Design Issues In Nitty Gritty Epidemiology

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Design Issues In Nitty Gritty Epidemiology

• Outcomes and Exposures
• Person, Place, and Time

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Web Site

• http//www.sph.uth.tmc.edu
• /courses
• choose epidemiology
• choose PH2610

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Nitty Gritty

• the actual state of things
• what is ultimately essential and true
• From Websters New Collegiate Dictionary, 1977.
  GC Merriam Co

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Epidemiology

• Epidemiology involves a way of thinking
• Training the mind to think critically
• Epidemiology is a systematic way of reasoning to
  solve a problem

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Epidemiology

• Assumption
• Disease does not occur randomly
• Disease has identifiable causes
• which can be altered
• and therefore prevent or delay the onset of
  disease
• Source Hennekens CH and Buring JE. Epidemiology
  in Medicine.
• Boston Little Brown Co. 1987

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Definition of Epidemiology

• The study of the distribution and determinants of
  health-related states or events in specified
  population, and the application of this study to
  control of health problems.
• source Last (ed.) Dictionary of Epidemiology,
  1995

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Specified populations

• any group, with identifiable characteristics
• adult men
• adolescent women
• children with cystic fibrosis
• neonates admitted to the NICU
• patients on dialysis
• individuals with hypertension

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Key components

• encompass all epidemiologic principles
• and methods
• Frequency
• Distribution
• Determinants

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Frequency

• Quantification of the presence of disease
  (health-related state)
• How to define disease?
• Careful definition and systematic counting is the
  prerequisite for any investigation of the
  patterns of occurrence

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What is a case

• Health-related states or events
• diseases, causes of death, behaviors, reactions
  to preventive regimens, and provision and use of
  health services
• Specifically defined outcomes
• Death (mortality) and Disease (morbidity)
• Disability, Discomfort
• Dissatisfaction (health related quality of life)
• Also good things

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Defining case

• Definition may be restrictive or broad depending
  on the scope of the study
• Detection and diagnosis
• may be difficult in latent or subclinical
  disease, so diagnostic criteria may be included
  in case definition criteria

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Case Definitions

• Chronic fatigue syndrome
• Toxic shock syndrome
• West Nile VirusWest Nile Virus Brochure - CDC
  Division of Vector-Borne Infectious Diseases
  (DVBID).htm
• Human Immunodeficiency Virus
• Pregnancy

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Cystic Fibrosis case definition

• Am. J. Epidemiol. 2002 156 397-401

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Alzheimers Disease definition

• Am. J. Epidemiol 2002 156445-453

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Distribution

• Distribution of cases in a defined population by
  person, place, and time
• define cases
• count
• proportions, rates, ratios, differences
• prevalence, incidence

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Prevalence

• the number of affected persons present in the
  population at a specific time, divided by the
  number of persons in the population at that time.

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Incidence

• the number of new cases of a disease that occur
  during a specified period of time in a population
  at risk for developing the disease.

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Determinants

• Determinants
• Causes of disease or health related states
• risk factors (smoking, alcohol use, obesity)
• preventive factors (seat-belt use, anger-control
  intervention)
• prognostic factors (spirituality, physical
  therapy)

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Determinants

• physical, biological, social, cultural, and
  behavioral factors that influence health
• Risk factors for disease include
• age, gender, SES, HTN, smoking, exercise
• genetic predisposition
• religious beliefs
• However for each of these there are potential
  errors in measurement

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• Questions designed to assess specific information
  on beverage consumption included the following
• caffeine-containing and decaffeinated beverages,
• weekday and weekend day consumption,
• the size of the container (accounting for ice),
• the method of brewing coffee or tea, and the
  portion of the container consumed.
• The following sources of caffeine were included
  coffee, tea, sodas, hot chocolate, chocolate
  snacks, and medications.
• To blind subjects to the purpose of the study,
  data were also collected on the consumption of
  other beverages that commonly changes during
  pregnancy, such as milk and alcohol. At the
  initial interview, examples of container sizes
  and pictorial representations of brewing methods
  were displayed. Am J Epidemiol 2002
  156428-437

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Steps of epidemiologic research

• To describe and measure the distribution of
  disease in the population
• Define a case / count cases (outcome)
• Describe who has the disease (person)
• Describe when and where the disease occurred
  (place and time)
• Finally, compare outcomes among groups

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Descriptive Epidemiology

• evaluate trends in health/disease
• compare across populations
• provide a basis for planning and evaluation of
  health services
• suggest areas for investigation
• identify problems by analytic methods

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Basic Tenets of Epidemiology

• The distribution of disease occurs in patterns in
  a community
• The pattern of disease in communities is
  predictable
• Characteristics of the pattern may suggest or
  lead to measures to control or prevent the disease

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Steps of epidemiologic research

• The research question - specific to
• Outcome definition
• Exposures definition
• Person
• Place
• Time

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Person

• Disease is often reported by three
  characteristics of person
• age
• sex
• race/ethnicity

