Transcript IN THE NAME OF GOD

I shall propose nothing dictated merely from theory ; but
shall confirm all by experience and facts, the surest and
most unerring guides.
James Lind,1753
-cohort studies:
Logical and direct way of studing risk
-Limitation:require a lot of time and effort can be
overcome by:
making modifications to cohort methods:
- retrospective cohort
- case-cohort
- another way of studying the
relationship between a potential risk
or protective factor and disease more
efficiently:case control studies
-despite the drawbacks the trade-off
between scientific strenght and feasibility
is
often worthwhile.
-Indeed, case-control studies are
indispensable for studying risk for very
uncommon diseases
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Example :Phenylpropanolamine and hemorrhagic
stroke
Example:Does habitual,vigorous physical activity
protect against primary cardiac arrest in people
without apparent heart disease?
The validity of case-control studies depends
on:
- Which cases and controls are seleted?
- How exposure is measured?
- How extraneous variables are controlled for?
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The cases should be new (incident) cases
not existing (prevalent) ones.
At best,the study should include all the cases
or a representative sample of all cases that
arise in a defined population.
Example:Is vasectomy a risk factor for prostate
cancer?
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The validity of a case-control study depends above
all on the comparability of cases and controls.
Cases and controls should be members of the
same base population and have equal opportunity
of being exposed
 The
population approach
-population-based case-control study
-nested case-control study
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Hospital and community controls
If cases are selected from a hospital ward, the
controls would be selected from patients with
different diseases , apparently unrelated to the
exposure and disease of interest, in the same
hospital.
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Another approach is to obtain Controls from the
community served by the hospital
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Multiple control groups
one group of controls is selected from the same
institution and another from the community.
Example: estrogen and endometrial cancer
Multiple controls per case
Having several control groups per case group
should not be confused with having several
controls per case. if the number of cases is limited,
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the study can provide more information when there
is more than one control per case.
-more controls produce a gain in the ability to detect
risk if it exists,a an increase or decrease in
property of a study called:”statistical power”
-If some characteristics seem especially
strongly related to either exposure or
disease,they can be matched
-With matching,for each case with a set of
characteristics,one on more controls
possess the same characteristics are .that
selected.(age,sex,residence)
-Overmatching
-Overmatching can bias the study toward
finding no effect
Example: NSAIDs and renal failure
The safest approach to measuring exposure is
to depend on complete, accurate records
that were collected before disease developed
Example : what are the risk factors for suicide
in China?
Bias:
Can occur in three general ways:
First,exposure (medical treatment) can be
affected by the presence of disease
Example: beta-blocker in hypertension and MI
prevention
Second,having the disease in question can
affect patients recollection of exposure(recall
bias)
Third, the presence of disease can also affect
the measurement or recording of exposure
-The odds ratio is approximately equal to the
relative risk only when the incidence of
disease is low
-If the frequency of exposure is higher among
cases,the odds ratio will exceed 1,indicating
increased risk
-If the frequency of exposure is lower among
cases,the odds ratio will be less than
1,indicating protection
- examine strata of patients who have
comparable characteristics.
- mathematically adjust for differences
between cases and controls
Use of the case-control method to identify
risk factors for chronic diseases,and the same
method is used to identify risk factors for
outbreaks of acute diseases
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It has been suggested that one should judge
the Validity of case-control study by
considering how a Randomized controlled
trial of the same question would have been
conducted
-As summarized,risk can be described in
quantative terms
-How effectively do clinicians communicate
risk to patients ?
-Communicatig risk is a thorney issue for
several reasons:
• most people are not numerate
• Framing
• personal experience
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