Transcript Leili salehi ,PhD

Types of studies
Leili Salehi,PhD
Public Health School
Alborz University of Medical Sciences
1
Types of Studies
A. Observational - no manipulation of study factor by
the investigator
B. Experimental - study factor is manipulated by the
investigator
Descriptive studies
Cannot establish causal relationships
 Still play an important role in describing
trends and generating hypotheses about
novel associations

Descriptive studies
Good descriptive reporting answers the five basic W questions:
Who, what, why, when, where
Who has the disease in question ? What is the condition or
disease being studied ? Why did the condition or disease
arise ? Where does or does not the disease or condition arise ?
Case report
Case-series reports
Descriptive studies
Correlation Study
Case Reports
Case Series
Correlation Study(ecological Study)
Use data from entire populations to compare
disease frequencies between different groups.
Example:
Correlation between per capita daily consumption
of meat and rates of colon cancer from a large
number of countries.
Ecological comparison (co relational)
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

Correlation of aggregated or group data
Association on the individual level is unknown and
may be different
Many relationships on global level are strictly
speaking of ecological nature
Advantages & Disadvantages


Can be done quicklyinexpensively
Often uses already
available information

First step in investigating a
possible exposure-disease
relationship

Inability to link exposure
with disease in particular
individuals

The lack of ability to control
for the effects of potential
confounding factors.
Example of an "ecological" comparison: The prevalence of HIV in TB patients
(y-axis) against the prevalence of HIV in adults (x-axis).
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Measured prevalence of HIV in TB patients (%)
80
MAL
ZIM
ETH
BOT
60
KEN
SOA
LES
CAF
IVC
BUU
TAN
HAI
BFA
IVC
40
COD
KEN CAE
RWA
MOZ
BFA
NIE
GHA
DJI
20 DJI
CNG
CAM
0
0
10
20
30
Estimated prevalence of HIV in adults (%)
40
Case Reports
Document unusual medical occurrence
Represent the first clues of new disease or adverse
effects of exposures
‫گزارش‌هاي موردي‬
‫)‪(Case Reports‬‬
‫‪12‬‬
‫گزارش يك مورد كوري ناش ي از بروسلوز‌‬
‫گزارش‌ موارد‬
‫)‪(Case Series‬‬
‫طغيان تيفوئيد در بين پناهندگان عراقي غرب ايران‬
Analytic Studies

Attempt to establish a causal link between
a predictor/risk factor and an outcome.
Analytical Studies
Research Designs in Analytic Epidemiology
‫مقطعي‬
Cross - Sectional
‫همگروهي‬
‫مورد ـ شاهدي‬
Cohort
Case - Control
Cross Sectional Studies
Population survey
Community Survey
Epidemiology Survey
Since exposure and disease status are assessed
at a single point in time
‫‪Cross-Sectional Study‬‬
‫‪‬‬
‫‪‬‬
‫در واقع در لحظه‌اي از زمان به بررس ي سالمت و بيماري‌‬
‫افراد مي‌پردازيم‬
‫موارد اندازه‌گيري شده‪ ،‬جزئي از شيوع به حساب مي‌آيد‪.‬‬
‫‪ ‬در گروه‌ها و جمعيت‌ها انجام مي شوند‬
‫‪ ‬آسان‪ ،‬سريع و كم هزينه هستند‬
‫‪ ‬با استفاده از اطالعات و آمار موجود قابل انجام مي‌باشند‬
‫‪ ‬زمينه ساز مطالعات بزركتر هستند‪.‬‬
‫کرج‬
‫‪ ‬مثال‪ :‬بررس ی ميزان آنمی فقرآهن در دختران دانش آموز شهر ‌‬
Disadvantages-Cross Sectional
Studies
Impractical for rare diseases
Not a useful type of study for establishing
causal relationships
Confounding is difficult to control
Miss diseases still in latent period

Cross-sectional study: minuses
- Cannot determine causality
Red Bull consumption
USMLE Score
time
Research Question
Is the regular consumption of Red Bull
associated with improved academic
performance among U.S. medical students?
Cross-sectional Study:


Descriptive value:
 How many UCSF medical students drink Red Bull?
 What is the age and sex distribution of UCSF medical students
who drink Red Bull?
Analytic value:
 Is there an association between regular Red Bull consumption
and test scores among UCSF med students?

Case Control Studies
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Case-control studies
 Case-Control Studies identify existing disease/s
and look back in previous years to identify previous
exposures to causal factors.
 Cases are those who have a disease.
 Controls are those without a disease.
 Analyses examine if exposure levels are different
between the groups.
Design of Case-Control Studies

Cases: persons/group with a given disease

Controls: persons/group without the given disease


Ascertain exposure or background of the two groups and
compare the proportion
Best suited for study of diseases where medical care usually
sought, (hip fracture, cancer) because this makes it easier to
identify cases
Case control studies selection of cases
Hospital
General Population
Case control studies selection of controls
Hospital
Friends-neighbors-relatives of cases
General Population Controls
Age, Sex, Socio-economical Status must be similar
with case group
Advantage
Relatively quick and inexpensive compared
with other analytic designs
Suitable for long latent periods and rare
diseases
Can examine multiple etiologic factors for a
single disease.

