week five Flashcards
What are the different types of observational studies
Descriptive: no comparison group
- Ecological
- Case study
Analytical: comparison group
- Cross sectional
- Cohort
- Case Control
What is an observational study
A study which allows nature to chose its own course
Ecological Studies
What are they
Examines populations, not individuals. ·
Compares disease frequencies between:
– Different populations during the same period of time, or
– Same population at different time periods
advantages of ecological
Fast, easy, cheap
Hypothesis generating
disadvantages of ecological
Highly susceptible to bias & confounding
Associations not causation
May confuse characteristics of group for characteristics of individuals
case studies
· Detailed report by one or more health professionals on the profile of a single patient
· Good for adverse events
case series
· Case series is a report on a series of patients with an outcome of interest – when case studies become common, there is a trend within them they get grouped together
Advantages of case studies
identification of rare disease/exposures, which leads to further rigorous studies
Disadvantages of case studies
Selection bias
Confounders
cross sectional studies
· Units of analysis are individuals (individual level, ecological is population level)
· Measures:
– Prevalence of disease, and
– Presence or absence of exposure
advantages of cross sectional
Cheap, easy
Estimate prevalence of outcome(s) of interest
Useful for understanding disease etiology and for the generation of hypotheses
disadvantages of cross sectional
Interpret associations with caution
Difficult to make causal inference
Only a snapshot in time
The situation may provide differing results if another time-frame had been chosen
Case control
· Simple method of investigating rare diseases
· Compares the occurrence of possible cause in cases and controls
· Data is collected at one point in time
· Exposures are collected at a previous point in time
They are retrospective
advantages of case control
Good for rare outcomes and ‘long diseases’
– Thalidomide available in the 1950s (morning sickness in pregnant women)
– The late 1950s and early 1960s saw an increase in birth defects (case control study was used to identify that this drug caused these defects)
Relatively easy, fast and cheap (as everything has already happened)
disadvantages of case control
Control selection crucial
– What to match?
Retrospective
– High probability of recall bias, selection bias and measurement error
Cohort studies
What are they
How do they work
- The incidence of an outcome is compared between those exposed and those not exposed to a risk factor during the study time
- Provides good evidence of cause and effect relationship (as we measure the exposure before the outcome, forward in time)
· Participants are selected on the basis of exposure
· Participants are then followed up to identify whether or not they have the outcome
how do we define a cohort
· Needs to be accessible
· Needs to be outcome free prior to exposure
· Needs to be some within the group who will potentially be exposed and thus may present with the outcome
· Need to be some within the group who don’t present with the outcome
follow up of a cohort
Length needed varies depending on – Objectives of the study – Outcome(s) being assessed · The longer they go for, the potential for more information but have the problem of loss to follow-up (people dropping out)
Advantages of cohort
– Identify the natural history of a disorder
– Identify the temporal sequence between cause and outcome
– Good for rare exposures and common outcomes
– Very rigorous epidemiological design
- Also high on the evidence pyramid
Disadvantages of cohort
– Selection bias – Exposed and non-exposed subjects differ on important predictors of outcome – Insufficient to study rare diseases – Loss to follow-up (exposure status) – Expensive and time consuming
Retrospective cohort studies
How do they work?
- participants are identified on the basis of a previously recorded exposure
Measures exposure first but looks back in time for the direction of inquiry
difference between case control and cohort studies
Case control: groups based on outcome
Cohort: groups are always divided up based on exposure which has already happened
Groups are based on exposure
