L19: Observational Studies Flashcards
What is an observational study?
investigator did not assign exposure
What is an observational study that has a comparison group vs no comparison group?
Have comparison group -> analytical study (cohort study, case control study and cross sectional study)
No comparison group -> descriptive study (case report, case series and ecologic study)
What is an ecologic study?
- unit is population, not individual
- uses aggregated/group level data
What is ecologic fallacy?
- data collected at group level assumed to apply to individual level
What are the strengths and limitations of an ecologic study?
Strengths
- inexpensive, easy to conduct (usually using secondary data eg published statistics)
- hypothesis generating
Limitations
- cannot link exposure to outcome in individuals
- confounding by other variables
What is a case report and case series?
- involves individuals
- careful, detailed report of the profile of a single patient (case report) or series of patients (case series), with respect to factors that could be related to illness or outcome -> usually on an unusual disease
- often the first alert by observant HCP
What are the strengths and limitations of case report and case series?
Strength
- hypothesis generating
Limitation
- no comparison group
What is a cross-sectional study?
- snapshot in time
- information on presence of absence of exposure and outcome of individuals assessed simultaneously at one point in time
- provides info on prevalence
What are the strengths and limitations of a cross-sectional study?
Strength:
- efficient in time and money
- many outcomes and risk factors can be assessed
- no loss to follow up
Limitations:
- unclear temporal relation between exposure and outcome -> difficult to establish causal relationship
What is a cohort study?
- follow 2 or more groups from exposure to outcome
- selection based on exposure status
What are the strengths and limitations of a cohort study?
Strength:
- clear temporal sequence between exposure and outcome
- can study several outcomes associated with a single exposure
- ideal for studying rare exposures
- can directly measure incidence of outcome
Limitations
- inefficient for studying rare outcomes (might not find it, need v large sample size)
- inefficient for outcomes that take v long to develop
- time consuming, costly
- potential for bias due to loss to follow up
What are two types of cohort studies?
Prospective cohort study - outcome has not occurred at initiation of study
Retrospective cohort study - outcome already occurred at initiation of study (but still look at exposure first)
What are the strengths and limitations of a prospective vs retrospective cohort study?
Prospective
- more control over quality and quantity of data -> less potential for bias
- more time consuming, expensive
Retrospective
- less control over quality and quantity of data -> greater potential for bias
- less time and money needed
What is a case control study?
- select based on outcome status
- look back to see if exposed or not (retrospective)
What are some sources of cases and controls?
Cases
- medical facilities
- disease registries
Controls
- hospitalized patients
- general population
What are the advantages and disadvantages of using hospitalized patients as controls?
adv:
- convenient
- relatively inexpensive to identify and interview
- cases and controls likely similar in accuracy of recall
- generally high level of cooperation
disadv:
- disease for which they hospitalized for may be associated with risk factors under study
- selection factors leading to hospitalization may differ between cases and controls (referral patterns)
What are the advantages and disadv of using general population as controls?
adv:
- ensures comparability
disadv:
- difficult to enumerate all members of population as basis for selecting individuals
- difficult to gain cooperation
- relatively expensive to identify and interview
- may not recall exposure w same degree of accuracy as cases
What are the strengths and limitations of a case control study?
strength:
- ideal for studying rare outcomes
- efficient for studying outcomes that take long time to develop
- can study multiple exposures for a single outcome
- efficient in time and money (bc outcome alr occurred)
limitations
- inefficient for rare exposures
- selection of appropriate control is difficult
- data on exposure may be difficult to obtain
- cannot estimate outcome rate directly, but can use estimate measures
- more potential for bias bc outcome status is known when assessing exposure status
What is a nested case control study?
case control study nested within a cohort study
What are the strengths and limitations of a case control study?
strengths;
- efficient in time and money (useful for costly measurements archived at beginning and preserved for later analysis for only cases and controls, not the whole cohort)
- preserves all adv of cohort studies that result from collecting exposure data first before outcome
- avoids potential bias of conventional case control studies that draw cases and controls from different populations
limitations:
- many research qns not amenable to strategy of storing materials for later analysis on a sample of the study subjects
- when data available for entire cohort at no additional cost, nothing is gained by studying only a sample of controls -> whole cohort should be used
What do we refer to when reporting observational studies?
STROBE statement