Case-control study-minuses
Causality still difficult to establish

Selection bias (appropriate controls)
Caffeine and Pancreatic cancer in the GI clinic

Recall bias: sampling (retrospective)
Abortion and risk of breast cancer in Sweden




Preliminary results from our cross-sectional and
case-control study suggest an association between
Red Bull consumption and improved academic
performance among medical students
What’s missing? - strengthening evidence for a
causal link between Red Bull consumption and
academic performance
Use results from our previous studies to apply for
funding for a prospective cohort study!
Cohort studies

A cohort study is a comparative, observational study
in which subjects are grouped by their exposure
status, i.e., whether or not the subject was exposed to
a suspected risk factor

The subjects, exposed and unexposed to the risk
factor, are followed forward in time to determine if
one or more new outcomes occur

The rates of disease incidence among the exposed
and unexposed groups are determined and compared.
Cohort Studies

Incidence studies

Follow up Studies

Longitudinal studies

Prospective studies
Cohort Studies
Disease
Present
Exposed
Time
Absent
Not exposed
Time
Present
Absent
‫‪Example:‬‬
‫بررس ی ميزان رخداد سرطان ریه در دو گروه از افرادی که در معدن آهن و‬
‫معدن روی کار می کنند‬
‫یا‬
‫بررس ی ميزان رخداد سرطان ریه در دو گروه از افرادی که در معدن آهن کار می‬
‫کنند و افرادی که در اداره کار می کنند‬
Two types of Cohort studies:
Prospective Cohort Study

Exposure(+)

Exposure(-)
beginning of study
Disease(?)
Disease(?)
Two types of Cohort studies:
Retrospective Cohort Study

Exposure(+)

Exposure(-)
beginning of study
Disease(?)
Disease(?)
Population
People who don’t have disease
Exposure(+)
Disease(+)
“a”
Exposure(-)
Disease(-)
Disease(+)
“b”
“c”
Disease(-)
“d”
Cohort Study disadvantages




If incidence rate is low and latent period is long ,it is
inefficient – large number ,more time are needed
If prospective ,can be expensive and time consuming
If retrospective require the availability of adequate
records
Loses to follow up
Cohort Study Advantages

Exposure-disease relation is very reliable

Can examine multiple effect of a single exposure

Prospective type minimize BIAS

Incidence rate and relative risk can be calculated
‫‪Cohort study‬‬
‫‪10/70‬‬
‫بيمار‬
‫بيمار‬
‫سالم‬
‫سالم‬
‫بيمار‬
‫بيمار‬
‫بيمار‬
‫سالم‬
‫سالم‬
‫سالم‬
‫بيمار‬
‫بيمار‬
‫سالم‬
‫جمعيت ‪ 200‬نفره‬
‫خوابگاه دختران‬
‫دانشجو‬
‫سالم‬
‫بيمار‬
‫تماس نيافتگان سالم و بيمار‬
‫سالم‬
‫بيمار‬
‫سالم‬
‫‪30/130‬‬
‫تماس يافتگان سالم و بيمار‬
Measures of association
Risk ratio (relative
risk)
Disease
Yes
No
Yes
A
B
No
C
D
Risk
Factor
A
A+B
C
C+D
Oral contraceptives
Ovarian cancer
Cohort Study
Cohort Study
Ovarian cancer
Use of oral contraceptives
Control group
Problem: adequate control group (matching) and recall bias. Because casecontrol studies lack denominators, incidence rates, relative risks or attributable
risks cannot be calculated. Instead odds ratios are the measures of association.
Comparison of the Characteristics of
Cohort Study
&
Usually very expensive
Complete source population
experienced tallied
Case-Control Studies
Usually less expensive
denominator
Can calculate incidence rates or risks and
their differences and ratios
Sampling from source population
Can usually calculate only the ratio of
incidence rates or risks
Convenient for studying many diseases
Convenient for studying many exposures

Is selection bias present ?

In a cohort study, are participants in the exposed
and unex-posed groups similar in all important
aspects except for the exposure ?

In a case-control study, are cases and controls
similar in all important aspects except for the
disease in question ?
Interventional study=Experimental study

In an experimental study's , the researcher
manipulate at least one independent variable,
controls other relevant ,and observer the effect on
one or more of the dependent variable.

Independent variable is called the experimental
variable .
Randomized controlled studies
Treatment
group
Follow up
Patients
Control
group
Random assignment=Random allocation
Follow up
Compare
Results
Experimental Design
outcome
Intervention
No outcome
Study
Population
outcome
Control
No outcome
Experimental Study
Clinical trials(patient)
Field trials(Healthy population)
Community trial(Small population)
Clinical Trials
Outcome
New
Treatment
Improved
Not improved
Patients
Comparison
treatment
Improved
Not improved
Hierarchy of Study Types
Descriptive
•Case report
•Case series
•Survey
Analytic
Observational
•Cross sectional
•Case-control
•Cohort studies
Strength of evidence for causality between a risk factor and outcome
Experimental
•Randomized
controlled trials
Experimental Studies



Clinical Trials (Involve humans)
Easier to identify (usually explicitly stated in the
abstract)
Two main categories of clinical trials:
1.
2.
Controlled trials
Uncontrolled trials
Clinical Trials
Randomized Controlled Studies
Experimental Studies
Controlled trials
Uncontrolled trials
Independent
concurrent
controls
Self controls
RCT
External controls
Non-randomized
Systematic Reviews &Meta-analyses
Study Pyramid
Best
Worst
Temporal direction of study designs
Lancet 2002; 359: 57-61