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Epidemiology I. What is epidemiology? What common measures are used in the field of epidemiology? What are the subject areas studied by epidemiologists?

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Presentation on theme: "Epidemiology I. What is epidemiology? What common measures are used in the field of epidemiology? What are the subject areas studied by epidemiologists?"— Presentation transcript:

1 Epidemiology I

2 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)?

3 Definition of Epidemiology Study of the distribution and determinants of diseases and injuries in human populations 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 Concerned with frequencies and types of injuries and illness in groups of people Focus is not on the individual Focus is not on the individual Concerned with factors that influence the distribution of illness and injuries Concerned with factors that influence the distribution of illness and injuries

4 Background Relatively new science – emerged in 19 th century Relatively new science – emerged in 19 th century In strictest terms – study of epidemics In strictest terms – study of epidemics Today: Today: Concerned with epidemic disease and all other forms of illness and bodily injury Concerned with epidemic disease and all other forms of illness and bodily injury Cancer, heart disease Cancer, heart disease HIV/AIDS HIV/AIDS Alcoholism, drug addiction Alcoholism, drug addiction Suicide Suicide Automobile accidents Automobile accidents …

5 Relationship Between Clinical Medicine and Epidemiology Focus in medicine is the individual patient Focus in medicine is the individual patient Community replaces the individual patient in epidemiology Community replaces the individual patient in epidemiology

6 Fundamental Assumptions in Epidemiology Disease doesn’t occur at random Disease doesn’t occur at random Disease has causal and preventive factors 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 Epidemiology uses systematic approach to study the differences in disease distribution in subgroups Allows for study of causal and preventive factors Allows for study of causal and preventive factors

7 Components of Epidemiology Measure of disease frequency Measure of disease frequency Quantification of existence or occurrence of disease Quantification of existence or occurrence of disease Distribution of disease - three questions Distribution of disease - three questions Who is getting disease? Who is getting disease? Where is disease occurring? Where is disease occurring? When is disease occurring? When is disease occurring?  Formulation of hypotheses concerning causal and preventive factors Determinants of disease Determinants of disease Hypothesis are tested using epidemiologic studies Hypothesis are tested using epidemiologic studies

8 Progression of Epidemiologic Reasoning 1. Suspicion that a factor may influence occurrence of disease - Observations in clinical practice - Are HC providers seeing unexpected illness patterns in their patients? - Examination of disease patterns - Do subpopulations have higher or lower rates? - Are disease rates increased in the presence of certain factors? - Observations in laboratory research - Theoretical speculation - What theories can be generated from existing knowledge of disease prevention and causation models?

9 Progression of Epidemiologic Reasoning (Cont.) 2. Formulation of specific hypotheses - Based on suspicions concerning influence of a particular factor on disease occurrence 3. Conduct study - Hypotheses are tested to determine if statistical associations between factors and disease occurrence exist - Study population is assembled from individuals with disease or outcome of interest and an appropriate comparison group - Data is collected and analyzed

10 Progression of Epidemiologic Reasoning (Cont.) 4. Assess validity of association - Does the observed association really exist? - Is the association valid? - Are there alternative explanations for the association? - Chance - Bias - Confounding

11 Progression of Epidemiologic Reasoning (Cont.) 5. Make a judgement of whether a cause- effect relation between factor (exposure) exists - What is the magnitude of the association? - Are the findings consistent with previous studies (or conflicting)? - Are the findings biologically credible? - Can underlying biological mechanisms that support the association be identified?

12 Historical Perspective Hippocrates - 5th century Hippocrates - 5th century Association between external environment and personal characteristics and health Association between external environment and personal characteristics and health

13 “Whoever wishes to investigate medicine properly should proceed thus: in the first place consider the seasons of the year, and what effects each of them produces. Then the winds, the hot and the cold, especially such are as common to all countries, and then such as are peculiar to each locality. In the same manner, when one comes into a city to which he is a stranger, he should consider its situation, how it lies as to the winds and the rising of the sun; for it influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. One should consider most attentively the waters which the inhabitants use, whether they be marshy and soft, or hard and running form elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hallow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor.” (Hippocrates, “On airs, waters and places” Medical Classics 3:19, 1938).

