What is epidemiology? What common measures are used in the field of epidemiology? What are the subject areas studied by epidemiologists? How has epidemiology evolved over time? What is the current focus of epidemiology? What are the health challenges of modern medicine (and focus of epidemiology)?
Definition of Epidemiology • Study of the distribution and determinants of diseases and injuries in human populations • Concerned with frequencies and types of injuries and illness in groups of people • Focus is not on the individual • Concerned with factors that influence the distribution of illness and injuries
Relationship Between Clinical Medicine and Epidemiology • Focus in medicine is the individual patient • Community replaces the individual patient in epidemiology
Fundamental Assumptions in Epidemiology • Disease doesn’t occur at random • Disease has causal and preventive factors • Disease is not randomly distributed throughout a population • Epidemiology uses systematic approach to study the differences in disease distribution in subgroups • Allows for study of causal and preventive factors
Components of Epidemiology • Measure of disease frequency • Quantification of existence or occurrence of disease • Distribution of disease - three questions • Who is getting disease? • Where is disease occurring? • When is disease occurring? • Formulation of hypotheses concerning causal and preventive factors • Determinants of disease • Hypothesis are tested using epidemiologic studies
Incidence • Measure of new cases of disease (or other events of interest) that develop in a population during a specified period of time • E.g. Annual incidence, five-year incidence • Measure of the probability that unaffected persons will develop the disease • Used when examining an outbreak of a health problem
Prevalence • Number of existing cases of disease or other condition • Proportion of individuals in a population with disease or condition at a specific point of time • Diabetes prevalence, smoking prevalence • Provides estimate of the probability or risk that one will be affected at a point in time • Provides an idea of how severe a problem may be – measures overall extent • Useful for planning health services (facilities, staff)
Ratio • Used to compare two quantities 1:1.1 ratio of female to male births • Used to show quantity of disease in a population cases population
Proportion • A specific type of ratio in which the numerator is included in the denominator, usually presented as a percentage
Calculation of proportion: Males undergoing bypass surgery at Hospital A Total patients undergoing bypass surgery at Hospital A 352 males undergoing bypass surgery 539 total patients undergoing bypass surgery 65.3% =
Rate • Special form of proportion that includes a specification of time • Most commonly used in epidemiology because it most clearly expresses probability or risk of disease or other events in a defined population over a specified period of time • 3 major types • Crude rates • Specific rates (age-specific, infant mortality) • Adjusted rates
Crude rates • Unadjusted, simple ratios cases in defined period of time x K population in defined period of time (k denotes units 100’s, 1,000, etc.) Crude mortality rate: Total deaths in 2003 x 1,000 = U.S. death rate Estimated U.S. pop in 2003
Adjusted or Standardized Rates • Allow for comparison of populations with different characteristics • Statistically constructed summary rates allow for appropriate comparisons by taking into account differences in populations (age, gender, etc.) • Example of use: Population in Arizona is much older than population in Alaska, so it would be inappropriate to compare mortality rates. Standardization allows for meaningful comparisons.
Incidence Rate • Also known as incidence density • Measure of incidence that is able to handle varying observation periods • Denominator is sum of person-time at risk
Relationship Between Incidence and Prevalence • Prevalence varies directly with both incidence and duration. • If incidence is low, but duration is long (chronic), prevalence will be large in relation to incidence. • If prevalence is low because of short duration (due to recovery, migration or death), prevalence will be small in relation to incidence.
Measures of Association • Calculations used to measure disease frequency relative to other factors • Indications of how more or less likely one is to develop disease as compared to another
Two by Two Tables Used to summarize frequencies of disease and exposure and used for calculation of association. Disease Yes No Total a b a + b Yes Exposure c d No c + d Total a + c b + d a + b + c + d
Relative Risk • Measure of association between incidence of disease and factor being investigated • Ratio of incidence rate for persons exposed to incidence rate for those not exposed Incidence rate among exposed RR = Incidence rate among unexposed • Estimate of magnitude of association between exposure and disease
Formula for relative risk: • Incidence rate among exposed • RR = • Incidence rate among unexposed • a / (a + b) • RR = • c / (c+ d) • Risk ratio • If RR calculated from cumulative incidence • Rate ratio • If RR calculated from incidence rate (person units of time)
RISK RATIO: Example Breast No Breast Cancer Cancer Total Alcohol 70 2,930 3,000 No alcohol 50 2,950 3,000 RR using Cumulative Incidence (CI): a/(a + b) 70 / 3,000 c/(c + d) 50 / 3,000 = = 1.4 =
Interpretation of Relative Risk • 1 = No association between exposure and disease • Incidence rates are identical between groups • > 1 = Positive association • < 1 = Negative association or protective effect • Example: .5 = half as likely to experience disease
Odds Ratio Breast No Breast Cancer Cancer Alcohol 70 100 No alcohol 50 140 a x d (70) (140) b x c (50) (100) * Used for case control studies because persons are selected based on disease status so you can’t calculate risk of getting disease = OR = = 2.0
Difference Measures • Attributable risk • # of cases among the exposed that could be eliminated if the exposure were removed = Incidence in exposed - Incidence in unexposed • Population attributable risk percent • Proportion of disease in the study population that could be eliminated if exposure were removed Incidence in total population - Incidence in unexposed incidence in total population =