Transcript 5 - descriptive studies.ppt

Study Designs
Observational
Descriptive
Case report
Analytical
Ecological
Animal
Experiment
Case control
Case series
Cross section
Experimental
Cohort
Human
Intervention
Clinical trial
Observational Studies
• non-experimental
• observational because there is no
individual intervention
• treatment and exposures occur in a
“non-controlled” environment
• individuals can be observed
prospectively, retrospectively, or
currently
Case Reports
• Careful and detailed report by one
or more clinicians of the profile of
a single patient
• e.g. previous un-described disease
• e.g. unexpected link between
diseases
• e.g. unexpected new therapeutic
effect
• e.g. adverse events
Case Reports
•
exposures (i.e. a case report gave the clue
OC use increases the risk of venous
thromboembolism.
• “Luck” in being the first to encounter an
interesting case.
• Rigor in diagnosis, testing and
documentation of clinical findings
Strengths
• over one million case reports indexed
on Medline.
• uses language that is familiar to
clinicians and easy to interpret.
• useful reminder about conditions,
diagnoses etc. that are rarely seen in
most practices.
• for researchers, case reports generate
hypotheses that can be tested using
other study designs.
Case Reports
Limitations:
• No appropriate comparison group.
• Cannot be used to test for presence
of a valid statistical association.
• Since based on the experience of
one person:
--presence of any risk factor
may be purely coincidental
--not a true epidemiologic
design
• Tendency to publish "gee whiz" reports
of strange conditions that have little
relevance to daily practice.
• Some authors erroneously try to imply
causation, therapeutic benefits, etc.
Case report from Medline
Authors: Smart ER. Macleod RI. Lawrence CM.
Title: Allergic reactions to rubber gloves in dental patients: report of three cases.
Source: British Dental Journal. 172(12):445-7, 1992 Jun 20.
Abstract:
Three cases of allergy to rubber are described, in which the patients
exhibited peri-oral rashes following dental treatment by personnel
wearing latex rubber gloves. Two of the patients were aware of possible allergy
to domestic rubber products but did not reveal this as part of their medical
history. With the increase in numbers of dentists wearing rubber gloves it is
probable that there will be many more such cases reported in the future. Rubber
products must then be added to the list of potential allergens which may be of
some import to the practice of dentistry.
Case Series
• Experience of a group of patients
with a similar diagnosis.
• Cases may be identified from a
single or multiple sources.
• Generally report on new/unique
condition.
• May be only realistic design for rare
disorders
Case Series
• Advantages
• Cannot study cause and effect
relationships.
• Cannot assess disease frequency.
Strengths:
Case Series
• Useful for hypothesis generation.
• Used as an early means to identify the
beginning or presence of an epidemic.
• Can suggest the emergence of a new disease
(i.e. AIDS).
• Informative for very rare disease with
few established risk factors.
Case Series
Limitations:
• Lack of an appropriate comparison
group
• Cannot be used to test for presence
of a valid statistical association
• Not a true epidemiologic design.
Cross-sectional studies
• An “observational” design that surveys
exposures and disease status at a
single point in time (a cross-section of
the population)
time
Study only exists at this point in time
Cross-sectional
Design
factor present
No Disease
factor absent
Study
population
factor present
Disease
factor absent
Prevalence
time
Study only exists at this point in time
Cross-sectional Studies
• Often used to study conditions that are
relatively frequent with long duration of
expression (nonfatal, chronic
conditions).
• It measures prevalence, not incidence of
disease.
• Not suitable for studying rare or highly
fatal diseases or a disease with short
duration of expression.
Cross-Sectional Studies
Strengths:
• Provides prevalence estimates of
exposure and disease for a well-defined
population.
• Easier to perform than studies that require
follow-up (hence relatively inexpensive).
• Can evaluate multiple risk (and protective)
factors and health outcomes at the same
point in time.
Strengths:
• May identify groups of persons at
high or low risk of disease
• Can be used to generate hypotheses
about associations between
predictive factors and disease
outcomes
Cross-Sectional Studies
Limitations:
• Prevalent rather than incident (new)
cases are used – the exposure could be
associated with survival after disease
occurrence, rather than development
of the disease
• Temporal sequence between exposure
and disease cannot be established
* i.e. Which came first, chicken or the egg?
Cross-sectional studies
• Usually don’t know when disease
occurred.
• Rare events a problem. Quickly
emerging diseases a problem
Health Survey
• Survey (n): Information gathered by
asking a group of individuals the same
questions related to their characteristics,
attributes.
• Survey (v): The process of collecting such
information
Knowing what to expect
• A good survey has the potential to reach a
large number of respondents;
• Generate standardized, quantifiable,
empirical data - as well as some qualitative
data; and offers confidentiality / anonymity
• Credible data, however, can be difficult to
generate
Surveys can be:
– Descriptive:
These surveys pretty much do what they
say - they describe. The goal is to get a
snapshot - of your ‘respondents’
– Explanatory:
These surveys go beyond description and
attempt to establish why things might be
the way they are
• Surveys can also involve populations
or samples of populations:
– Census: This is a survey that does not rely
on a sample. A census surveys every
single person in a defined or target
population
– Cross-sectional surveys: This type of
survey uses a sample or cross-section of
respondents selected to represent a target
population
• Surveys administration:
– Face to face
– Telephone
– Self-administered.
– Electronic e.g. E mail
• Conducting a survey capable of
generating credible data requires:
– thorough planning
– meticulous instrument construction
– comprehensive piloting
– reflexive redevelopment
– deliberate execution
– and appropriate analysis
• Survey questions can either be open or
closed:
– Open questions: Open questions can generate
rich and candid data, but it can be data that is
difficult to code and analyze.
– Closed questions: These questions force
respondents to choose from a range of
predetermined responses, and are generally easy
to code and statistically analyze.
Considerations in Survey Construction
• Providing clear background information
and lucid instructions.
• Logical organization.
• Comprehensive coverage without undue
length.
• User friendly and aesthetically pleasing
layout and design.
Ecologic Studies
• Measures that represent characteristics of
entire populations are used to describe
disease and to postulate causal
associations.
• Measure of interest is correlation between
exposure rates and disease rates among
different groups.
• Correlation coefficient (denoted as r)
Range of r is from –1.0 to 1.0
R evaluated in relation to difference
from 0.
Ecologic Studies
Strengths:
• Cheap, quick, and simple (generally
make use of secondary data)
Limitations:
• Cannot link exposure-disease
relationship at the individual level
• Uses average exposure levels rather
than actual levels of exposure
• Inability to control for confounding
factors
EXAMPLE:
Prevalence-Hypertension
Average Salt Consumption
Country A
Person
Salt Intake Hyp.
1
1
Yes
2
1
Yes
3
1
Yes
4
3
No
5
3
No
6
3
No
7
3
No
8
5
No
9
5
No
10
5
No
Avg.
3.0
30%
Country A:
30%
Moderate
Country B
20%
Low
Country B
Salt Intake Hyp.
1
Yes
1
Yes
1
No
1
No
1
No
2
No
2
No
2
No
2
No
2
No
1.5
20%
Ecologic Studies
The “Ecologic Fallacy”:
• Erroneous conclusions based on
grouped data
• Patterns observed on the aggregate
level are not observed on the
individual level