14 Historical Perspective John Graunt – 1662 (Hennekins and Buring 1987) John Graunt – 1662 (Hennekins and Buring 1987) The Nature and Political Observations Made Upon the Bills of Mortality Systematic statistical approach Systematic statistical approach Analyzed births and deaths in London Analyzed births and deaths in London Excess of males born, higher mortality for males Excess of males born, higher mortality for males Infant mortality is very high Infant mortality is very high Seasonal variation for mortality Seasonal variation for mortality Importance of routinely collected information for study of human illness Importance of routinely collected information for study of human illness William Farr William Farr Examined mortality and occupation and marital status Examined mortality and occupation and marital status Identified important issues in epidemiological investigations Identified important issues in epidemiological investigations Use of comparison population, influence of multiple factors on disease Use of comparison population, influence of multiple factors on disease

15 Historical Perspective John Snow (1854) – Father of modern epidemiology John Snow (1854) – Father of modern epidemiology Established modern epidemiologic methods Established modern epidemiologic methods Cholera epidemic in London Cholera epidemic in London Plotted geographical location of all cases – deaths from cholera Plotted geographical location of all cases – deaths from cholera

16 From The Visual Display of Quantitative Data, Edward R. Tufte

17 John Snow (cont) Went door to door, collecting information on daily habits Went door to door, collecting information on daily habits Suspected water supply as source of epidemic Suspected water supply as source of epidemic Broad street pump closed, epidemic stopped Broad street pump closed, epidemic stopped Mode of investigation – “shoe leather” Mode of investigation – “shoe leather” Practical application of epidemiology – use epidemiological investigation to impact a health problem Practical application of epidemiology – use epidemiological investigation to impact a health problem

18 How the Epidemiologist Works Studies origin and distribution of a health problem Studies origin and distribution of a health problem Collection of data Collection of data Constructs a logical chain of inferences to explain the various factors in a society or segment of society that cause a health problem to exist Constructs a logical chain of inferences to explain the various factors in a society or segment of society that cause a health problem to exist Likened to a detective investigating the scene of a crime looking for clues Likened to a detective investigating the scene of a crime looking for clues Starts with examination of sick person(s) Starts with examination of sick person(s) Extends investigation to the setting where illness is occurring Extends investigation to the setting where illness is occurring Looks for common denominator that links all the affected so that the cause of the problem can be eliminated or controlled Looks for common denominator that links all the affected so that the cause of the problem can be eliminated or controlled

19 Epidemiologic Analyses – Areas of Study Causal agents related to disease: 1. Biological agents – bacteria, viruses, insects 2. Nutritional agents – diet (fats, carbohydrates, food nutrients) 3. Chemical agents – gases, toxic agents 4. Physical agents – climate, vegetation, chemical pollutants (air, water, food) 5. Social agents – occupation, stress, social class, lifestyle, location of residence

20 Epidemiologist studies: Host characteristics: Host characteristics: Biological factors Biological factors Age, sex, degree of immunity, other physical attributes that promote resistance or susceptibility Age, sex, degree of immunity, other physical attributes that promote resistance or susceptibility Behavioral factors Behavioral factors Habits, culture, lifestyle Habits, culture, lifestyle Social environment Social environment Living conditions such as poverty, crowding Living conditions such as poverty, crowding Norms, values and attitudes Norms, values and attitudes Socially prescribed standards of living Socially prescribed standards of living Use of food and water, food handling practices Use of food and water, food handling practices Household and personal hygiene Household and personal hygiene

21 Eras of Epidemiology 1. Sanitary era – early 19 th century 2. Infectious disease era – between late 19 th century and early 20 th century 3. Chronic disease era – 2 nd half of 20 th century 4. Eco-epidemiology era – 21 st century

22 Definitions

23 Case Episode of disorder, illness, or injury affecting an individual Episode of disorder, illness, or injury affecting an individual Case of measles Case of measles Cancer case Cancer case TB case TB case Food poisoning event Food poisoning event Various sources provide case information Various sources provide case information Interviews or surveys Interviews or surveys Medical providers Medical providers Institutions or agencies Institutions or agencies

