L9-10 Exposure Flashcards
Why do we measure?
To obtain quantitative data to help: (3)
- Describe the distribution of disease
- Identify determinants of health and disease
- Identify the impact of health technologies
What do we measure? (4)
- Attributes and statuses
- Behaviour and events (including exposures)
- Beliefs and knowledge
- Attitudes, opinions and reasons
Read
Attributes and statuses
- What people are:
- Age
- Gender
- Marital/civil status
- Ethnic group
- Blood pressure, cholesterol level, other clinical parameters
- Personal and familial medical history etc.
Read
Behaviour and events
- What people do
- Smoking
- Food and alcohol consumption
- Mode of travel to work etc.
- What has happened to people
- Being exposed to asbestos
- Having an operation
- Participating in health education event etc.
Read
Beliefs and knowledge
- What people think is true
- No implicit value judgement about what is good or bad
- Knowledge of causes of disease
- Knowledge of what constitutes ‘healthy behaviour’
- Self-efficacy etc.
Read
Attitudes, opinions and reasons
- What people say they want
- How people feel about something
- Essentially evaluative, largely subjective
- Satisfaction with health care services
- Reasons for engaging in risky behaviour
Read
Proxy variables
- Often a proxy is used for the variable of real interest, if it is difficult or infeasible to measure the latter
- Examples:
- Body mass index for adiposity
- Fasting insulin for insulin resistance
- Indirect blood pressure measurement
- Occupation or educational level for social class
How do we measure? (3)
- Measuring instrument, e.g. in a hospital, workplace or the general environment
- Recording events, e.g. treatments given
- Responses to questionnaires, etc.
Read
Sources of measurement variation
- Characteristics being measured (lack of constancy)
- Subject
- Observer (lack of objectivity)
- Measurement procedure (lack of precision between readings or congruency between instruments)
- Instrument or other aspects of the method of data collection
- Assay or other aspects of analysis o Environment
Define validity.
The degree to which an instrument is capable of measuring accurately what it purports to measure.
Define reliability.
The degree of stability exhibited when a measurement is repeated under identical conditions.
How can reliability be measured?
By performing two or more independent measurements and comparing the findings using an appropriate statistical test.
Characteristics of exposure
Define identity.
What the agent is (chemical, dust, physical agent, infection, etc.).
Characteristics of exposure
Define intensity.
Concentration or level of exposure.
Characteristics of exposure
Define duration.
Period of time during which the exposure occurs.
Characteristics of exposure
Define persistence.
How long agent remains in an active state in the environment or body.
Exposure and dose definitions for individual monitoring
Define exposure.
Presence of a substance in the environment.
Exposure and dose definitions for individual monitoring
Define concentration.
Amount of substance (often per unit of environmental medium).
Exposure and dose definitions for individual monitoring
Define burden.
Amount of substance in the body (organ tissue) at a point in time (e.g from serum).
Exposure and dose definitions for individual monitoring
Define dose.
Amount of substance in the body during a specified time-interval.
- Usually approximated by cumulative exposure
Define dose reconstruction.
The collecting of other information to estimate the dose received.
Define biological monitoring.
The measurement of exposure agents in bodily fluids or tissues.
Read
Problems of personal level exposure assessment in epidemiology – summary
- There may be multiple environmental media (air, water, soil, food) and multiple routes of exposure
- It can be expensive and time consuming
- Individual-level dosimeter measurements are often not possible; biological monitoring is the exception rather than the rule
- There may be mixtures of exposures (e.g., contaminated water may contain metals, solvents, pesticides, microbes)
Misclassification
The erroneous classification of an individual, a value, or an attribute into a ____1____ (usually disease or exposure) other than that to which it should be assigned. The term is usually applied to categorical variables but the concept is equally applicable to ____2____ variables (usually refer here to measurement error). Both ____3____ and imprecision in measurement lead to misclassification.
- Category
- Continuous
- Bias
State and describe the types of misclassification.
Differential
- The misclassification with respect to exposure (or disease) is dependent on the individual’s disease (or exposure) status.
Non-differential
- The misclassification with respect to exposure (or disease) is independent of the individual’s disease (or exposure) status.
What are the effects of differential misclassification in identifying associations between exposure & disease? (2)
- Can bias results in any direction
- May suggest an association when none is there, or vice versa
What are the effects of non-differential misclassification in identifying associations between exposure & disease?
Often biases results towards the null (i.e. to diminish the magnitude of any association) – BUT NOT ALWAYS
Minimising the impact of exposure misclassification
Differential
- Examine whether the level of misclassification might be larger for those with higher (or lower) exposures – can any corrections be applied to these values?
- Consider potential for bias associated with, e.g. retrospective collection of information about past events from people with and without the disease under investigation
- Can use be made of contemporary records or measurements, rather than relying solely or largely on retrospective assessments? – BUT need to check quality of such information, including whether it is available only for a subgroup of the study population (or varies in quality across the population)
Non-differential
- Can exposures be assessed with higher validity/reliability? (E.g. using a different type of detector?) Also need to consider costs and logistics of alternative methods of exposure assessment
- Even if not, it would be good to assess the level of measurement variability, since this could be used to adjust analyses of the relationship between exposure and disease
