what is an exposure? what is a disease ? how do we measure them ?

What is an exposure?What is a disease?

How do we measure them?

Epidemiology matters: a new introduction to methodological foundationsChapter 3

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

1. What is a variable?

2. What are health indicators?

3. What is an exposure?

4. Measuring exposure and disease

5. Summary

What is a variable?

A variable is any measured characteristic of individuals that differs across individuals

Variable examples

Age Sex Place of birth Occupation Education Ethnicity Cigarette smoking Diet

Alcohol consumption Blood pressure Gun ownership Diabetes Pancreatic cancer Depression

5. Summary

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

Defining health indicators

1. Binary2. Ordinal3. Continuous

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

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

Continuous health indicators Variable with continuous response options Examples

Age Weeks of pregnancy Diastolic and systolic blood pressure Cholesterol level Viral load Cancer stage

5. Summary

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

Types of exposures

1. Acute2. Chronic or stable3. Time-varying

Acute exposures

Occur for a relatively short duration Do not repeat Examples

Natural disasters Motor vehicle accident

Chronic exposures

Stable over time May be present at birth Examples

Pollution Poverty Policies and laws Biological sex Race and ethnicity DNA sequence

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

Non-diseased Diseased

Smoking and exercise

Summary: exposure

Exposure: wide range of potential variables that individuals are ‘exposed to’ Age Sex Education

Water consumption Individual attendance at lecture today

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?

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

1. Duration of exposure

2. Latency and critical windows

Characterizing exposures

Exposure duration

Duration that individual is exposed matters for production of adverse health for certain exposures

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

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

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

Examples, exposure timing

5. Summary

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

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

Measurement

1. Be clear about the construct being measured2. Assess the reliability of the measures3. Assess the validity of the measures

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?”

Reliability and validity of measures

Not valid or reliable

Valid and reliable

Reliable not valid

Not valid or reliableScale does not work

Valid and reliableScale works perfectly

Reliable not validScale consistently weighs people 5 pounds more than they weight

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?

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?

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

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

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

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?

5. Summary

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

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

epidemiologymatters.org

what is an exposure? what is a disease ? how do we measure them ?

population health

health indicatorstake

result matters

values health outcomes

adverse health states

illnesshealth indicators

measures of exposures

absentexamples individual

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