chp400: community health program- li research methodology study designs observational / analytical...

CHP400:Community Health Program- lI

Research Methodology

STUDY DESIGNSObservational / Analytical Studies

Case ControlStudies Present:

Disease Past: Exposure

Cross - section Cohort

Case - control

Present:Disease & Exposure

Present: Exposure Future: Disease

nbmmb

Time is Key

Content

Definition and importance of Case - Control Studies

Design Selecting Cases Selecting Controls Assessing Exposure: Odds Ratio (OR) Advantages Disadvantages

STUDY DESIGNS

ExperimentalObservational

Animal

Experiment

HumanInterventio

nClinical trial

Analytical

Case control

Descriptive

Case report

Case series

Cross section

Ecological

Cohort


Case ControlStudies

STUDY DESIGNS


Observational / Analytical StudiesCase - Control Studies

Definition:It is a type of observational analytic epidemiologic investigation in which subjects are selected on the basis of whether they do or do not have the particular disease under study.

STUDY DESIGNS



Importance: The most frequently undertaken analytical

epidemiological studies The only practical approach for identifying

risk factors for rare diseases They are best suited to the study of

diseases for which medical care is sought, such as cancers or hip fracture

Design :

At baseline:

Selection of cases (disease) and controls (no disease) based on disease status

Exposure status is unknown

Retrospective design – lacks temporality!

STUDY DESIGNS



Design :

Target Population

Diseased(Cases)

Not Diseased(Controls)

Exposed

Not Exposed

Exposed

Not Exposed

STUDY DESIGNS



Design :

YES

NO

TIMEOUTCOME/DISEASEEXPOSURE

?

?

STUDY DESIGNS



Selecting Cases: Select cases after the diagnostic

criteria and definition of the disease is clearly established

Selected cases should be representative of all cases

STUDY DESIGNS



The study need not include all cases in the population

Cases may be selected from hospitals, clinics, disease registries, screenings, etc.

STUDY DESIGNS



Selecting Cases:

Selecting Cases: Incident cases are preferable to prevalent

cases for reducing (a) recall bias and (b) over-representation of cases of

long duration The most desirable way to obtain cases is

to include all incident cases in a defined population over a specified period of time

STUDY DESIGNS



Selecting Controls: Controls should come from the same

population at risk for the disease as the cases

Controls should be representative of the target population

STUDY DESIGNS



Selecting Controls: Multiple controls can be used to add

statistical power when cases are difficult to obtain

Using more than one control group lends credibility to the results

More than 3 controls for a case is usually not cost-efficient

STUDY DESIGNS



Selecting Controls:Sources of cases and controls

Spouses, siblings or associates of cases

Any of the above methods

Sample of patients in the same hospital who do not have the disease

All cases diagnosed in a single hospital

Sample of patients in all hospitals who do not have the disease

All cases diagnosed in all hospitals

Non-cases in a sample of the population

All cases diagnosed in a sample of the population

Sample of general population

All cases diagnosed in the community

CONTROLSCASES

Analysis:

Odds Ratio (OR)

A ratio that measures the odds of exposure for cases compared to controls

Odds of exposure = number exposed number unexposed

OR Numerator: Odds of exposure for cases OR Denominator: Odds of exposure for

controls

STUDY DESIGNS



Analysis:

Odds Ratio (OR)

400200Total

22488Non-

smoker

176112SmokerExposure Status

No CHD(Controls)

CHD cases(Cases)

Disease Status

Odds Ratio = =adbc

112 x 224176 x 88

= 1.62

a bc d

STUDY DESIGNS



ac

bd=

Exposure increases

disease risk(Risk factor)

Exposure is not a risk factor nor

a protectivefactor

Exposure reduces

disease risk(Protective

factor)

Exposure as a risk factor for the disease?

Odds of exposure for cases are

greater than the odds of exposure

for controls

Odds of exposure are equal among

cases and controls

Odds of exposure for cases are less than the odds of

exposure for controls

Odds comparison between cases and controls

OR>1OR=1OR<1

Interpreting the Odds Ratio (e.g)

Analysis:

Odds Ratio (OR)

The odds of exposure for cases are 1.62 times the odds of exposure for controls.

Those with CHD are 1.62 times more likely to be smokers than those without CHD

Interpreting the Odds Ratio

STUDY DESIGNS



Possible Sources of Bias and Error

Information on the potential risk factor

(exposure) may not be available:

- either from records

- or the study subjects’ memories

STUDY DESIGNS



Cases may search for a cause for their

disease and thereby be more likely to report

an exposure than controls (recall bias)

The investigator may be unable to determine

with certainty whether the suspected agent

caused the disease or whether the

occurrence of the disease caused the person

to be exposed to the agent

STUDY DESIGNS


Observational / Analytical StudiesCase - Control Studies cont.

Identifying and assembling a case group

representative of all cases may be unduly

difficult

Identifying and assembling an appropriate

control group may be unduly difficult

STUDY DESIGNS


Observational / Analytical StudiesCase - Control Studies cont.

Advantages:

Quick and easy to complete, cost effective

Most efficient design for rare diseases

Usually requires a smaller study population than a cohort study

Several exposures can be studied.

STUDY DESIGNS



Disadvantages:

Uncertainty of exposure-disease time relationship

Inability to provide a direct estimate of risk

Not suitable for studying rare exposures

Subject to biases (recall & selection bias)

STUDY DESIGNS



Summary

Case-control study:Observational, analytic study.Most frequently undertaken analytical studiesQuick and easy to complete, cost effectiveMost efficient design for rare diseasesSubjects are selected on the basis of presence or absence disease under studyOdds Ratio (OR)

chp400: community health program- li research methodology study designs observational / analytical...

Documents