Epi - L4 - Cohort Studies Flashcards
what is a cohort-study?
they are OBSERVATIONAL studies allowing researchers to be passive observers of natural events occurring in naturally-exposed and unexposed (comparison) groups
what is the group allocation based on?
- exposure status
or - group membership (something in common), not group allocation based
when is this mostly useful?
when studying a RARE EXPOSURE
what are other known names for cohort studies?
incidence studies
longitudinal studies
how do you come up with a measure of association?
cohort studies commonly generate the Risk of disease/ outcome for each, then a Risk Ratio/ Relative Risk (RR) as measure of association
why would you choose to do a Cohort-Study?
- unable to force group allocation
- limited resources
- the EXPOSURE OF INTEREST IS RARE and little is known about its associations/ outcomes
- more interested in incidence rates or risks for outcome of interest (more than effects of interventions)
what are the 3 ways you can conduct a cohort study?
prospective
retrospective
am bidirectional
*group assignment is STILL based on EXPOSURE status or Group Membership (something in common)
Prospective Cohort Studies
exposure group is selected on the basis of a past or current exposure and both groups (exposed and non-exposed) followed INTO FUTURE to assess for outcomes of interest (which has yet to occur), and then compared
Retrospective cohort studies
aka historical
at the start of the study, both exposure and outcome of interest have already occurred, BUT groups still allocated based on past history of exposure
- retrospectively start at time of exposure and follow forward to the point of outcome occurrence (known), in the present
- exposure still has to occur BEFORE outcome of interest and group allocation is based on exposure status, not disease
ambidirectional cohort studies
uses RETROSPECTIVE design to assess past differences (up to present), but also ADDS FUTURE DATA collected on additional outcomes PROSPECTIVELY from start of the study
- looking for outcomes in the past, and known in the present, but also into the future
what is a cohort and what are some examples?
a cohort refers to a group with something in common:
- birth cohort: individuals assembled based on being born in a geographic region in a given time period
- inception cohort: individuals assembled at a given point BASED ON SOME COMMON FACTOR; useful for single-group assessments for incidence rate determination
exposure cohort: individuals assembled based on some common exposure
cohort sizes may or may not change over time, what are the names suggesting the numbers?
fixed
closed
open (dynamic)
fixed cohort
a cohort which can’t gain members but CAN have loss-to-follow-ups; fixed at the start, can only lose numbers
closed cohort
a fixed cohort with NO loss-to-follow-ups; this is made possible if the study is quick, leaving no room for people to die or leave the study
open (dynamic) cohort
a cohort with new additions and some LTFUs
how do you select an exposed study population?
allocate subjects based on pre-defined criteria of ‘exposure’
- scientifically and consistently determined
how do you best select an unexposed study population?
make the groups as close as possible (coming from the same cohort/ population (yet not exposed))
what 3 sources can the unexposed group come from?
- internal
- general population
- comparison cohort
Internal
best, if feasible
patients from the same ‘cohort,’ yet who are unexposed (most similar)
if there are only levels of exposure, you may have to use the lowest exposure group as comparator (if there is not a ‘no exposure’ group internally-available)
general population
used as a second choice;
comparison cohort
least acceptable
simply attempt to match groups as close as possible on numerous personal characteristics (can’t control for other potentially harmful exposures in comparison cohort; also causing disease)
what are some of the strengths of Cohort-Studies?
- good for assessing MULTIPLE OUTCOMES of ONE exposure; hard to control for other exposures if more than one plausible for being associated with an outcome
- useful when EXPOSURES are rare
- useful when calculating Risk and RR’s
- less expensive
- good when ethical issues limit use of interventional
- good for long induction/ latent periods (retrospective)
- able to represent ‘temporality’ (prospective)
what are some weaknesses of cohort-studies?
- can’t demonstrate causation
- hard to control for other exposures if more than one plausible for being associated with an outcome (primarily retrospective)
- retrospective; cannot control for other Exposures (if not known/assessed) or potential changes in amount of study-exposure during study frame
- not good for long induction/ latent periods (prospective; retrospective much better here)
- can be impacted by unassisted confounders (more so with retrospective)
- can be impacted by various biases, SELECTION and RECALL BIASES
- limited by available data
Advantages of Prospective Cohort studies:
- can obtain a greater amount of study-important information from patients (more control over specific data collection process; interviews/ lab assessments/ physicals)
- follow up/ tracking of patients may be easier (IF you plan ahead!)
- better at giving answer to ‘temporality’ (simple association vs. causal inference [hill’s criteria])
- may look at multiple outcomes from a supposed single exposure
- can calculate incidence and incidence rates
what are some disadvantages of Prospective cohort studies?
- time, expense, and LTFU
- not efficient for rare disease (use Case-Control study in this situation)
- not suited for long induction/ latency conditions
- exposure may change over time
what are some advantages of retrospective cohort studies?
- best for long induction/ latency conditions
- able to study rare exposures
- useful if the data already exists
- saves time and money compared to prospective studies
what are some disadvantages of retrospective cohort studies?
- requires access to charts, databases, employment records (may not be complete/ thorough enough for study)
- “information” may not factor in or control for other exposures to harmful elements during study period or over time
- patients may not be available for interview if contact necessary for missing or incomplete data
- exposure (or its ‘amount’) may have changed over time
what are they key biases with cohort-studies?
healthy worker effect and
selection bias: HOW exposure status is defined/ determined
