Choosing your study design

1. Choosing the appropriate
study design
Research Week 2014
Health Cluster UKM

2. USUAL STUDY DESIGNS
OBSERVATIONAL
STUDY
EXPERIMENTAL
STUDY
DESCRIPTIVE
STUDY
ANALYTIC STUDY
CROSS-SECTIONAL
CASE CONTROL
COHORT
CLINICAL
TRIAL
COMMUNITY
TRIAL

3. Difference Between
Study Designs
Study Design Intervention Temporal Sequence Sampling
Cross Sectional Absent
Risk Factor and
Outcome at same time.
Yes
Case Control Absent
Outcome first, then risk
factor (retrospectively).
Matching
Cohort Absent
Risk Factor first, then
Outcome.
Maybe
Clinical/
Community Trial
Present
Intervene, then measure
Outcome.
Randomisation

4. Which design is appropriate for
your study?

5. 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

6. 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.

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

8. Clinical Trial
Time
Intervention
Confounders
Outcome
Look forward
in time
Starts with Intervention, then measure the future Outcome

9. EXPERIMENTAL STUDY
Eligible
Participants
T +
T -
C +
C +
C -
C -
Future
Randomisation
Study
Population
Selection

10. 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

11. 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

12. CROSS-SECTIONAL STUDY
Also known as Prevalence Study
or Survey. Easiest study design.

13. 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.

14. Exposure & Outcome
Time
Risk Factor
Confounders
Outcome

15. Cross-Sectional Study
Time
Risk Factor
Confounders
Outcome
Both Risk Factor & Outcome measured at the same time.

16. 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

17. 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)

18. Imagine a result like this
HIV+ HIV- Total
Risk Factor + 3 97 100
Risk 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.

19. CASE-CONTROL STUDY
For rare diseases. Consider if
prevalence rate below 5%.

20. 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

21. Case-Control Study
Time
Exposure
Confounders
Outcome
Look back in
time
Starts with Outcome, then trace the retrospective exposure

22. Case-Control Study
Cataract
Normal
vision
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

23. 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

24. 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.

25. 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

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

27. 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.

28. Cohort Study
Time
Exposure
Confounders
Outcome
Look forward
in time
Starts with Risk Factor, then measure the future Outcome

29. COHORT STUDY
DISEASE
Free from
disease
EXPOSED GROUP
UNEXPOSED GROUP
DISEASE
NONDISEASE
NONDISEASE
FOLLOW-UP

30. 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

31. 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.

32. 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

33. Next
Calculate sample size