24 Incidence Measure of new cases of disease (or other events of interest) that develop in a population during a specified period of time 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 E.g. Annual incidence, five-year incidence Measure of the probability that unaffected persons will develop the disease Measure of the probability that unaffected persons will develop the disease Used when examining an outbreak of a health problem Used when examining an outbreak of a health problem

25 Prevalence Number of existing cases of disease or other condition 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 Proportion of individuals in a population with disease or condition at a specific point of time Diabetes prevalence, smoking prevalence Diabetes prevalence, smoking prevalence Provides estimate of the probability or risk that one will be affected at a point in time 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 Provides an idea of how severe a problem may be – measures overall extent Useful for planning health services (facilities, staff) Useful for planning health services (facilities, staff)

26 Epidemic, Endemic and Pandemic Epidemic Epidemic Any significant increase in the number of persons affected by a disease Any significant increase in the number of persons affected by a disease The first occurrence of a new disease The first occurrence of a new disease Endemic Endemic A disease that is established within a population that remain at a fairly stable prevalence A disease that is established within a population that remain at a fairly stable prevalence Pandemic Pandemic Widespread, universal disease penetration over a wide geographic area Widespread, universal disease penetration over a wide geographic area

27 More Terms Morbidity – illnesses, symptoms, impairments Morbidity – illnesses, symptoms, impairments Mortality - deaths Mortality - deaths Acute disease – diseases that strike and disappear quickly, within a month or so (chicken pox, colds) Acute disease – diseases that strike and disappear quickly, within a month or so (chicken pox, colds) Chronic disease – long term or lifelong diseases, incurable Chronic disease – long term or lifelong diseases, incurable

28 More Terms Birth cohort Birth cohort Persons born in a given year Persons born in a given year Life expectancy (LE) Life expectancy (LE) Average number of years of life remaining to a person at a particular age Average number of years of life remaining to a person at a particular age Based on mortality rates and personal characteristics (e.g. gender, race) Based on mortality rates and personal characteristics (e.g. gender, race) Years of potential life lost (YPLL) Years of potential life lost (YPLL) Measure of premature mortality Measure of premature mortality Death before age 75 Death before age 75

29 Epidemiologic Measures

30 Ratio Used to compare two quantities Used to compare two quantities 1:1.1 ratio of female to male births Used to show quantity of disease in a population Used to show quantity of disease in a population cases cases population population

31 Proportion A specific type of ratio in which the numerator is included in the denominator, usually presented as a percentage A specific type of ratio in which the numerator is included in the denominator, usually presented as a percentage

32 352 males undergoing bypass surgery 539 total patients undergoing bypass surgery = Calculation of proportion: Males undergoing bypass surgery at Hospital A Total patients undergoing bypass surgery at Hospital A 65.3%

33 Rate Special form of proportion that includes a specification of time 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 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 3 major types Crude rates Crude rates Specific rates (age-specific, infant mortality) Specific rates (age-specific, infant mortality) Adjusted rates Adjusted rates

34 Crude rates Unadjusted, simple ratios 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 x 1,000 = U.S. death rate Estimated U.S. pop in 2003

35 9,981 deaths in Detroit in ,270 total population in Detroit 2000 = per 1, per 100,000 Calculation of rates: Number of events in a specified time period Population at risk of these events in a specified time period X k k is used to denote the units of population such as per 1,000 or per 100,000

36 Detroit Population N=951,270 in 2000 N=951,270 in deaths 9981 deaths 15,892 births 15,892 births 7,181 to single “named” parent 7,181 to single “named” parent Is Detroit population declining, stable or increasing? Is Detroit population declining, stable or increasing?

37 Specific Rates Capture effects of specific variables or social characteristics Capture effects of specific variables or social characteristics Age-specific, gender-specific, gender and race- specific Age-specific, gender-specific, gender and race- specific  Example – infant mortality – deaths within the 1 st year of life Total # of deaths in 2003 among persons age less than 1 year persons age less than 1 year x 1,000 = 2003 infant x 1,000 = 2003 infant Number of live births during 2003 mortality rate

38

39 Adjusted or Standardized Rates Allow for comparison of populations with different characteristics 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.) 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. 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.

