دکتر خلیلی

1

Lucid the way to Lucid the way to ““Research”Research”

And Follow And Follow an “an “Evidence Based Evidence Based Medicine”Medicine”

Sampling

Study Design

Measurement

National

International

Published

Unpublished?!

We w

ill d

iscu

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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Unpublished documents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)

Iran Medical Indexan Iranian private company includes 155 national journals

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