Model 3 of science

1. Overview of Epidemiology

• Definition: The study of when and where diseases occur in human populations, and who within the population develops disease, to determine how diseases occur.

• Primary Objective: Prevention or control of illness, injury, death, and disability.

• Requirement: Accurate descriptions and measurements of population health are essential to achieve prevention goals.

2. Descriptive Epidemiology

• Time: Measures when the disease or health event occurs or occurred.

  • Can use any timeframe (hours, days, weeks, months, years).

  • A single year is the standard timeframe for most epidemiological data.

• Place: Measures where the health event occurs or occurred.

  • Environmental factors impacting occurrence:

  • Natural environment

  • Built environment

  • Occupational environment

  • Socioeconomic environment

• Person: Evaluates characteristics of individuals affected.

  • Common characteristics evaluated:

  • Age

  • Sex/Gender

  • Race and Ethnicity

  • Socioeconomic status

  • Occupation

  • Educational status

3. Health Status Indicators (HSIs)

• Definition: Quantitative measurements of a population's health and disease burden.

• Purposes:

  • Identify population health problems.

  • Evaluate the effectiveness of prevention strategies.

  • Monitor health trends over time.

  • Enable comparisons between different populations.

• Common HSIs:

  • Life Expectancy: Average number of years a person from a specific cohort is projected to live from a given point in time.

  • Health-Adjusted Life Expectancy (HALE): Average number of healthy years expected in a given population.

  • Disability-Adjusted Life Years (DALYs): Combined measure of premature death and lost healthy life due to illness or disability.

  • $1\,\text{DALY} = 1\,\text{lost year of healthy life}$

  • Categorized by cause to measure overall disease burden across a population.

4. Epidemiological Rates

• Definition: A measure of a health-related event in a given population over a specified time period, expressed per unit of population (e.g., per $1,000$ or $100,000$ people).

• General Formula:   $\text{Rate} = \frac{\text{Total \# of health events in a given time}}{\text{Total population at risk at that time}} \times 10^x$

• Utility:

  • Contextualizes events by time, place, and person.

  • Adjusts for total population size.

  • Provides a standardized unit for valid population comparisons.

5. Types of Health Rates

• Birth (Natality) Rates:

  • Crude Birth Rate:     $\text{Crude Birth Rate} = \frac{\text{\# of live births in a given year}}{\text{Total population at midyear}} \times \text{Unit Size}$

  • Birth Rate:     $\text{Birth Rate} = \frac{\text{\# of live births in a given year}}{\text{Total women aged } 15\text{--}44 \text{ in same year}} \times \text{Unit Size}$

• Mortality Rates:

  • Derived from highly reliable vital statistics (death certificates as numerator) and census data (population counts as denominator).

  • Cause-Specific Mortality Rate:     $\text{Cause-Specific Mortality Rate} = \frac{\text{Total \# of deaths from a specific cause in a year}}{\text{Total population at midyear}} \times \text{Unit Size}$

  • Infant Mortality Rate:     $\text{Infant Mortality Rate} = \frac{\text{Total \# of deaths of infants } 0\text{--}364 \text{ days old in a year}}{\text{Total live births in same year}} \times \text{Unit Size}$

  • Maternal Mortality Rate/Ratio:     $\text{Maternal Mortality Rate} = \frac{\text{Total \# of deaths from pregnancy/birth causes in a year}}{\text{Total live births in same year}} \times \text{Unit Size}$

• Morbidity Rates:

  • Measure disease occurrence in a population; less reliable than mortality data due to unreported cases.

  • Numerators are gathered via disease surveillance, record searches, surveys, and reporting by clinics or health departments.

  • Incidence Rate: Measures new cases during a specified period; assesses the risk of developing the disease.     $\text{Incidence Rate} = \frac{\text{Total \# of NEW cases in a given time period}}{\text{Total population at risk in that time period}} \times \text{Unit Size}$

  • Prevalence Rate: Measures total existing cases at a given point in time regardless of diagnosis date; estimates the overall burden of disease for health services planning.     $\text{Prevalence Rate} = \frac{\text{Total \# of existing cases at a given point in time}}{\text{Total population at that point in time}} \times \text{Unit Size}$

6. Contextual Analysis: Raw Numbers vs. Rates

• Case Example: Drug arrests on $5$ Ohio college campuses ($2009$).

• Ranking by Total Arrests (Misleading):

  1. Kent State University: $105$ arrests

  2. Ohio University: $86$ arrests

  3. Ohio State University: $68$ arrests

  4. Miami University: $60$ arrests

  5. Ohio Wesleyan University: $13$ arrests

• Calculation Example (Kent State):   $\text{Fraction} = 0.0053 \implies 0.0053 \times 1,000 = 5.3\,\text{arrests per } 1,000\,\text{students}$

• Ranking by Rate per $1,000$ Students (Accurate):

  1. Ohio Wesleyan University: $6.9$ per $1,000$ students

  2. Kent State University: $5.3$ per $1,000$ students

  3. Ohio University: $4.6$ per $1,000$ students

  4. Miami University: $4.1$ per $1,000$ students

  5. Ohio State University: $1.6$ per $1,000$ students

• Key Takeaway: Ranking by raw numbers without adjusting for population size can create misleading conclusions.