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Study designs
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Recall from previous session
Selection bias (sampling bias) (includingBerksons bias )
Non respondent bias / volunteer effect
Measurement bias (including Hawthorne effect) Experimenter expectancy bias
Lead time bias
Recall bias
Late look bias
Confounding bias
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Type of bias Definition Important
Associations
Solutions
Selection Sample not
representative
Berksons bias, Non
respondent bias
Random
independent
sample
Measurement Gathering the
information,
distorts it
Hawthorne effect Control group/
Placebo group
Experimenter
expectancy
Researchers belief
affects the outcome
Pygmalion effect
(on students)
Double blind
studies
Lead time Early detection in
screening confused
with increasedsurvival
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Question pattern 2 : What type of study design ?
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Types of study designs
Key distinction : Observational studies vsExperimental studies (clinical trials)
Observation InterventionObservational study : When the researcher simply
observes and records any characteristic. Eg.. Number
of teenagers who drop out of school or Number of
people dying of a particular disease etc.
Experimental study : When there is some sort of
intervention to the study group. Eg.. A new drug,
placebo, education, new surgery, new diagnostic test
etc..
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Exam guidance
These 2 broad types Observational andexperimental will further be divided into types orsubtypes.
However, when a question asks you to identifythe study design used, make sure that youidentify whether it is an observational orexperimental study first following this you will
have the option to eliminate any of the optionswhich belong to the other category Thus, afterthat even if you guess the name of a subtype, itwill be an intelligent guess with increased
peobability
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Types of Observational Studies
Case report : A brief objective report of a clinical
characteristic or outcome from a single clinical subjector event; n = 1. there is no control group.
- E.g., A 23 year old male with syphilitic lesions on the
skin.- Some of the most prestigious scientific journals like
The lancetpublishes a case report in every issue.
- It is the beginning of a possibility of something thatmay be occurring e.g. The first reported case of HIV orthe first reported case of hepatitis B. It all starts witha case report.
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Case series report: Objective report or outcome froma group of clinical subjects. N>1
Eg. Patients at a local hospital with syphilitic lesions.
No control group
- Looking for the common elements of a diseaseamongst people who have it.
- Eg.. As some of your books may say; the patientssuffering from angina experience chest pain referredto the shoulder, Nausea, dizziness etc.. These arethe common clinical elements seen for a disease in
various patients
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Next question to be answered How wide spread
is this disease?- Cross sectional study : A particular point in time
- The presence or absence of disease or othervariables are determined in each member of the
study population or a representative sample at aparticular time. The co-occurrence of a diseaseand a variable can be examined.
- Disease prevalence is recorded (NOT INCIDENCE)
- The temporal sequence of cause and effectcannot be determine
- There is no time frame.
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To increase our knowledge further we need to
know what causes, potentiates or facilitates the
disease process Case-control studies : Retrospective study of
disease vs non-disease.
Identifies a group of people with the disease andcompares them with a suitable comparison group
without the disease. Done retrospectively.
Incidence or prevalence CANNOT be assessed Can help determine causal relationships
Very useful for studying conditions with very low
incidence or prevalence
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Following this I would like to actually observe howthe disease occurs, the process, the factors and theeffect.
Cohort study : Prospective study of groupsfollowed over time.
Population group of those who have been exposed
to risk factor is identified and followed over timeand compared with a group not exposed to therisk factor.
- Prospective subjects tracked forward in time- Can determine incidence and causal relationship
- Must follow population long enough for incidenceto appear
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Therefore in a cohort study INCIDENCE DATA
CAN BE GENERATED
Not the Prevalence.
Therefore if you are given incidence data in a
question, it has to be a COHORT study
It is the best of all the observational study for
understanding a disease process
DOWN SIDE Time consuming and expensive
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Crosssectional
Case control Cohort
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Most feasible study for clinicians who wish to
do research ?
Case control study
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Analysis of the studies
For Cohort study :
Relative risk (RR) :-
Divide : Ie / Iu (I = Incidence, e = Exposed, u =
unexposed)
How much more likely
Absolute risk reduction :
Subtract : Ie - IuHow many more cases