Cohort Studies Flashcards
Define the term “cohort” as it applies to epidemiology studies. what do these types of studies already know before the study starts?
A group of people that are rounded up for a study that all have something in common at the beginning of the study
these types of studies also already know who was exposed and unexposed
What are the 3 “perspectives” that a cohort study can be designed to have? explain their differences.
Retrospective: groups are selected based on past history of exposure, and follows along “in the past” to the point of outcome (that has already occurred at the start of the study)
Prospective: groups are selected based on past/current exposure and the groups are followed “into the future” to assess for outcomes that have yet to occur
Ambidirectional: uses retrospective design to asses past differences up until the present time AND adds future data collected on additional outcomes after the start of the study (prospective portion)
what are the 2 things that comparison groups are based on in a cohort study design?
Exposure status (group allocation)
The common factor that defines the cohort being studied (group membership)
What are the advantages and disadvantages of prospective, retrospective, and ambidirectional cohort studies?
Prospective
Ad: can determine causation
Disad: difficult to conduct and more costly
Retrospective
Ad: less expensive and less ethically difficult
Disad: cannot determine causation
Ambidirectional
Ad: best of both
Disad: very costly/difficult to conduct (takes a lot of time)
Differentiate between a fixed, closed, and open cohort. give examples of each
Fixed Cohort: cohort from an irrevocable event which can’t gain members but CAN have loss-to-follow-ups
Ex. long term health effects on 9-11 responders
Closed Cohort: has NO loss-to-follow-ups (due to short study design)
Ex. an acute stomach virus (it is a short acting disease so there will be no LTFU)
Open Cohort: cohort with new additions and some loss-to-follow-ups
Ex. If people migrate into or out of the region that determines the cohort that is being used
What are the strengths and weaknesses of prospective cohort studies?
Strengths
Can obtain more relevant data from the subjects
Follow-up/tracking may be easier
Gives a more valid answer the to issue of “temporality”
May look at multiple outcomes from a (supposed) single exposure
Can calculate incidence and incidence rates
Weaknesses
Time, expense, and loss-to-follow-up’s
Not efficient for rare diseases
Not suited for long induction periods
Exposure (or the amount of exposure) may change over time
What are the strengths (3) and weaknesses (3) of retrospective cohort studies?
Strengths
Best for long induction/latency conditions
Able to study rare diseases
saves time and money compared to prospective studies
Weaknesses
requires access to charts/databases with necessary info (info might not factor in control for other exposures)
you may not be able to contact subjects to “fill in the blank”
exposure, or it’s amount, may have changed over time
When selecting the unexposed (control) group for a cohort study, what are the 3 sources these subjects are gathered from? explain their levels of preference
Internal: best if feasible
If there are only “levels of exposure” then the lowest level of exposure group must be used as the unexposed group
General population: 2nd choice
From the same population
Comparison Cohort: least acceptable group
Selects controls based on numerous personal characteristics
Cite and delineate how “lost to follow-up” can impact cohort studies & ways researchers can minimize this issue.
Lowers sample size (power) in prospective cohort studies
The LTFU’s (loss to follow up’s) may not be equal between groups, yielding uneven groups
Researchers must use any means they have to prevent as much LTFU as possible
Define the term “healthy-worker effect” and describe how it may impact a cohort study.
When you are studying a workplace and only the healthy workers (exposed or unexposed) become subjects bc those who were workers but became too unhealthy to continue working, and therefore were not included as subjects
loosely define cohort studies.
observational studies that allow the researcher to be a passive observer of the natural events occurring in naturally exposed and unexposed comparison groups
What is a “birth cohort”?
individuals based on being born in geographic region in a given time period.
What is an “inception cohort”?
individuals assembled at a given point based on some common factor
ex. where people live, or work, or an event they all experienced at one time.
what is an “exposure cohort”?
individuals assembled based on some common exposure
ex. flu at a work conference, new yorkers during 9/11
Define matching. what type of ratio might this occur at?
a way to strive to make groups as equal as possible on known/potential confounders
1:1 or greater ratios
