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Lucid the way to Lucid the way to ““Research”Research”
And Follow And Follow an “an “Evidence Based Evidence Based Medicine”Medicine”
A systematic review is a literature review focused on a single question which tries to identify, appraise, select and synthesize all high quality research evidence relevant to that question.
Published documents
national (i.e. IRANMEDEX) and international (i.e. MEDLINE) data banks
Unpublished documents
???
MEDLARS on Line
Medical Literature Analysis and Retrieval
System
For NLM
National Library of
Medicine
(United States)
QualitativeQualitative QuantitativeQuantitative
BasicBasic AppliedApplied
ObservationalObservational ExperimentalExperimental
DescriptiveDescriptive AnalyticAnalytic
Observational
Experimental (Randomized Control Trial - RCT)
Descriptive
Analytic
o Cross-
Sectional
o Case-Control
o Cohort
Case Report Case series Cross sectional Longitudinal Normative research Secondary data analysis
(summaries, meta-analysis) Ecological
Who? When? Where?
3 important questions to consider:
Definition of the Population
Definition of Case
Definition of risk factors
Are cases and non-cases from an unbiased sample of the population?
Cross-Sectional StudyCross-Sectional Studyor prevalence studyor prevalence study
Case / Control
Target population
Prevalence of Dis. = No. of disease / No. of subjects
Prevalence of R.F. = No. of risk factor / No. of subjects
Some uses of Cross-sectional StudySome uses of Cross-sectional Study
Identify and describe a problem
Information about socio-demographic characteristics
Evaluate utilisation rates of services
Monitoring health status of a community by regular repeated surveys
Collect information for planning e.g. surveys of immunisation or prenatal care coverage
Trend Design in Cross-Sectional studies
Population
Disease Prevalence
Risk Factor
Present
Sampling
Future
Risk Factor
Disease Prevalence
Sampling
In cross-sectional studiesIn cross-sectional studies
Advantages
Useful for descriptive studies
Rapid, inexpensive, can provide analytic clues.
Less prone to error about exposure recall bias
Disadvantages
• Prone to sample distortion bias.
• Unable to sort out what came first exposure or outcome
• Prone to seasonal and time to time variations
Case-control StudyCase-control Study
Population at risk
Cases
Controls
Yes
No
Yes
No
people who get the diseasepeople who do not get the diseaseCompare
past "exposures“
In case control studies
In case control studies
Case-selection usually clinic- or hospital-based.
Controls may also be clinic- or hospital-based, or population-sampled.
First S
tep
First S
tep
Selection of cases
Precise definition of ‘case’.
Inclusion / Exclusion criteria.
How are cases to be identified? How recruited?
Second
Second
StepStep
Selection of Controls
Source ( hospital patients without disease;
neighborhood controls; random sample of
population; sibs).
Inclusion / exclusion criteria.
Controls must be related to the same population as the cases are.
Third Step
Third Step
Collection of information
Identify risk factor of interest
Method of collection of information ( questionnaire;
medical records; employment records)
Same procedure to be used for cases and controls
Interviewer should be unaware who is a case and who
a control.
Results of a Case-Control StudyResults of a Case-Control Study
Risk factorDisease
Yes (cases) No (controls)
Yes a c
No b d
Total N1 N2
N1 and N2 are fixed numbers
Some PointsSome Points
Two Methods of SelectionTwo Methods of Selection
Select new cases (i.e. incident) as they come up. Controls are selected from those in the same setting at the same time.
• Select existing cases (prevalent) from a defined population. From the same population a larger number of controls are identified.
The Incident type of case-control study is stronger because diagnosis of cases and ascertainment of exposure is being done by the researcher.
Some PointsSome Points
Matched Case-Control StudiesMatched Case-Control Studies
cases
Non-cases
cases
Non-cases
Cases
Cases
Controls
Controls
Matched controls
cases
Non-cases
Advantages and Disadvantages of Case-control Advantages and Disadvantages of Case-control StudiesStudies
Relatively cheap compared to cohort studies
Relatively quick Useful for study of rare
diseases. No ethical problems Useful for diseases with
long latent period.
• Estimate of disease
incidence cannot be done
• At times difficult to measure
exposure accurately
• Open to selection bias.
• Difficult to interpret.
Advantages Disadvantages