40 Number of existing cases of disease P = at a given point in time Total population at risk Calculating prevalence: 2176 DNW pts with asthma encounter P = = DNW pts = 7 asthmatics per 100 pts = 7 %

41 Number of existing cases of disease P = at a given point in time Total population at risk Prevalence calculation exercise: Pediatric Asthma at DNW 2159 DNW pts < 19 with asthma encounter P = = 9173 DNW pts < 19 =

42 Types of Prevalence Point prevalence: number of cases that exist at a given point in time Point prevalence: number of cases that exist at a given point in time Lifetime prevalence: proportion of the population that has a history of a given disorder at some point in time Lifetime prevalence: proportion of the population that has a history of a given disorder at some point in time Period prevalence: number of cases that exist in a population during a specified period of time Period prevalence: number of cases that exist in a population during a specified period of time

43 Cumulative Incidence The proportion of individuals who become diseased during a specified time period. The proportion of individuals who become diseased during a specified time period. Time period can be a calendar year, 6 months, 3 years, 5 years, etc. Time period can be a calendar year, 6 months, 3 years, 5 years, etc.

44 Number of new cases of disease during a given time period CI = Total population at risk Formula for cumulative incidence: 70 new cases of breast cancer in a 5 year period CI = 3,000 women at risk = = 23 cases per 1,000 women during 5 years

45 Incidence Rate Also known as incidence density Also known as incidence density Measure of incidence that is able to handle varying observation periods Measure of incidence that is able to handle varying observation periods Denominator is sum of person-time at risk Denominator is sum of person-time at risk

46 Number of new cases of disease during a given time period ID = Total person-time at risk Formula for incidence rate or incidence density: 70 new cases of breast cancer ID = 13,000 women-years of observation = = 5.4 cases / 1,000 women years

47 Relationship Between Incidence and Prevalence Prevalence varies directly with both incidence and duration. 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 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. If prevalence is low because of short duration (due to recovery, migration or death), prevalence will be small in relation to incidence.

48 Special Types of Incidence Rates Morbidity rate - number of nonfatal cases in the population at risk during a specified period of time Morbidity rate - number of nonfatal cases in the population at risk during a specified period of time Mortality rate - number of deaths in a population at risk during a specified period of time Mortality rate - number of deaths in a population at risk during a specified period of time Cause-specific mortality - death from a specific cause Cause-specific mortality - death from a specific cause Case fatality rate - number of deaths from a disease divided by all case of the disease Case fatality rate - number of deaths from a disease divided by all case of the disease Attack rate - cumulative incidence expressing the risk of disease among a population observed for a specified period of time Attack rate - cumulative incidence expressing the risk of disease among a population observed for a specified period of time

49 Special types of incidence and prevalence measures RateTypeNumeratorDenominator Morbidity rate Incidence New cases of nonfatal disease Total population at risk Mortality rate Incidence Number of deaths from a disease or all causes Total population Case-fatality rate Incidence Number of deaths from a disease Number of cases of that disease Attack rate Incidence Number of cases of a disease Total population at risk, for a limited period of observation Disease rate at autopsy Prevalence Number of cases of a disease Number of persons autopsied Birth defect rate Prevalence Number of babies with a given abnormality Number of live births

50 Measures of Association Calculations used to measure disease frequency relative to other factors 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 Indications of how more or less likely one is to develop disease as compared to another

51 Two by Two Tables Used to summarize frequencies of disease and exposure and used for calculation of association. Disease Exposure YesNo Yes No Total a cd b a + b c + d a + c b + da + b + c + d

52 Two by Two Tables: Contents of Cells a = number of individuals who are exposed and have the disease b = number who are exposed and do not have the disease c = number who are not exposed and have the disease d = number who are both non-exposed and non-diseased *************************************************** a + b = the total number of individuals exposed c + d = the total number of unexposed a + c = the total number with the disease b + d = the total number without the disease a + b + c + d = sum of all four cells and the total sample size for the study

