Week 4: Epidemiological Designs Flashcards
What are the features of epidemiological studies?
Observe people in their natural surrounding
-over a specific period of time
-at one point in time
-retrospectively or prospectively
The objective is to describe the relationship between a disease (or outcome of interest) and exposure to a treatment if risk factor
Retrospective vs prospective. Explain them both
Prospective:
-variable measured in direct recording in the present
-the researcher follows subjects as they progress through their treatment or evaluation (subjects are followed forward in time)
-possible to ID factors which precede a given outcome
Retrospective:
-examination of data that has been collected in the past (eg medical records)
-researchers look back in time
prospective studies thought to be more valid than retrospective as there is greater control over data collection methods and ability to document a temporal sequence of events (EXPOSURE AND OUTCOME)
Longitudinal vs cross-sectional
Longitudinal: over time
-researchers follow a cohort if subjects over time
Cross sectional:
-researchers studies a group of subjects at one point in time
-comparisons made between different groups in the cohort eg males and females
What’s the difference between a case and a control?
Case: individuals with the disease
Control: individuals without the disease
They are both compared
What are some of the common designs used in rpidemologic research?
-cross sectional studies
-case control studies
-cohort studies
Which one they use depends on
-the frequency if the disease condition
-the availability of human and economic resources
Cross-sectional studies aka prevalence studies
Assesses the health status and exposure levels of persons in a population at one point in time
-cases must actively manifest the diseases to be included
-cases with developing conditions that have not yet been diagnosed are bit counted as cases
-exposure and outcome are determined at the same time.
PURPOSE:
-to determine if there is an association between suspected causal factor and a condition
-they are useful to discover associations, but incapable if determining if one factor caused the other
-case-control or cohort studies are often used to verify their results
-the are attractive to researchers as they are quick and easy to carry out and are inexpensive. Consequently they are often the initial research tools used to investigate exposures to risk factors and their relationships to disease.
Cross-sectional study design advantages and disadvantages
Advantages:
-relatively quick and inexpensive
-no loss to follow up
-may be the only practical design
Disadvantages:
-cannot establish a temporal sequence between exposure and outcome
-difficult to establish causal relationship
-unsuitable for the study of rare outcomes.
Case-control studies
A study with 2 groups of subjects
-one has the disease or condition
-the other group is free from the disease or condition
Prior exposure of the cases are compared with those of the controls to see if the exposures influence the odds of developing the disease
Case-control studies are retrospective
Exposure levels are determined by looking back in time before the person became a case or a control.
Because they are retrospective
-you can’t determine the risk of developing a disease
-but can estimate the odds I’d developing a disease given that a person was exposed to the risk factor: the odds ratio
Bias in case control studies
This design is vulnerable to a number of biases because cases and controls are selected after the disease outcome and the exposure have occurred
Recall bias:
Systematic differences between the way cases and controls recall their past exposures
Selection bias:
-any systematic difference between the groups which can lead to conclusions which are different to the truth
Case-control studies and rare diseases
Case-control studies are usually the best way to study rare diseases
-prospective studies typically require many subjects, which isn’t realistic with a rare disease. It is much easier to select a group with the disease then look back in time
What are the advantages of case-control studies?
- good for investigating rare diseases (outcomes)-can be performed quickly and I expensively
- useful for studying diseases with long latency period between exposure and manifestation
- existing records can often be used
- can assist in establishing cause and effect
What are the disadvantages of case-control studies?
- no information regarding temporal relationship between exposure and outcome
- typically rely on patients recall of past exposures
- difficult to validate information on exposure
- not appropriate for studying rare exposures not diseases
What are cohort studies?
- Studies that follow groups of subjects over time and compare their outcomes
- one group is exposed to a known suspected cause if disease while the other group is not exposed
- or one group was exposed to the risk factor in the past
- a population I’d identified, from which two if more groups of patients are selected
- the groups are called cohorts
- outcomes are assessed on both groups prospectively over a defined period of time
- the objective
Timing of cohort studies
Retrospective
Prospective
Retrospective:
- cheap, faster
- efficient with disease with long latent period
- exposure data may be inadequate
Prospective:
- more expensive, time consuming
- not efficient for diseases with long latent periods
- better exposure and confounder data
- less vulnerable to bias
