What is an exposure? What is a disease ? How do we measure them ?
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What is an exposure?What is a disease?
How do we measure them?
Epidemiology matters: a new introduction to methodological foundationsChapter 3
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Epidemiology Matters – Chapter 1 2
Seven steps
1. Define the population of interest2. Conceptualize and create measures of exposures and health
indicators3. Take a sample of the population4. Estimate measures of association between exposures and health
indicators of interest5. Rigorously evaluate whether the association observed suggests a
causal association6. Assess the evidence for causes working together7. Assess the extent to which the result matters, is externally valid, to
other populations
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Epidemiology Matters – Chapter 3 3
1. What is a variable?
2. What are health indicators?
3. What is an exposure?
4. Measuring exposure and disease
5. Summary
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Epidemiology Matters – Chapter 3 4
1. What is a variable?
2. What are health indicators?
3. What is an exposure?
4. Measuring exposure and disease
5. Summary
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Epidemiology Matters – Chapter 3 5
What is a variable?
A variable is any measured characteristic of individuals that differs across individuals
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Epidemiology Matters – Chapter 3 6
Variable examples
Age Sex Place of birth Occupation Education Ethnicity Cigarette smoking Diet
Alcohol consumption Blood pressure Gun ownership Diabetes Pancreatic cancer Depression
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Epidemiology Matters – Chapter 3 7
1. What is a variable?
2. What are health indicators?
3. What is an exposure?
4. Measuring exposure and disease
5. Summary
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Epidemiology Matters – Chapter 3 8
What are health indicators? Population health is often defined by the absence of the
occurrence of disease Health indicators are typically measures of the occurrence of
infections, syndromes, symptoms, and biological or subclinical markers
Health indicators can be measured over the life course and include measures of, for example, disability associated with adverse health states, potential years of life lost due to an illness
Health indicators can also be positive, e.g., well-being
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Epidemiology Matters – Chapter 3 9
Defining health indicators
1. Binary2. Ordinal3. Continuous
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Epidemiology Matters – Chapter 3 10
Binary health indicators
Variable that takes on two values Health outcomes: present or absent
Examples Individual has diabetes Individual does not have cancer Individual has Alzheimer’s disease Individual does not have HIV
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Epidemiology Matters – Chapter 3 11
Ordinal health indicators Variable that takes on multiple (>2) graded values Examples
Individual health rating Question: How would you rate your health? Response options: Excellent, Good, Fair, or Poor
Symptom frequency Question: How often do you experience night sweats? Response options: Always, Often, Rarely, or Never
Ability to perform health-related activity Question: How difficult is it to climb a flight of stairs? Response options: Very difficult, Somewhat difficult, or Not
difficult
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Epidemiology Matters – Chapter 3 12
Continuous health indicators Variable with continuous response options Examples
Age Weeks of pregnancy Diastolic and systolic blood pressure Cholesterol level Viral load Cancer stage
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Epidemiology Matters – Chapter 3 13
1. What is a variable?
2. What are health indicators?
3. What is an exposure?
4. Measuring exposure and disease
5. Summary
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Epidemiology Matters – Chapter 3 14
Exposure
Any measurable variable that affects or is associated with health
Variable can be from macro social environment to the molecular level
Examples Policies and laws: areas with higher taxes on alcohol have
lower alcohol consumption rates Biological sex: Men die, on average, younger than women
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Epidemiology Matters – Chapter 3 15
Types of exposures
1. Acute2. Chronic or stable3. Time-varying
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Epidemiology Matters – Chapter 3 16
Acute exposures
Occur for a relatively short duration Do not repeat Examples
Natural disasters Motor vehicle accident
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Epidemiology Matters – Chapter 3 17
Chronic exposures
Stable over time May be present at birth Examples
Pollution Poverty Policies and laws Biological sex Race and ethnicity DNA sequence
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Epidemiology Matters – Chapter 3 18
Time-varying exposures
Vary across the life course of an individual Examples
Diet Exercise Smoking Alcohol consumption
Epidemiologists capture variation over time with different measures of exposure
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Epidemiology Matters – Chapter 3 19
Non-diseased Diseased
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Epidemiology Matters – Chapter 3 20
Smoking and exercise
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Epidemiology Matters – Chapter 3 21
Smoking and exercise
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Epidemiology Matters – Chapter 3 22
Smoking and exercise
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Epidemiology Matters – Chapter 3 23
Smoking and exercise
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Epidemiology Matters – Chapter 3 24
Smoking and exercise
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Epidemiology Matters – Chapter 3 25
Smoking and exercise
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Epidemiology Matters – Chapter 3 26
Smoking and exercise
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Epidemiology Matters – Chapter 3 27
Smoking and exercise
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Epidemiology Matters – Chapter 3 28
Summary: exposure
Exposure: wide range of potential variables that individuals are ‘exposed to’ Age Sex Education
Water consumption Individual attendance at lecture today
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Epidemiology Matters – Chapter 3 29
Summary: exposure
Exposure: wide range of potential variables that individuals are ‘exposed to’ Age Sex Education
Water consumption Individual attendance at lecture today
What type of exposures are
these?
