Lecture 17 Cross sectional studies Flashcards
descriptive epidemiology definition
distribution of health-related
states or events in specified populations…”
Analytic epidemiology definition
determinants of health-related states or events in specified populations…”
what is descriptive epidemiology?
- ‘Person, place and time’
- Observational
- What
- Who
- Where
- When
what is Analytic Epidemiology?
• Associations: exposures and outcomes • Causation • Observational or intervention studies
• Why
What is a cross-sectional study?
Measures exposures and/or outcomes at one point in time
What do we mean by a ‘point in time?’
A particular date eg (14 August 2019)
A specific event eg (visit to doctor, retirement)
A specific period of time eg (in the past 12 months)
Some examples of cross-sectional studies
New Zealand Health Survey
Census of Populations and Dwellings
Youth 2000 National Youth Health and Wellbeing Survey Series
why are Cross sectional studies used?
Describe Prevalence of exposures or health conditions in a population
Compare prevalence amongst different groups in a population
What do cross-sectional studies measure?
Prevalence: the proportion of a defined population who have a disease at a point in time
Prevalence is affected by
incidence AND duration
What are cross-sectional studies used for?
To describe:
Prevalence of exposures or health conditions in a population
E.g. What is the prevalence of osteoarthritis in NZ?
To compare:
prevalence amongst different groups in a population
E.g. what is the prevalence of osteoarthritis in NZ for people of different ages?
To compare:
Compare repeated studies over time
E.g. Has the prevalence of smoking changed over time among high school students?
To generate hypotheses:
E.g. Which factors are associated with low back pain among nurses?
To plan:
health services
E.g. Population of Nelson Marlborough DHB Population by age 2015/16
Recap: Cross-sectional studies measures
exposures and/or outcomes at one point in time
prevalence (affected by incidence and duration)
Recap: Cross-sectional studies used to
describe prevalence
compare prevalence
generate hypotheses
plan (e.g. health service delivery)
GATE frame
Graphic Appraisal Tool for Epidemiological studies:
Gate frame used for
Describe main elements of epidemiological study
Gate Frame Triangle Circle Square Time
Population
- Source
- Sample
Exposure/Comparison
- Exposed group (top circle) (exposure / potential risk factor)
- Comparison group (bottom circle)
Outcome
Duration of study
Cross sectional study exposure and outcome occurs
at same time
Time barrier changes accordingly
What is the prevalence of “outcome”?
a + b / a + b + c + d
What is the prevalence of “outcome” among those who have “exposure”?
a / a + b
What is the prevalence of knee pain among those with “comparison group”?
c / c + d
what is a Sample population?
Subset of source population who meet inclusion criteria in study and enters study
Hypothesis generation
People who were overweight or obese were 1.47 times as likely to have knee pain as people with a normal BMI
BUT
Which came first?
Knee pain or increased BMI?
Exposure and outcome were assessed at the same time
Limitations of cross sectional study
Unable to determine temporal sequencing between exposure and the outcome (exposure and outcome occurs at same time)
Measures prevalence not incidence
- unable to tell us about onset of disease
Not good for studying rare outcomes or exposures
Not good for assessing variable and transient exposures or outcomes
(Findings may be different depending when you do cross sectional study)
Transient
Some things interested in may last short time
why do cross-sectional studies?
- Depend on research question may be most appropriate
- Only conduct data at one point in time
- Can assess multiple exposures and outcomes
- Can be less expensive than some other study designs
- Relatively quick
what are some appropriate research question for cross-sectional studies?
- Prevalence, and distribution of prevalence in the population
- Stable exposures and outcomes
- Hypothesis generating
Ecological studies
Compare exposures and outcomes across GROUPS not individuals
What are ecological studies used for?
- To compare between populations
- To assess population level factors
- To consider hypotheses
Limitations of ecological studies
Ecological fallacy
Cannot control for confounding
Cannot show causation
Ecological fallacy
- Unable to describe characteristics of group to the individuals in that group
- Measure for that group
- “…derives from ascribing to individuals the characteristics of the group.”
Cofounding
Relationships looking at is muddled up by another factor
why do ecological studies?
Depends on the research question
- Population level exposures
- Consideration of hypotheses
Data is often routinely collected
- May be relatively easy to do
- May be relatively inexpensive
hypothesis of ecological study
may not be correct
Not used to confirm hypotheses Because can’t infer at the level of individual
recap what is Cross-sectional studies?
- Assess prevalence
- Temporal sequence
- Not good for rare, variable or transient exposures or outcomes
recap what is Ecological studies?
- Group level not individual level
- Every data point is a population (not individual people)
- Ecological fallacy
Income inequality use
Group measure
Use ecological study
