2. Study Design: Evidence for Global Health Flashcards
Why may inconclusive study results arise?
Because avoidable flaws have arisen in:
- Study design
- Data Analysis
- Data Interpretation
What is the importance of the research questions?
The formulation of a problem is often more essential than its solution, which may be merely be a matter of mathematical or experimental skill. - Einstein
Thus vague problems will be difficult to answer.
What is PICO?
P - What is the Patient, Problem or Population
I - Intervention or exposure or test being considered
C - Is there a Comparative intervention (Control)
O - What is the Outcome
How can a study performed on a small sample be generalisable to a population?
• For the study results to be generalisable, the
study should ideally be performed on a random
sample of the relevant individuals
• However, the target population may be too wide
ranging to do this
What is Sampling Bias?
In statistics, sampling bias is a bias in which a sample is collected in such a way that some members of the intended population are less likely to be included than others.
Occurs if some members of the eligible
population are more likely to be included
than others, i.e. the sample is not
random.
What is the ‘Healthy Worker Effect’?
“HWE is a phenomenon initially observed in studies of occupational diseases: Workers usually exhibit lower overall death rates than the general population because the severely ill and chronically disabled are ordinarily excluded from employment” – Last, 1995
How can sampling bias be minimised?
To reduce sampling bias, the two most important steps when designing a study or an experiment are:
- To avoid judgment or convenience sampling
- To ensure that the target population is properly defined and that the sample frame matches it as much as possible.
Minimise it through careful study design
Record as much information as possible
about subjects that refuse to participate
– This will allow assessment of the extent to
which the study sample represents the target
population
– May allow adjustment for bias in analysis
What are confounding factors?
Factors that interact with an exposure and can effect an outcome. In statistics, a confounder is a variable that influences both the dependent variable and independent variable causing a spurious association.
What is a descriptive study?
Descriptive studies are primarily designed to
describe the distribution of existing variables that
can be used for the generation of broad
hypotheses
What is an analytical study?
Analytical studies examine an association, i.e.
the relationship between a risk factor and a
disease in detail and conduct a statistical test of
the corresponding hypothesis.
What is the aim of a comparison group?
The control group is defined as the group in an experiment or study that does not receive treatment by the researchers and is then used as a benchmark to measure how the other tested subjects do.
What is an observational study?
The study has not changed anything but observes the situation or differences between groups/over time.
Name an example of an observational study
cohort, casecontrol,
crosssectional,
ecological
What is an experimental study?
The researcher has
changed something
Give an example of an experimental study
Randomised
Controlled Trials
Name the difference between prospective and retrospective studies.
A prospective study collects information at the time of the study and so may be more reliable. Whilst a retrospective study looks back in time, this may be open to recall bias.
What are Case Reports and Case Series?
A collection of patients with common characteristics used to describe some clinical, pathophysiological or operational aspects of a disease, treatment or diagnostic procedures.
Case reports are considered the lowest level of evidence, but they are also the first line of evidence, because they are where new issues and ideas emerge. This is why they form the base of our pyramid. A good case report will be clear about the importance of the observation being reported.
If multiple case reports show something similar, the next step might be a case-control study to determine if there is a relationship between the relevant variables.
Give an example of a case report or case study.
This case report was published by eight physicians in New York city who had unexpectedly seen eight male patients with Kaposi’s sarcoma (KS). Prior to this, KS was very rare in the U.S. and occurred primarily in the lower extremities of older patients. These cases were decades younger, had generalized KS, and a much lower rate of survival. This was before the discovery of HIV or the use of the term AIDS and this case report was one of the first published items about AIDS patients.
(Hymes et al. 1981)
What are design pitfalls for case studies/reports?
- The patient should be described in detail, allowing others to identify patients with similar characteristics.
- Case reports could provide measurements and/or recorded observation that are wrong or subject to bias
- The CR/S could confirm a phenomenom instead of infer (it cannot prove with no analysis)
What are the advantages of Case studies/series?
- Allows reporting of
unusual medical cases, additionally, can help in the identification of new trends or diseases - Can generate
hypotheses and
indications of possible
new diseases - Provides rapid
feedback of events in
the medical community - Can help detect new drug side effects and potential uses (adverse or beneficial)
- Educational – a way of sharing lessons learned
- Identifies rare manifestations of a disease
