventions.

Positionality

• The concept of positionality in research recognizes the social lens through which one approaches research and is a part of the research project itself and must be considered when studying vulnerable populations.
• Analyzing the role of social positioning in research processes is necessary to ensure ethical sensitivity toward the subjects and communities being studied
• Ownership, Control, Access, Possession (OCAP):*
• First Nations principles of OCAP outline how Indigenous data and information should be collected, protected, used, and shared, emphasizing Indigenous ownership and stewardship.
• OCAP is essential for responsible research involving Indigenous communities, particularly to address the potential for ethical harm through research.

Collaboration with Community

• A collaborative effort that involves community members as co-investigators and advisors is key to ethical and meaningful research.
• The Two-Eyed Seeing Approach involves viewing the world from both Indigenous and Western ways of knowing.

Certainty (Quality of Evidence):

• GRADE (Grading of Recommendations Assessment Development and Evaluation) framework evaluates the certainty of evidence to classify the strength of conclusions in a research study.
• Factors that can downgrade or upgrade the certainty level include consistency with other knowledge and risk of bias.

Bradford Hill Criteria:

• Bradford Hill criteria assesses whether an exposure is likely causal to an outcome (specifically related to epidemiological research).
• The criteria considers aspects such as temporal relationship, strength of association, other factors, specificity and consistency, and alternative explanations.

Bias in Studies

• The concept of Bias in Studies considers aspects of the studies such as errors in result interpretation that often occur through the selection and/or reporting of studies.
• Target Trials use a similar structured design to Randomized Controlled Trials (RCTs) except it uses observed data (i.e., observational data) not experimental data.
• Target trials use established trial principles in attempting to determine the best treatment strategy or approach to an observed situation.

Sensitivity and Specificity

• Sensitivity is the proportion of people with the disease that test positive.
• Specificity is the proportion of people without the disease that test negative.
• Positive predictive value (PPV) is the probability of having a disease when the test is positive, while negative predictive value (NPV) is the probability of not having a disease when the test is negative.

Analyzing Diet Impact on Disease

• Total energy intake plays a substantial role, requiring adjustment in studies if it is a variable in the analysis and will affect the study's conclusions if researchers do not recognize this potential influence.

Mendelian Randomization

• Mendelian randomization uses genetic variants associated with a certain exposure to infer causal relationships between that exposure and a particular outcome.
• Presentation End*

This quiz covers key calculations and definitions in epidemiology. Test your knowledge on incidence, prevalence, case fatality rates, and vaccine efficacy. Additionally, explore the types of epidemiological questions and their applications in public health.