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Person - AGE

• Age is the most important determinant among
  person variables
• age-specific death rates are J shaped
• higher death rates at very young ages
• lower death rates from about 5-24
• steadily increasing rates from 25

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Mortality by Age
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Incidence of disease by age
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Infection rate by age
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Person - Sex

• Sex
• physiologic
• Gender
• sociologic

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Person - Sex
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Person - Race/Ethnicity

• African American
• higher mortality
• American Indian
• Diabetes Mellitus, obesity
• Asian
• Lower mortality
• Hispanic/Latino
• Diabetes Mellitus in Mexican Americans
• Migration - Foreign born vs. American born

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Death by race/ethnicity
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Person - Socioeconomic Status

• Low income/social class have excess mortality,
  morbidity and disability rate
• negative health effects
• Factors include poor housing/crowding, racial
  disadvantage, poor education/ unemployment,
  environmental hazards exposure, negative
  lifestyle, lack of access to health care

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Person - Socioeconomic Status

• Measurement of social class
• occupation
• social position
• education
• income

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Sever illness by social class
Source. J Clin Pathol 21(S2)31, 1968
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Person

• Married individuals have lower rates of disease
• A protective factor in health, due to a
  beneficial environment, lifestyle factors, social
  support
• Religion
• Important influence on lifestyle behavioral
  factors, social support

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Place geographic area

• natural boundaries
• city blocks
• neighborhoods
• cities
• counties
• regions
• countries

• urban / rural
• resident or nonresident
• legal or undocumented
• place of occupation
• place of residence

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Time

• Cyclic fluctuation
• Epidemic
• Change over time
• Cohort effect
• a group of persons who share something in common
• particular or general exposure

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Time

• time after certain exposure
• time of day
• day of week
• time of year
• over time (secular trends)

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Outbreak Investigation
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Outbreak Investigation

• Verify that there is a problem
• Define the numerator (cases)
• Develop a case definition and verify cases
• Clinical features
• Serologic markers
• Cultural aspects
• Develop hypotheses

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Outbreak investigation

• Collect and analyze data
• Define the denominator
• What is the population at risk?
• Calculate the attack rates
• Refine hypotheses
• Implement control measures
• Evaluate effectiveness of control
• Develop prevention strategies

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Example

• Outbreak investigation
• Food-borne disease associated with church picnic
• Illness 6 to 24 hours after a church picnic
• vomiting diarrhea
• chills nausea
• fever

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Example

• Case definition
• vomiting (more than 2 times in 24 hours) 6 or
  more hours after picnic
• nausea, chills and
• diarrhea (more than 3 bowel movements in 24
  hours) 6 or more hours after picnic

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Example

• Who, When, Where
• How many attended picnic
• How many got sick
• When and what the people ate and drank

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Example
This can help us determine what the sick
individuals ate, but can it help us identify the
probable cause of the illness? What is missing?

• 80 attended
• 55 got sick within 6 to 10 hours
• ate/ill
• potato salad 55/55
• ham sandwiches 10/55
• hamburgers 4/55
• chicken salad 2/55

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Example
Would percentages or rates be easier to compare?

• 80 attended
• 55 got sick within 6 to 10 hours
• Ill Not Ill
• potato salad 55/55 25/25
• ham sandwiches 47/55 8/25
• hamburgers 4/55 13/25
• chicken salad 2/55 2/25

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Example

• 80 attended
• 55 got sick within 6 to 10
• Ill Not
  Ill
• potato salad 55/55 100 25/25 100
• ham sandwiches 47/55 85 8/25 32
• hamburgers 4/55 7 13/25 52
• chicken salad 2/55 4 2/25 8

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Methods

• 35 people in a small rural community became ill
  with diarrhea, abdominal cramps, nausea, and
  vomiting during a period of 36 hours.
• Investigation has indicated that all cases
  attended a picnic on Sunday, and had eaten at
  approximately 4 p.m.

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Investigate

• 35 people were affected and 50 ate at the church
  dinner.

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Epidemic

• A bar-graph was made showing the number of cases
  vs. the time after which the meal was eaten.
• To investigate the effect of each food on
  outbreak of disease, food-specific attack rates
  were calculated.

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Attack Curve
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Food-Specific Attack Rates
AR Attack Rate RD risk difference RR
Relative Risk
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Food-Specific Attack Rates
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Food-Specific Attack Rates
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Food-Specific Attack Rates
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Investigating food borne illness
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Critical Reasoning

• These all started with outbreak investigation
  reports and simple non-experimental
  epidemiologic studies
• Lyme Disease (1975)
• Legionellosis (1976)
• AIDS (1981)
• Hantavirus (1993)
• DES exposure (1989)
• EMS/ tryptophan (1970)
• TSS (1980)

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Defining case

• Natural history
• the course (stages) of disease from exposure to
  ultimate resolution without intervention
• critical in detection, diagnosis, establishment
  of causality, and prevention/control

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Defining case

• Natural course of disease
• Exposure ? onset ? symptoms ? dx ? outcome

Incubation period
Recovery Death Chronic disease
Clinical Stage
Subclinical Stage