Epidemiology Classically speaking Classically speaking EPI DEMO LOGOS Upon,on,befall People,population,man the Study of The study of anything that happens to people That which befalls man Definition Generally Epidemiology is a fundamental medical science that focuses on the distribution and determinants of disease frequency in human populations. Why does a disease developed in some people and not in others ? Definition Study of the occurrence and distribution of health-related diseases or events in specified populations, and the application of this knowledge to control the health problem Definition Objectives To identify the etiology or the cause of a disease and the risk factors (factors that increase a persons risk for a disease) To determine the extent of disease found in the community (burden of disease) To study the natural history and progress of disease Objectives Evaluate both existing and new preventive and therapeutic measures, and modes of care delivery To provide the foundation for developing public policy Descriptive Epidemiology Examining, identifying, and reporting on the frequency and distribution of disease in a population. Learning the basic features of its distribution. Analytic Epidemiology Identifying factors underlying disease or health events. Testing a hypothesis by studying how exposures relate to outcomes 7 Practices Developing interventions to reduce disease or improve health in the community Using information from analytical studies, develop strategies centered around an important exposure factor. Test these strategies with clinical trials. Program Evaluation Examining the effectiveness of programs for disease control in the community 8 Practices Epidemiological Triad Epidemiologist have created a model to help explain the multifaceted phenomena of disease occurrance: the epidemiological triad Descriptive Epidemiological Triad: Person Place Time Analytical Epidemiological Triad: Agent Host Environment 25 February 2016Triads & Natural History10 Epidemiological Triad Descriptive epidemiology Descriptive epidemiology is the first step in epidemiological study. Careful observation of available information alone have led many successful preventive measures in the past. Describing health or disease occurrence according to place, person and time can lead to very meaningful hypothesis formulations. Epidemiological Triad Basic Triad of Descriptive Epidemiology TIME PLACE PERSON Why now? Why now? Why here? Why here? Why in this group? Why in this group? Epidemiological Triad Agent: Amount, infectivity, pathogenicity, virulence,. Environment: Physical, biological, social Host: Intrinsic factors, genetic, physiologic factors, psychological factors, immunity Health or Illness ? 25 February 2016Triads & Natural History Analytic Triad Epidemiological Triad Many diseases rely on an agent or single factor for an infectious disease to occur. Epidemiologist use an ecological view to assess the interaction of various elements and factors in the environment and disease-related implications When more than a single cause must be present for a disease to occur, this is called multiple causation Epidemiological Triad Counts Proportions Ratios Rates Tools of Measurement Counts Limited Interpretation New Cases Locationof Disease YearPopulation City A City B Annual Rate of Occurrence City A:20 / 100 = 1 / 5 City B:100 / 1000 = 1 / 10 Tools of Measurement Persons included in the numerator are always included in the denominator: A Proportion: A + B Indicates the magnitude of a part, related to the total. In epidemiology, tells us the fraction of the population that is affected. Proportions Tools of Measurement Proportions - Example ABTotal (A + B) # persons with hypertension # persons without hypertension Total study population 1,4009,65011,050 P = A / (A + B) = (1,400 / 11,050) = Tools of Measurement Like a proportion, is a fraction, BUT without a specified relationship between the numerator and denominator Example: Occurrence of Major Depression Female cases = =----2:1 female to male Male cases = Ratios Tools of Measurement Rates are a special form of a ratio which represents the probability of a certain event. The numerator is the number of occurrences of an event during a time period, and the denominator is the number of persons exposed to that event in the time period. In other wards it expresses the relative frequency of an event per unit time (risk) To be a true rate we must try to have only those at risk in the denominator. Sometimes this is difficult to do so we use approximations or use the total population. Rates Tools of Measurement Rate: a measure of the occurrence of a health event in a population group at a specified time period Number of events in time period Number at risk for the event in period Numerator Denominator : Tools of Measurement In epidemiology, the occurrence of a disease or condition can be measured using rates and proportions. We use these measures to express the extent of these outcomes in a community or other population. Rates tell us how fast the disease is occurring in a population. Proportions tell us what fraction of the population is affected. Tools of Measurement Measuring Epidemiologic Outcomes Health Outcomes Death Recovery Ongoing Disease Stable disease with treatment Progressive disease disability Re-infection, Recurrence 24 Related to Prognosis or the Evaluation of Health Care Interventions 25 Measurement of Mortality (death) Measurement of Morbidity (incidence, prevalence) Health outcomes in epidemiology can be expressed through multiple types of measures Incidence The development of new cases of a disease that occur during a specified period of time in previously disease-free or condition-free (at risk) individuals. 26 Morbidity Measures There are two fundamental approaches to considering the incidence of disease or a health condition - Cumulative Incidence - Incidence Rate (Incidence Density) Morbidity Measures Incidence is always calculated for a given period of time An attack rate is an incidence rate calculated for a specific disease for a limited period of time during an epidemic Population at risk during that time period X 1,000 Number of new events occurring during a time period Incidence Rate = Morbidity Measures Prevalence: It quantifies the burden of disease Number of new &old cases of disease in population in time period Population at risk during that time period = Prevalence Rate X K Morbidity Measures Point prevalence measures the frequency of all current events (old and new) at a given instant in time Period prevalence measures the frequency of all current events (old and new) for a prescribed period of time Morbidity Measures P ID High prevalence may reflect: High risk Prolonged survival without cure In migration of diseased persons Low prevalence may reflect: Low risk Rapid fatal disease progression Rapid cure Emigration of cases P= ID