choosing your study design

Choosing the appropriate study design

Research Week 2014Health Cluster UKM

USUAL STUDY DESIGNS

OBSERVATIONAL STUDY

EXPERIMENTAL STUDY

DESCRIPTIVE STUDY

ANALYTIC STUDY

CROSS-SECTIONAL

CASE CONTROL

COHORT

CLINICAL TRIAL

COMMUNITY TRIAL

Difference Between Study Designs

Study Design Intervention Temporal Sequence Sampling

Cross Sectional AbsentRisk Factor and Outcome at same time.

Yes

Case Control AbsentOutcome first, then risk factor (retrospectively).

Matching

Cohort AbsentRisk Factor first, then Outcome.

Maybe

Clinical/ Community Trial

PresentIntervene, then measure Outcome.

Randomisation

Which design is appropriate for your study?

Usual study design

Commonly chosen in PPUKM

• Clinical trial• Cross-sectional • Case-control• Cohort (PURE &

Malaysian Cohort)

Specific for Patho/Diagnostic

• Diagnostic testing• Sensitivity,

Specificity & ROC• Kappa/ Agreement

Which one?

• If you have an intervention, then it is a clinical trial.

• If there is no intervention, then it is one of the observational studies, usually cross-sectional.

• If the disease is rare, less than 5% prevalence rate, then it is a case-control study.

• If you are looking forward in time for new incidence of the disease, then it is a cohort study.

Clinical Trials

If you have an intervention, then probably it is a clinical trial.

Clinical Trial

Time

Intervention

Confounders

OutcomeLook forward in time

Starts with Intervention, then measure the future Outcome

EXPERIMENTAL STUDY

Eligible Participants

T +

T -

C +

C +

C -

C -

Future

Randomisation

Study Population

Selection

Clinical Trial

Drug Sogood

Drug Feelgood

No improvement (25%)

Improved (75%)

Improved (70%)

No improvement (30%)

Sample ratio (1:1)Depressed

patients

Intervention Outcome-Improved?

Starts with Intervention, then measure the future Outcome

Time Future

PROCEDURE• Assignment of subjects to treatments• Interview and examinations• Methods of assessment• Lab. Studies• Treatment schedules: number of units per visit,

rules for changing dosage, compliance checks• Adverse reactions: definition and grading, inquiry,

management.• Drops out: definition, handling and recording,

terminating and extending study

CROSS-SECTIONAL STUDY

Also known as Prevalence Study or Survey. Easiest study design.

Cross-Sectional Study

• Measures the relationship of variables in a defined population at one particular time

• Both risk factors (exposure) and disease outcome are observed at the same (point in) time in a sample (or the entire population) of subjects.

• i.e. Studying the effect of overweight on the prevalence rate of diabetes mellitus.

Exposure & Outcome

Time

Risk Factor

Confounders

Outcome

Cross-Sectional Study

Time

Risk Factor

Confounders

Outcome

Both Risk Factor & Outcome measured at the same time.

Cross-Sectional Study

Overweight (40%)

Normal (60%)

Disease - (68%)

Disease + (32%)

Disease + (8%)

Disease - (92%)

Sample ratio

Risk Factor -Overweight Outcome-Diabetes Mellitus

Both Risk Factor & Outcome measured at the same time.

Time

Reminder

• If the prevalence/incidence rate is below 5%, please do not choose a cross-sectional study design. Your number of cases would be too low to do any analysis.

• For example, doing a cross-sectional study of HIV cases among Malaysian pregnant mothers. The rate of HIV+ is only 0.04%. Therefore even with 10,000 samples, you will only get 4 HIV+.(http://www.moh.gov.my/attachments/3886.pdf)

Imagine a result like this

HIV+ HIV- TotalRisk Factor + 3 97 100Risk Factor - 1 9899 9900

4 9996 10000

Not much analysis could be done with a result like this. So instead do a case-control study for rare diseases.

CASE-CONTROL STUDY

For rare diseases. Consider if prevalence rate below 5%.

CASE CONTROL STUDY- CONCEPT

• comparison of group of diseased person (cases) with another group of non-diseased person (control) for past exposure to a suspected cause of the disease.

• arises because of hypothesis that the risk factor (exposure) causes the disease

Case-Control Study

Time

Exposure

Confounders

Outcome

Look back in time

Starts with Outcome, then trace the retrospective exposure

Case-Control Study

Cataract

Normalvision

DM - (50%)

DM + (50%)

DM + (8%)

DM - (92%)

Sample ratio (1:1)

Outcome-Cataract Risk Factor-Diabetes Mellitus

Starts with Outcome, then trace the retrospective exposure

Time Past

CASE SELECTION• Determine clear and reproducible definitions of

the health problems to be studied (avoid misclassification bias)

• source of cases All persons with the disease seen in particular

facility(ies) in a specified period of time. All persons with the disease found in general

population.

• Incidence cases (newly diagnosed cases) preferred

CHOICE OF CONTROLS• Controls should ideally be selected from the same

population gave rise to cases• Similar to cases in regard to past potential exposure• Free from study disease• If controls are patient with other diseases then

select only diseases that are not known to have relationship with factors under study.

ADVANTAGES

• able to study rare diseases• can explore multiple exposures• relatively inexpensive• can calculate Odds Ratio• can support causation but not prove it• easy to get cases

COHORT STUDY- ignore, none of you will do this.

COHORT STUDY

BASIC CONCEPT• Group or groups of individuals are

studied over time as to onset of new cases of disease and factors associated with the onset of disease.

• Synonyms : incidence study, longitudinal study, prospective study.

Cohort Study

Time

Exposure

Confounders

OutcomeLook forward in time

Starts with Risk Factor, then measure the future Outcome

COHORT STUDY

DISEASE

Free from disease

EXPOSED GROUP

UNEXPOSED GROUP

DISEASE

NONDISEASE

NONDISEASE

FOLLOW-UP

Cohort Study

Normal

Overweight

DM - (68%)

DM + (32%)

DM + (7%)

DM - (93%)

Sample ratio (1:1)

Risk Factor-Weight Outcome-Diabetes Mellitus

Starts with Risk Factor, then measure the future Outcome

Time Future

Free from DM

Recruitment

• Those recruited must be free from the disease of interest at the beginning of the study.

• Those with sub-clinical presentations of the disease may miss from being excluded. This is one of the challenges.

Follow-up

• Keep participation at > 90%• Must have equal ability to detect disease in all

subjects and all groups, with standard measurement• Active vs Passive follow-up• Verbal Autopsy• Blinding of the assessor• Assess both primary and secondary outcomes

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