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Introduction to Epidemiology

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1 Introduction to Epidemiology
Introduction to Research Methods In the Internet Era Introduction to Epidemiology Epidemiology as a Population Science Basic Epidemiology Measures All lectures from Workshop - This project is made possible by the support of the American people through the United States Agency for International Development (USAID). The contents are the sole responsibility of the University of Pittsburgh and do not necessarily reflect the views of USAID or the United States  Government. Thomas Songer, PhD

2 Key Lecture Concepts Understanding epidemiology as a science focused on populations Samples of the population are taken to assess health issues Health outcomes data can be expressed through multiple measures These measures can be expressed as differing metrics The fundamental concepts of this lecture are outlined here. 2

3 What is Epidemiology? 3

4 Epidemiology Epidemiology is the study of the determinants, distribution, and frequency of disease in human populations Who gets disease and why Epidemiologists study sick and well people to determine the crucial difference between those who get disease and those who are spared 4

5 Purpose of Epidemiology
To provide a basis for developing disease control and prevention measures for groups at risk. This translates into developing public health measures to prevent or control disease. 5

6 Population Focus The focus of epidemiology is on the occurrence of health and disease in the population.  What happens to many The population approach contrasts with clinical medicine’s primary concern with health and disease in the individual.  What happens to one 6

7 Health as a manifestation of individuals in social groups
Humans are social animals. Many diseases are caused only by the interaction of individuals within and between populations. Disease and health outcome patterns are generated in and by populations and need to be described, explained and predicted in a population setting. Bhopal 2002 7

8 What is a Population? The common definition of a population is “All the inhabitants of a given country or area considered together;…”  A “population” can also be groups of individuals that share a common thread Clinical populations Subgroups of the population by age, race, etc… 8

9 Epidemiology as a population science
Diseases are expressed biologically in individuals, however, no epidemiological study can be done on one person Epidemiology studies humans in the aggregate (i.e. groups) Conclusions are directly applicable to the groups studied Conclusions are only indirectly applicable to individuals Bhopal 2002 9

10 Epidemiology is… The study of disease and its treatment, control, and prevention in a population of individuals. Whole populations may be examined, but… More frequently, samples of the population may be examined. Samples that are studied must be representative of the population for the results to be generalized to the total population. Epidemiology is distinguished by its focus on the study of disease in populations (large groups) or communities. It is often impossible to study every single person in the population, so epidemiologists will assess the importance of disease by focusing on samples of the population. These samples must be representative (or reflect the characteristics of the overall population) for the results of any work to be valid. If the samples are representative, then one can say that is it possible to generalize the results of the epidemiology study from the sample to the total population. An important issue here is understanding the difference between populations and population samples. Almost all epidemiology work is done on samples, and then the results are assumed to apply to the whole population. This assumption can only be true if the sample represents the characteristics of the total population to a good degree. Torrence 1997 10

11 Descriptive & Inferential Statistics
Descriptive Statistics deal with the enumeration, organization and graphical representation of data from a sample Inferential Statistics deal with reaching conclusions from incomplete information, that is, generalizing from the specific sample Inferential statistics use available information in a sample to draw inferences about the population from which the sample was selected Rahbar

12 Background Different types of activities and practices are undertaken in epidemiology to develop disease control and prevention measures for groups at risk. 12

13 Broad Characterizations of Epidemiology Practices
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 13

14 Broad Characterizations of Epidemiology 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 14

15 There is a logical sequence to the practice of epidemiology in disease prevention
Descriptive Analytical Interventions Programs Disease prevention 15

16 Basic Question in Research
Are exposure and disease/outcome linked? Is there an association between them? E D Exposure Disease / Health Outcome 16

17 Health outcomes in research studies may be expressed through multiple types of measures

18 Basic Measurements of Disease or Health Outcome Frequency in Epidemiology
Measurement of Mortality (death) Measurement of Morbidity (incidence, prevalence) 18

19 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. 19 19

20 - Cumulative Incidence
There are two fundamental approaches to considering the incidence of disease or a health condition - Cumulative Incidence - Incidence Rate (Incidence Density) Approaches 20

21 Number of existing cases of disease in population
Prevalence: is another major measure of disease in the population It quantifies the “burden” of disease Number of existing cases of disease in population in time period Prevalence Rate = Persons in population in same time period 21

22 Epidemiology is frequently used in clinical populations….
To identify the response of health problems to health care solutions Assess the impact of health care on health problems (i.e. does treatment work?) Assist in the development of health services and programs 22

