Transcript 5-Study Designs for 305.ppt

Study Designs
Ashry Gad Mohamed,
MB.ChB, MPH, DrPH
Professor of Epidemiology
College of Medicine & KKUH
Objectives
At the end of the presentation each
participant should be able to:
1-know main designs for biomedical researches.
2-Select the proper design for each research
question.
3-list areas of strength and weakness of each
design.
Study Designs
Observational
Descriptive
Case report
Analytical
Ecological
Animal
Experiment
Case control
Case series
Cross section
Experimental
Cohort
Human
Intervention
Clinical trial
Case Report
• Detailed presentation of a single case
Generally report a new or unique finding
• Previous unknown disease
• Unexpected link between diseases
• Unexpected new therapeutic effect
• Adverse events
Case Series
• Experience of a group of patients
with a similar diagnosis
• Cases may be identified from a
single or multiple sources
• Generally report on new/unique
condition
• May be only realistic design for rare
disorders
Case Series
• Advantages
• Useful for hypothesis generation
• Informative for very rare diseases
with few established risk factors
• Disadvantages
• Cannot study cause and effect
relationships
• Cannot assess disease frequency
Cross-sectional Study
• Data collected at a single point in time
• Describes associations
• Prevalence
Cross-sectional Studies
• Frequent conditions with long
duration of expression (nonfatal,
chronic conditions)
• It measures prevalence, not
incidence of disease
• Not suitable for studying rare or
highly fatal diseases or a disease
with short duration of expression
Prevalence vs. Incidence
• Prevalence
– The total number of cases at a point in time
– Includes both new and old cases
• Incidence
– The number of new cases over time
Cross-sectional Study
Sample of Population
Physically
active life
style
Prevalence of
IHD
Sedentary life
style
Prevalence of
IHD
Time Frame = Present
Cross-sectional studies
• Disadvantages
• Weakest observational design,
(it measures prevalence, not incidence of
disease).
• The temporal sequence of exposure and
effect may be difficult or impossible to
determine
• Usually don’t know when disease occurred
• Rare events a problem. Quickly emerging
diseases a problem
Case-Control Study
High fish Diet
Low fish Diet
High fish Diet
Low fish Diet
Past
Cases
Patients with CAD
Controls
Patients w/o CAD
Present
Case-Control Studies:
Strengths
• Good for rare outcomes: cancer
• Can examine many exposures
• Useful to generate hypothesis
• Fast
• Cheap
Case-Control Studies:
Weaknesses
• Cannot measure
– Incidence
– Prevalence
– Relative Risk
• Can only study one outcome
• High susceptibility to bias
Analysis of case control study
• Because population at risk is absent we
can not calculate relative risk as it is
based on incidence, however it can be
estimated by means of odds ratio (OR)
which is the ratio of odds of exposure
among diseased to the odds of
exposure among controls.
Exposure for each case and control
Disease status
Cases
Exposure
Controls
Yes
A
B
NO
C
D
A+C
B+D
• Calculating Odds (number exposed 
number unexposed)
Odds (Cases) = A/C
Odds (controls) = B/D
Odds Ratio = (A/C) / (B/D) = AD/BC
Example
Disease Status
Odds Ratio=
AD
BC
= 176 X 88
112 X 224
= 0.62
Fish diet
CHD
(Cases)
No CHD
(Controls)
88
224
112
176
200
400
No fish
Total
Cohort Study
• Begin with disease-free patients
• Classify patients as exposed/unexposed
• Record outcomes in both groups
• Compare outcomes using relative risk
Cohort Study: Strengths
• Provides incidence data
• Establishes time sequence for causality
• Eliminates recall bias
• Allows for accurate measurement of
exposure variables
Cohort Study: Weaknesses
• Exposure may change over time
• Disease may have a long pre-clinical
phase
• Attrition of study population
Measuring the effect of risk factor
Relative Risk = (a/a+b) / (c/c+d)
Risk
factor
Outcome
Total
present
absent
Present
a
b
a +b
Absent
c
d
c +d
a +c
b+d
N
Total
Obesity & Diabetes
Obesity
Diabetes
Total
present
absent
Present
43
369
412
Absent
29
601
630
Total
72
b+d
1042
RR= (43/412) / (29/630) =0.104 / 0.046 =2.26
Clinical trial
Definition
a planned experiment on humans. The setting is
in health institutions environment. It usually
involves patients.
Rationale
Before a new treatment method is made
available to the public it must be studied
and tested for safety and effectiveness.
Clinical trials provide the “gold standard”
of determining the relationship between
garlic and cardiovascular disease
prevention.
Clinical Trial
Study
sample
R
a
n
d
o
m
i
z
e
Treatment
Group
Outcomes
Control
Group
Outcomes
Allocation of regimens
Intervention
versus
Randomization
Aim
Methods
Placebo
Current treatment
Nothing
Blinding
One or more of the people involved in the
trial is unaware of the intervention.
1- Open trial
2- Single- blind trial
3- Double blind trial
4-Double blind double dummy trial
5- Triple and quadruple blind
Follow up
• Quantity
• Quality
• Compliance