Epidemiology I

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Epidemiology I
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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)?
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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

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Background

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

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Relationship Between Clinical Medicine and
Epidemiology

• Focus in medicine is the individual patient
• Community replaces the individual patient in
  epidemiology

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Fundamental Assumptions in Epidemiology

• Disease doesnt 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

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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

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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?

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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

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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

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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?

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Historical Perspective

• Hippocrates - 5th century
• Association between external environment and
  personal characteristics and health

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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 319, 1938).
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Historical Perspective

• John Graunt 1662 (Hennekins and Buring 1987)
• The Nature and Political Observations Made Upon
  the Bills of Mortality
• Systematic statistical approach
• Analyzed births and deaths in London
• Excess of males born, higher mortality for males
• Infant mortality is very high
• Seasonal variation for mortality
• Importance of routinely collected information for
  study of human illness
• William Farr - 1839
• Examined mortality and occupation and marital
  status
• Identified important issues in epidemiological
  investigations
• Use of comparison population, influence of
  multiple factors on disease

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Historical Perspective

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

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From The Visual Display of Quantitative Data,
Edward R. Tufte
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John Snow (cont)

• Went door to door, collecting information on
  daily habits
• Suspected water supply as source of epidemic
• Broad street pump closed, epidemic stopped
• Mode of investigation shoe leather
• Practical application of epidemiology use
  epidemiological investigation to impact a health
  problem

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How the Epidemiologist Works

• Studies origin and distribution of a health
  problem
• 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
• Likened to a detective investigating the scene of
  a crime looking for clues
• Starts with examination of sick person(s)
• 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

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Epidemiologic Analyses Areas of Study

• Causal agents related to disease
• Biological agents bacteria, viruses, insects
• Nutritional agents diet (fats, carbohydrates,
  food nutrients)
• Chemical agents gases, toxic agents
• Physical agents climate, vegetation, chemical
  pollutants (air, water, food)
• Social agents occupation, stress, social class,
  lifestyle, location of residence

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Epidemiologist studies

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

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Eras of Epidemiology

• Sanitary era early 19th century
• Infectious disease era between late 19th
  century and early 20th century
• Chronic disease era 2nd half of 20th century
• Eco-epidemiology era 21st century

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Definitions
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Case

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

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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

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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)

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Epidemic, Endemic and Pandemic

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

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More Terms

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

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More Terms

• Birth cohort
• Persons born in a given year
• Life expectancy (LE)
• 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)
• Years of potential life lost (YPLL)
• Measure of premature mortality
• Death before age 75

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Epidemiologic Measures
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Ratio

• Used to compare two quantities
• 11.1 ratio of female to male births
• Used to show quantity of disease in a population
• cases
• population

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Proportion

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

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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

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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

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Crude rates

• Unadjusted, simple ratios
• cases in defined period of time
• x K
• population in defined period of time
• (k denotes units 100s, 1,000, etc.)
• Crude mortality rate
• Total deaths in 2003
• x 1,000 U.S. death rate
• Estimated U.S. pop in 2003

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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

9,981 deaths in Detroit in 2000 951,270 total
population in Detroit 2000
10.49 per 1,000 1049 per 100,000
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Detroit Population

• N951,270 in 2000
• 9981 deaths
• 15,892 births
• 7,181 to single named parent
• Is Detroit population declining, stable or
  increasing?

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Specific Rates

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

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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.

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Calculating prevalence
Number of existing cases of disease P

at a given point in time
Total population at risk
2176 DNW pts with asthma encounter
P
.07 31005
DNW pts 7 asthmatics per 100 pts 7
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Prevalence calculation exercise Pediatric
Asthma at DNW
Number of existing cases of disease P

at a given point in time
Total population at risk
2159 DNW pts encounter P

9173 DNW pts
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Types of Prevalence

• 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
• Period prevalence number of cases that exist in
  a population during a specified period of time

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Cumulative Incidence

• 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.

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Formula for cumulative incidence
Number of new cases of disease during a
given time period CI
Total population at risk
70 new cases of breast cancer in a
5 year period CI 3,000
women at risk 0.023 23 cases per 1,000
women during 5 years
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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

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Formula for incidence rate or incidence density
Number of new cases of disease during a
given time period ID
Total person-time at risk
70 new cases of breast
cancer ID 13,000
women-years of observation 0.0054 5.4
cases / 1,000 women years
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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.

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Special Types of Incidence Rates

• 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
• Cause-specific mortality - death from a specific
  cause
• 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

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Special types of incidence and prevalence measures
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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

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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
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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

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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

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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)

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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

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Interpretation of Relative Risk

• 1 No association between exposure and disease
• Incidence rates are identical between groups
• 1 Positive association
• Example .5 half as likely to experience
  disease

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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 cant
  calculate risk of getting disease

OR

2.0
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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

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Impact of Modernization on Health

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

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Changing Mortality Patterns

• 1900
• Pneumonia/Influenza
• Tuberculosis
• Heart disease
• Stroke
• Diarrhea/enteritis
• Nephritis
• Cancer
• Accidents
• Diphtheria

• 1990s
• Heart disease
• Cancer
• Stroke
• Chronic lung disease
• Unintentional injuries
• Pneumonia/influenza
• Diabetes mellitus
• HIV/AIDS
• Suicide

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U. S. Life Expectancy
Health, United States, 2002
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U.S. Infant Mortality Rate
Deaths per 1,000 infants

• Significance
• Measure of societys sanitary and medical
  standards
• Health care
• Diet
• Living conditions

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Factors Influencing Changing Pattern

• Improvements due to industrialization
• Nutrition
• Environmental
• Sanitation
• Water supply
• Housing
• Medical advancements
• Antibiotics
• Immunization
• Disease surveillance programs

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Factors Influencing Changing Patterns

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

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Challenges of Modern Medicine

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