23 Health Outcomes May be intermediate in the clinical course of a disease or treatment Short-term events May be the end-result of the clinical course of a disease or treatment Longer-term events 23

24 Health Outcomes Death Recovery Ongoing Disease
Related to Prognosis or the Evaluation of Health Care Interventions Death Recovery Ongoing Disease Stable disease with treatment Progressive disease disability HRQOL; health related quality of life Re-infection, Recurrence 24

25 = Mortality: is one of the major measures of disease in the population
information available from death certificates (required by law) Crude Death rate: Number of deaths in time period Mortality rates are the most common measure of disease frequency used in epidemiology. This is due to the registration of all death on death certificates. The death certificate lists the cause of death which outlines the disease causing the death. The use of standards in this process allows public health practitioners to use death certificate data to assess disease frequency in different populations. Death certificate data, though, only portray numerator data. To calculate a mortality rate, you also need to consider the denominator, or the population at risk for dying. = Number at risk of dying in period 25

26 = Number of deaths under age 1 year in time period
Infant Mortality Rate: assesses the risk of dying during the first year of life Number of deaths under age 1 year in time period = Two other types of mortality rates have gathered attention in the public health community. These include the infant mortality rate. The calculation of the infant mortality rate is illustrated here. Note the denominator is slightly different here (the number of live births rather than the number in the population at the mid year point). This is because most infant mortality occurs very early after birth. Number of live births in time period 26

27 = Number of deaths among persons with disease Number of persons
Case-fatality: the frequency in which cases of disease die Case-fatality rate: the proportion of persons who die from a disease Number of deaths among persons with disease Another mortality rate used in public health is the case fatality rate. It is generally used to represent the severity of the disease. A disease with a high case fatality rate (like inhalation anthrax) is considered as a very severe disease. The case fatality rate also has a different denominator. Here the denominator is all persons with the disease. This is in contrast to other measures where epidemiologists usually consider both the number of persons with disease and the number who are healthy. The case fatality rate is also a misnomer in name. It is really a proportion, rather than a rate. = Number of persons with the disease 27

28 • The cause of mortality is common or relatively common, AND
Years of Potential Life Lost (YPLL): Measure of the loss of future productive years resulting from a specific cause of death. Indicates the potential burden related to when death occurs. YPLL are highest when: • The cause of mortality is common or relatively common, AND • Deaths occur at an early age. 28

29 Other Health Outcomes Disability rates Nutrition-related indicators
Health Care Utilization rates Indicators of social and mental health Salama 29

30 Outcome measures in research studies may be expressed in differing units, though most often as rates

31 Tools of Measurement • Counts • Proportions • Ratios • Rates 31

32 Case Counts Measuring disease or health or health care frequency starts with counting cases Simplest and most frequently gathered measure in epidemiology 32

33 Counts • Refers to the number of cases of a disease or
other health phenomenon being studied i.e. Number of cases of influenza in Astana in January 2012 • Can be useful for allocation of health resources • Limited usefulness for epidemiologic purposes without knowing size of the source population 33

34 Counts – Limited Interpretation
New Cases Location of Disease Year Population City A City B Annual Rate of Occurrence City A: 20 / = 1 / 5 City B: 100 / = 1 / 10 Understanding the source population is important. In this example, a decision based upon the count data alone might lead one to value City B over City A (100 cases of disease vs. 20 cases). However, the populations differ markedly between these cities. In fact, the disease is more important in City A (affecting 20% of the population vs. 10%). 34

35 Proportions 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. 35

36 Proportions - Example A B Total (A + B) # persons with hypertension
# persons without hypertension Total study population 1,400 9,650 11,050 P = A / (A + B) = (1,400 / 11,050) = 0.127 36

37 Ratios Like a proportion, is a fraction, BUT without a specified relationship between the numerator and denominator Example: Occurrence of Major Depression Female cases = = :1 female to male Male cases = 37

38 Rates A ratio in which TIME forms part of the denominator
Epidemiologic rates contain the following elements: • health issue frequency (in the numerator) • unit size of population • time period during which an event occurs 38

39 numerator denominator :
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 numerator : In epidemiology, the calculation of a rate generally involves some measure of the frequency of disease in the numerator and the population at risk in the denominator. denominator Number at risk for the event in period 39

40 Rates are the basic tool of epidemiologic practice
Why are rates important? because they provide more complete information to describe or assess the impact of a health issue in a community or population Rates are the fundamental tool of epidemiology because they give more complete information about the importance of disease in a community. 40

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