Class 22-23 Flashcards
(22 cards)
Cohort studies
Observational studies allowing researcher to be a passive observer of natural events occurring in natural-exposed and unexposed (comparison) groups
***Group allocation based on EXPOSURE-status OR Group Membership (something in common)
Useful when studying a rare exposure
Cohort studies are also termed:
Incidence studies/Follow-up studies/Longitudinal studies
Commonly generates the RISK of disease/outcome for each, then a Risk Ratio/Relative Risk (RR) as a measure of association
Reasons to select a Cohort design
Unable to force group allocation (‘randomize’)
-Unethical/Not feasible
Limited resources
-Time/Money/Subjects
The exposure of interest is rare in occurrence and little is known about its associations/outcomes
More interested in incidence rates or risks for outcome of interest (more than effects of interventions)
Timing of Cohort studies
Can be conducted in Prospective, Retrospective (or Historical), or Ambidirectional fashion
-Group assignment is STILL based on EXPOSURE
Prospective Cohort Studies
Exposure group is selected on the basis of a past or current exposure and both groups (exposure and non-exposure) followed into the future to assess for outcome(s) of interest (which has yet to occur), and then compared
Retrospective Cohort studies
At the start of the study, both the exposure and the outcome of interest have already occurred
Retrospectively start at time of exposure (historically) 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 status
Go back in time to find exposure. At the mercy of information that is available. Can’t easily control for confounders
The exposure is what occurred retrospectively
Ambidirectional Cohort studies
Uses retrospective design to assess past differences but adds all data collected on additional outcomes prospectively from start of study
Looking for outcomes in the past and into the future
Cohorts
A cohort also refers to a group with something in common
Example ‘cohorts’:
1) Birth cohort
- Individuals assembled based on being in a certain place at a certain time (everyone born in KC in 2014)
2) Inception cohort
-Individuals assembled at a given point based on some common factor
~Where people live or where they work, or something they have in common
~Useful for single-group assessments for incidence rate determination
-A single health-care system
-A single payer of health-care coverage
-Example:
~Framingham Heart Study
3) Exposure cohort
-Individuals assembled based on some common exposure
~Frequency connected to environmental or other 1-time events
Cohort sizes may/may not change over time:
1) Fixed Cohort:
- A cohort (derived from an irrevocable event) which can’t gain members but CAN have loss-to-follow-ups
- Fixed on the front end
2) Closed Cohort:
- A Fixed cohort with NO loss-to-follow-ups
- Fixed on both ends; no one leaves, no one dies, etc., relatively short study
3) Open (or Dynamic) Cohort:
- A cohort with new additional additions and some loss-to-follow-ups
- Cohort can increase or decrease over time
- Ex: birth cohort; babies are born every day, some die
How to select an Exposed study population for Cohort studies
This is the easier part!
Allocate subjects based on pre-defined criteria of “exposure”
-Scientifically and consistently determined
How to select Unexposed study population in Cohort studies
Make the groups as close as possible (coming from the same cohort/population (yet not exposed))
If exposure truly has no effect, then risk will be exactly the same for both groups and RR will be 1.0 (no difference)
Unexposed group for Cohort Studies can come from 1 of 3 sources:
1) Internal
2) General Population
3) Comparison Cohort
Internal Unexposed Group
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 a comparator (if there is no “no” exposure group internally-available)
General Population Unexposed Group
Used as a second choice when the best-possible comparison group (internal) is not realistically possible (ex: everyone is exposed, of the exposure subjects were drawn from the general population)
Comparison Cohort Unexposed Group
Least acceptable group (but can still be utilized)
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 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 in calculating risk and RR’s
Less expensive than interventional trials
Good when ethical issues limit use of intervention
Good for long Induction/Latent periods (retrospective)
Able to represent “Temporality” (Prospective)
**Weaknesses of Cohort studies may be the opposite of these general points listed above
Advantages of Prospective Cohort Studies
Can obtain a greater amount of study-important information from patients
- More control over specific data collection process
- Interview/Lab assessments/Physical exams
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
Disadvantages of Prospective Cohort Studies
Time, Expense, & Lost-to-follow-ups
Not efficient for rare diseases
-use Case-Control study in this situation
Not suited for long Induction/Latency conditions
Exposure (or its ‘amount’) may change over time
A comment on Loss to Follow-up (LTFU)
Possible with Prospective Cohorts
Lowers Sample Size (Power)
- Just at it does with Interventional studies!
- Increased risk of Type 2 error
- Loss of study participation (follow-up) may not be = b/t groups
- Authors MUST list LTFU’s by group (exposes/un-exposed)
Do ALL you can to limit LTFU’s
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
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
Patients may not be available for interview if contact necessary for missing or incomplete data
Exposure (or its “amount”) may have changed over time
Cohort study designs: Matching
A way to strive to make groups as equal as possible on known/potential confounders
Can match on a 1:1 or even higher (1:5) ratio [exposed to unexposed]
Key biases with Cohort Studies
1) Healthy-worker effect:
If healthy, you work (even if exposed). If too ill to work (due to exposure?) you may be unemployed (now part of non-working general population)
2) Selection bias:
How exposure status is defined/determined (less of an issue with exposure status)
**#1 common bias we worry about