53 Relative Risk Measure of association between incidence of disease and factor being investigated 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 Ratio of incidence rate for persons exposed to incidence rate for those not exposed Incidence rate among exposed Incidence rate among exposed RR = Incidence rate among unexposed Incidence rate among unexposed Estimate of magnitude of association between exposure and disease Estimate of magnitude of association between exposure and disease

54 Incidence rate among exposed RR = Incidence rate among unexposed Formula for relative risk: 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)

55 RISK RATIO: Example Breast No Breast Breast No Breast Cancer Cancer Total 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 a/(a + b) 70 / 3,000 c/(c + d) 50 / 3,000 c/(c + d) 50 / 3,000 = = = 1.4

56 Interpretation of Relative Risk 1 = No association between exposure and disease 1 = No association between exposure and disease Incidence rates are identical between groups Incidence rates are identical between groups > 1 = Positive association > 1 = Positive association < 1 = Negative association or protective effect < 1 = Negative association or protective effect Example:.5 = half as likely to experience disease Example:.5 = half as likely to experience disease

57 Odds Ratio Breast No Breast Breast No Breast Cancer Cancer Cancer Cancer Alcohol No alcohol a x d (70) (140) a x d (70) (140) b x c (50) (100) 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

58 Difference Measures Attributable risk Attributable risk # of cases among the exposed that could be eliminated if the exposure were removed # of cases among the exposed that could be eliminated if the exposure were removed = Incidence in exposed - Incidence in unexposed Population attributable risk percent Population attributable risk percent Proportion of disease in the study population that could be eliminated if exposure were removed 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 - Incidence in unexposed incidence in total population incidence in total population =

59 Impact of Modernization on Health Infant mortality decreased Infant mortality decreased Life expectancy greatly increased during 20 th century Life expectancy greatly increased during 20 th century Males Males Increased from 48 to 74 years Increased from 48 to 74 years Females Females Increased from 51 to 79 years Increased from 51 to 79 years Persons living longer with multiple illnesses Persons living longer with multiple illnesses Chronic and degenerative diseases Chronic and degenerative diseases Illness with social causes requiring social solutions Illness with social causes requiring social solutions

60 Changing Mortality Patterns Pneumonia/InfluenzaTuberculosis Heart disease StrokeDiarrhea/enteritisNephritisCancerAccidentsDiphtheria 1990s 1990s Heart disease CancerStroke Chronic lung disease Unintentional injuries Pneumonia/influenza Diabetes mellitus HIV/AIDSSuicide

61 U. S. Life Expectancy Remaining life expectancy in yearsMalesFemales 1900 – at birth – at birth – at age – at birth – at age – at age Health, United States, 2002

62 U.S. Infant Mortality Rate Significance: Significance: Measure of society’s sanitary and medical standards Measure of society’s sanitary and medical standards Health care Health care Diet Diet Living conditions Living conditions Deaths per 1,000 infants

63 Factors Influencing Changing Pattern Improvements due to industrialization Improvements due to industrialization Nutrition Nutrition Environmental Environmental Sanitation Sanitation Water supply Water supply Housing Housing Medical advancements Medical advancements Antibiotics Antibiotics Immunization Immunization Disease surveillance programs Disease surveillance programs

64 Factors Influencing Changing Patterns Problems associated with industrialization Problems associated with industrialization Environmental pollutants Environmental pollutants Increase in smoking Increase in smoking Excess consumption of calories and dietary fats Excess consumption of calories and dietary fats Lack of exercise, physical activity Lack of exercise, physical activity Stress Stress Alcohol, drug use Alcohol, drug use

65 Challenges of Modern Medicine Behavioral aspects of health Behavioral aspects of health Promotion of healthy lifestyles Promotion of healthy lifestyles Diet Diet Exercise Exercise Tobacco, alcohol, drugs Tobacco, alcohol, drugs Sexual behavior Sexual behavior Management of stress Management of stress New diseases – AIDS, SARS, West Nile Virus, bioterrorism New diseases – AIDS, SARS, West Nile Virus, bioterrorism


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