Lecture 7 Flashcards
Cohort Studies
Involve the formation of a cohort, which is a group of individuals followed over
time.
4 Points to Cohort Studies
- Those who enter the cohort should be outcome free. The exposure is determined before the outcome happens.
- Selecting exposed and comparison groups
- Following up participant
- Determining outcome status
What bias does cohort study design protect against? The benefits to cohort studies:
➢ Better exposure and confounder data
➢ Less vulnerable to information bias
The downsides of cohort studies:
> more expensive
Not efficient for diseases with long latent periods
What kind of study is this?
Purpose: To examine the relationship between working night shifts and breast cancer.
Participants: 78,562 female nurses aged 30-55 years with no history of cancer.
Exposure: Outcome: Years of night shift working obtained by questionnaire in 1988.
Incident cases of breast cancer between 1988 and 1998. Non-fatal cases were
identified using medical records. Fatal cases were identified in the National Death Index and by
contacting next of kin. Also death due to other reasons.
Cohort study
If you are doing a cohort study of the relationship between multi morbidity and depression you must start with people who:
Do not have depression
Prospective vs Retrospective cohort studies
> In prospective cohort studies, researchers follow a group over time, starting before any outcomes (like diseases) occur, collecting data on exposures and observing who develops the outcome. These studies are accurate for showing causality but take more time and money. > In retrospective cohort studies, researchers look back at existing records to study past exposures and outcomes. They are quicker and cheaper but may have less reliable data, making it harder to determine causation.
- Good retrospective studies are very rare. Because the data needs to be there, be specific and be accessible.
The pros and cons of retrospective studies:
Pros:
- cheap
-efficient with diseases with long latent period
Cons:
- more vulnerable to bias
- exposure and confounder data may be inadequate
-simply don’t have access to the reports
Established recording systems
military, occupational, birth cohorts
Relative Risk in Cohort Studies is measured using the
Incidence (rate) Ratio
(a/(a+b))/ (c/(c+d))
Different design of cohort studies
> Common Baseline Time Point. Common Outcome Assessment Time
Common Baseline Time Point.
Outcome Measured Throughout Follow-Up.
Rolling Entry Point (variable baseline) Outcome Measured Throughout Follow-Up.
Concept of person-time: people are in the study for varied durations
➢ Different times under exposure
➢ Contribute differentially to the study
What is the PTU# in cohort study
Person-Time Units
measure represents the cumulative amount of time each person in the study contributes while they are at risk of developing the outcome of interest.
Cohort Study Advantages
➢ Valuable when exposure is rare
➢ Can examine multiple effects of a single
exposure
➢ Easier to determine the temporal relationship
between exposure and outcome
➢ Allows measurement of incidence
Cohort Study Disadvantages
➢ Validity affected by losses to follow-up (selection bias)
➢ Other factors may not be distributed evenly between exposure groups (confounding)
➢ Inefficient for evaluation of rare diseases
➢ Can be expensive and time-consuming (need for large numbers and long follow-up)
➢ If retrospective they require good records
Recall Cohort Study Limitations
➢ May need large numbers of subjects to be followed for long periods of time so logistically
difficult, time consuming, expensive (especially prospective cohort studies)
➢ Loss-to-follow has potential to undermine validity
➢ Not good for rare diseases or those with long latency
➢ Not good when exposure data are expensive to obtain
The design of a Case Control study
- Define population from source population
- Cases and controls are sampled without regard to their exposure status
- Classify participants according to their outcome status.
> Cases (+disease)
> Controls (-disease) - Then compare prior exposure
What to do in a situation where you can’t do a recall cohort study?
Case control study
The problem with case control studies
the denominator has no meaning. Because the size of the control was chosen.
However the ODDS RATIO has much value!!!
➢ Cannot calculate measures of occurrence: risks and rates; why
➢ No longer have entire denominator of population at risk (because controls represent a selected SAMPLE of the total
population with an arbitrary size)
➢ 20/32 is NOT the prevalence of disease in exposed, we decided to select 20% (36 controls)
➢ No idea what the real prevalence is in exposed and unexposed, hence no information on risk
➢ Not a defect, good feature: efficiency
➢ Can only calculate the odds of exposure in cases and controls;
➢ Therefore, the only ODDS RATIO is the proper measure of association
Only ‘Odds’ can be estimated from case-control studies. OR of Exposure is:
(A/C)/(B/B) = (A/B)/(C/D) = OR of outcome
In cohort studies Odds Ratio of Exposure is equal to
Odds Ratio of Outcome
Case control studies are good for
rare diseases
Relative risk is
Prevalence Ratio
OR
Incidence Ratio
Depending on what kind of study you are conducting
Why can you not calculate measures of occurrence: risks and rates in a case control study
In a case-control study, you cannot calculate measures of occurrence like risks and rates because this design lacks a true measure of person-time or population at risk over time. Here’s why:
Sampling Based on Outcome: Case-control studies start by selecting participants based on whether they have the outcome (cases) or not (controls). Unlike cohort studies, you’re not observing the entire population over time to see who develops the outcome. Instead, you’re retrospectively examining exposures in those who already have the outcome versus those who don’t.
No Follow-Up Period: Since case-control studies don’t follow participants over time, they lack the temporal dimension needed to calculate incidence rates or risks. You need to know how many people were at risk of the outcome over a period to calculate these measures accurately.
Unknown Denominator: Risks and rates require a known denominator (the total population or the person-time at risk). In a case-control study, you only have cases and controls, not a full at-risk population, making it impossible to establish the true occurrence rate.
However, case-control studies are useful for estimating the odds ratio, which approximates the risk ratio when the outcome is rare. This is often used to infer associations between exposures and outcomes indirectly.
Selection of cases in case control study
➢Very clear case definition required
➢who exactly are the cases of disease in your study?
➢Ideally, case selection will involve direct sampling of cases within a source
population
➢All people in the source population who develop the disease of interest will be
included as cases
➢random sample if large number of cases, or logistical constraints
➢If someone in the source population developed the disease of interest, would
they (meet the case definition and) be included as a case in the study?