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Epidemiology Matters – Chapter 3 30
Summary: exposure
Exposure: wide range of potential variables that individuals ‘exposed to’ Age continuous chronic Sex binary chronic Education ordinal chronic Water consumption binary time-varying Individual attendance at lecture today binary acute
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Epidemiology Matters – Chapter 3 31
1. Duration of exposure
2. Latency and critical windows
Characterizing exposures
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Epidemiology Matters – Chapter 3 32
Exposure duration
Duration that individual is exposed matters for production of adverse health for certain exposures
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Epidemiology Matters – Chapter 3 33
Exposure duration, examplesSmoking
Smoking a cigarette is unlikely to have long-term health consequences
Smoking > a pack of cigarettes per day for 40 years is likely to have long-term health consequences
Trans fat One trans fat and calorie laden meal is unlikely to affect health Years of unhealthy eating is likely to accumulate to adversely
impact health
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Epidemiology Matters – Chapter 3 34
Exposure timing
Timing of the exposure across the life course may also be important for the production of health
Core concepts: Latency and critical windows
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Epidemiology Matters – Chapter 3 35
Exposure timing, examples
LatencyLow birth weight associated with the development of chronic diseases in adulthood
Critical windowExtreme caloric restriction during first trimester of fetal development associated with schizophrenia development in adulthood
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Epidemiology Matters – Chapter 3 36
Examples, exposure timing
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Epidemiology Matters – Chapter 3 37
Examples, exposure timing
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Epidemiology Matters – Chapter 3 38
Examples, exposure timing
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Epidemiology Matters – Chapter 3 39
Examples, exposure timing
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Epidemiology Matters – Chapter 3 40
Examples, exposure timing
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Epidemiology Matters – Chapter 3 41
1. What is a variable?
2. What are health indicators?
3. What is an exposure?
4. Measuring exposure and disease
5. Summary
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Epidemiology Matters – Chapter 3 42
Measuring exposure and disease In previous sections we have conceptualized the
exposures and health indicators of interest Now we are interested in measuring these factors Good measurement of variables is critical for
epidemiologists
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Epidemiology Matters – Chapter 3 43
Measurement example Research question
Are individuals who have depression more likely to be overweight than individuals without depression?
Measuring depression Constellation of symptoms Condition characterized by disabling feelings of
hopelessness, sadness, and loss of interest in activities Measuring overweight
Obesity = Body Mass Index (BMI) ≥ 30
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Epidemiology Matters – Chapter 3 44
Measurement
1. Be clear about the construct being measured2. Assess the reliability of the measures3. Assess the validity of the measures
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Epidemiology Matters – Chapter 3 45
Measurement example, clarity
1. Be clear about the construct being measured Depression: validated scale Obesity: BMI ≥ 30
2. If measurements include respondent answered questions, make sure questions are easily interpretable, short, clear, and precise.
Instead of “Are you depressed?” Try “In the past week have you felt happy most of the
time?”
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Epidemiology Matters – Chapter 3 46
Reliability and validity of measures
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Epidemiology Matters – Chapter 3 47
Reliability and validity of measures
Not valid or reliable
Valid and reliable
Reliable not valid
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Epidemiology Matters – Chapter 3 48
Reliability and validity of measures
Not valid or reliableScale does not work
Valid and reliableScale works perfectly
Reliable not validScale consistently weighs people 5 pounds more than they weight
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Epidemiology Matters – Chapter 3 49
Dimensions of reliability
Test-retest reliability: Would the respondent answer the question similarly if asked at ≥ 2 time points?
Internal consistency: Are all the items used to assess the construct indicative of that construct?
Inter-rater reliability: Would ≥ 2 independent raters all rate the response the same?
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Epidemiology Matters – Chapter 3 50
Measurement validity
Questions to consider when assessing validity What is the gold standard? What are the sensitivity and specificity of our
measure as compared to the gold standard?
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Sensitivity, key question: Among those who have blood cotinine ≥300 ng/mL, what proportion report that they smoke ≥20 cigarettes per day?
≥20 cigarettes per day self-report smokers with ≥300 ng/mL cotinine / all smokers with ≥300 ng/mL cotinine
20/(20+10)=0.67 or 67%
Interpretation: 67% of people who actually smoked a pack of cigarettes in the past 24 hours reported that they smoked a pack of cigarettes in the past 24 hours
Measurement, sensitivity
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Specificity, key question: Among those who have blood cotinine <300 ng/mL, what proportion report that they smoke < 20 cigarettes per day?
<20 cigarettes per day self-report smokers and <300 ng/mL cotinine / all with <300 ng/mL cotinine
168/(2+168)=0.99 or 99%.
Interpretation: 99% of people who actually did not smoke a pack of cigarettes in the past 24 hours reported that they did not smoke a pack of cigarettes in the past 24 hours
Measurement, specificity
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Epidemiology Matters – Chapter 3 53
Summary: sensitivity and specificity
Provides an assessment of the validity of our measures
Sensitivity: proportion who are accurately identified as positive on the measure
Specificity: proportion who are accurately identified as negative on the measure
Requires a gold standard
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Epidemiology Matters – Chapter 3 54
Measurement, validity
Questions to consider when assessing validity What is the gold standard? What are the sensitivity and specificity of our
measure as compared to the gold standard?
What if there is no gold standard?
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Epidemiology Matters – Chapter 3 55
1. What is a variable?
2. What are health indicators?
3. What is an exposure?
4. Measuring exposure and disease
5. Summary
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In summary Conceptualization and measurement of health in
populations is critical to improving population health Health indicators are presence of disease, symptoms,
syndromes, disability, wellness, quality of life, and other health-related states
Exposures are potential influences on these health-related exposures and can be acute or chronic, long or short in duration, have impact only at a critical point in human development or accumulate
Epidemiology Matters – Chapter 3
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Epidemiology Matters – Chapter 1 57
Seven steps
1. Define the population of interest2. Conceptualize and create measures of exposures and health
indicators3. Take a sample of the population4. Estimate measures of association between exposures and health
indicators of interest5. Rigorously evaluate whether the association observed suggests a
causal association6. Assess the evidence for causes working together7. Assess the extent to which the result matters, is externally valid, to
other populations
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Epidemiology Matters – Chapter 1 58
epidemiologymatters.org