Lectures #22-23 Flashcards
Observational, analytical studies allowing researcher to be a passive observer of natural events occurring in naturally-exposed and unexposed (comparison) groups
Cohort Studies
How is group-allocation based in a cohort study
on exposure-status or group membership (something in common)
When are Cohort studies most useful
When studying a rare exposure
Incidence studies/follow-up studies/longitudinal studies are different names for
Cohort Studies
Cohort studies commonly generate the ______ as measure of association
Risk Ration (RR)
What is the difference in measure of association generated by cohort and Case-control studies
Cohort- Risk Ratio (RR)
Case-control- Odds Ration (OR)
At the start of a cohort study we know what part of the 2x2 table
Row totals
What are the reasons to select a cohort study
- unable to “randomize”: this can be do to unethical/illegal/otherwise not feasible
- The exposure of interest is rare in occurrence and little is known about its associations/outcomes
- more interested in incidence rates/predictors of or risks for outcome of interest (more than effects of interventions)
Cohort studies can be conducted in what fashion (s)
Prospective, Retrospective (historical), or Ambidirectional fasion
In what kind of cohort study have both the exposure and outcome of interest already occurred at the start of the study
Restrospective (historical) cohort studies (note that exposure still has to occur before outcome of interest and group allocation is based on exposure status, not disease status)
Cohort study that uses retrospective design to assess past differences but adds all data collected on additional outcomes prospectively from start of study
Ambidrectional cohort study
What is a Birth cohort
individuals assembled based on being born in a geographic region in a given time period
What is an inception cohort
individuals assembled at a given point based on some common factor: such as where people live or where they work, or something they have in common
When is a inception cohort useful
for single-group non-comparisons for incidence rate determination
What is an exposure cohort
individuals assembled based on some common exposure
- most connected to environmental or other one-time events
What is a fixed cohort
A cohort (derived form an irrevocable event) which can’t gain members but can have loss-to-follow-ups
What is a closed cohort
A fixed cohort with no loss-to-follow-ups
What is an open (or Dynamic) Cohort
A cohort with new additions and some loss to follow ups
How to select exposed study population for cohort study
allocate subjects based on pre-defined criteria of “exposure”
How to select unexposed for cohort study
make the groups as close as possible (coming from the same cohort/population (yet not exposed))
If exposure truly has no effect than
the risk will be exactly the same for both groups and the risk ration (RR) will be 1.0
What are the three sources the unexposed group in a cohort study can come from
Internal (best, if feasible)
- same cohort but are unexposed
- if there are only levels of exposure, you may have to
use the lowest exposure group as comparison
General Population
- second choice
- used when internal group is not possible (e.g. everyone is exposed, or the exposure subjects were drawn from the general population)
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
Strengths of a Cohort study
- Good for assessing multiple outcomes of one exposure
- Useful when exposures are rare
- Useful in calculating risk and RR’s
- Less expensive than interventional studies (in general)
- good when ethical issues limit use of interventional studies
- Good for long induction/latent periods (retrospective studies)
- Able to represent “temporality” (Prospective)
What are the advantages to a prospective cohort study
- Can obtain a greater amount of study-important information from patients
- more control over specific data collection processes
- Follow-up/Tracking of patients may be easier
- Better at giving answer to “Temporality”
- May look at multiple outcomes form a (supposed) single exposure
- Can calculate incidence and Incidence rates
What are the disadvantages of a prospective Cohort
Time, Expense and lost-to-follow up
Not efficient for rare diseases
Not suited for long induction/Latency conditions
Exposure (or its “amount”) may change over time
What kind of observational study is used for rare diseases
Case-Control study
Loss to follow up (LTFU’s) increases what type of error
Type 2 error
Do authors’s have to list the LTFU’s by group (exposed/unexposed)
yes
What are the advantages of a retrospective cohort study
Best for long induction/latency conditions
able to study rare exposures
useful if the data already exists
save time and money compared to prospective studies
What are the disadvantages of retrospective cohort studies
- Requires access to charts, databases, employment records
- “information” may not factor in or control for other exposures to harmful elements
- patients may not be available for interview if contact necessary for missing or incomplete data
- exposure (or its “amount”) may have changed over time
issues affecting outcome occurrence in groups of cohort studies
- Levels of exposure
- Induction period
- Latency Period
What are the two key biases with cohort studies
- Healthy-worker effect
- Selection bias
