Epidemiological studies

Dr. Dalia El-Shafei
Lecturer, CommunityMedicineDepartment, Zagazig
University
http://www.slideshare.net/daliaelshafei

Epidemiology is derived from the Greek,
Epi: On or upon.
Demos: people.
Logos : the study of
Epidemiology is the basic science
of Public Health

Definition of Epidemiology
The STUDY of the DISTRIBUTION and
DETERMINANTS of HEALTH-RELATED
STATES in specified POPULATIONS, and
the application of this study to
CONTROL of health problems."

Is the basic science of public health
Provides insight regarding the nature,
causes, and extent of health and disease
Provides information needed to plan and
target resources appropriately
So, Epidemiology

Non-experimental
studies = Observational
studies:
-Investigator does not
intervene.
-The investigator observes
natural course of events,
observing who is exposed
and who is not, who is
diseased and who is
healthy.
-The non-experimental
studies can be either
descriptive or analytical.
Experimental studies =
Interventional studies:
- Involve an active trial to
change disease determinant
by the investigator who
controls the exposure.
-Investigator allocates the
exposure and follows the
subjects.
- Participant are identified on
the basis of their exposure
status and followed to
determine whether they
develop the outcome or not.
Epidemiological methods

Descriptive Epidemiological studies
To Know the situation: (what is the
problem? What are its manifestations?)
Or
 To describe the general characteristics of a
disease /or health problem in relation to
(time – place –person).

□ Person: Who is getting sick?
□ Place: Where is the sickness occurring?
□ Time: When is the sickness occurring?
PPT = person, place, time

Descriptive Studies
1- Case Report:
Example:
Intestinal obstruction was reported in a young child.. Documents
showed that this child received Rota virus vaccine three months ago. A
detailed report about this unusual event and exposure was published
in a medical journal. The investigator formulated a hypothesis that
Rota virus vaccine may have been responsible for the rare occurrence
of this event.

The features of the Case Report:
 It consists of a careful and detailed report (published
in medical journals) by one or more clinicians of
unusual medical condition.
 It represents the first clue in the identification of a new
disease.
 It leads to formulation of a new hypothesis.

2-Case Series:
It is the only study which depends on Routine Surveillance.
What is surveillance?
Example of the case series study:
•During 1950 , 8 cases of cancer lung were admitted to different
hospitals during the same period of time. Taking history from these
patients showed that they were miners . This unusual circumstance
suggested that the miners may been exposed to something. Investigating
this circumstance showed high concentration of radon gas. A hypothesis
was formulated that lung cancer is related to exposure to radon.

The benefits of case report & case series:
They identify a new case and/ or an unusual variation
of a disease occurrence.
•They formulate a new hypothesis for disease
occurrence.
•They act as trigger as they stimulate the start of
analytic studies to be conducted to identify the risk
factors of the disease.
•Modification of the case series to be a case control
study can be obtained by using a comparison
group.

The limitations of the case report & case series:
 For the case report, the presence of any
exposure may be coincidental because it is based
on a single experience .
 Lack of the comparison group in case series can
either obscure the relationship or suggest an
association which is not actually exist.
 Both of them cannot be used to show the causal
association, i.e. can not be used to test the
hypothesis.

3-Correlation study: ( Ecological study)
The source of data is the entire population .
It compares disease frequencies:
- between different population during the
same period of time Or
- In the same population at different in time .
It compares 2 quantitative variables.

Correlation between one of climatic indicator
(Temp.) & frequency of cerebrovascular storks.
Figure 1 shows the correlation between the
average regional temp. & the frequency of
CVSs in different countries. Countries with the
highest average temp. have the highest rates of
CVSs and vice versa.

100 The average regional temp. & the frequency of CVSs
80
+ve Correlation (r = +1)
60
40
20
0
10°C 15°C 20°C 25°C 30°C 35°C 40°C
The Average Regional Temperature

Example 2:
The average number of mammography carried for
women above 50 years of age per year & the
mortality from cancer breast.
This can be presented by the following figures.

The average number of mammography per year for woman
above 50 & the mortality from cancer breast
100 Negative Correlation (r = -1)
80
60
40
20
0
3 4 5 6 7 8 9
The average number of mammography per year for woman above 50

The advantages of the correlation study:
1- Formulates new hypothesis.
2- Quick & Cheap.

The limitations of correlation studies:
1.As the value of exposure is quantified by the
average, it is impossible to link the exposure & the
disease in a particular individual. It is not possible to
tell that the person who gets cerebro-vascular stroke
is the one who is exposed to high temperature.
2. They cannot be used for testing the hypothesis.
3.Lack of the ability to control for the effects of the
confounding factors.

Confounding factors:
These are factors other than the studied one
that disturb the relation between the studied
exposure and the disease of interest.
For example: The association between the
average family size and the frequency of iron
deficiency anemia may be due to other factors
such as the pattern of diet, the infectious
diseases , the socioeconomic conditions and
parasitic infections.

Impacts of the Confounding Factors
Large Family size
(Exposure)
Iron deficiency anemia
(condition)
Parasitic Infection
Pattern of Diet
Mothers Awareness
Mothers Education
(Confounding factors)

4- Cross sectional study (Prevalence study):
Population Sample
Without Exposure & without disease
Without Exposure & with disease
With Exposure &without disease
With Exposure & with disease

Example:
During the year 2004 , a representative sample of
secondary school pupils in a city x (n=400) were
asked about consumption of high caloric diet &
examined to detect obesity.
Questions:
Draw the flow chart.
Tabulate the data.
Write the title of the table.

No consumption of high
Caloric diet without obesity
n=304
No consumption of high
Caloric diet with obesity
n=16
Consumption of high
Caloric diet without obesity
n=60
Consumption of high
caloric diet with obesity
n=20
Secondary
school pupils
Sample
n=400
The flow Chart:

Distribution of the studied sample of secondary school
pupils in the city X during the year 2004 according to
consumption of high caloric diet & obesity.
Consumption
of high caloric
diet
With
obesity
Without
obesity
Total
Yes 20 60 80
No 16 304 320
Total 36 364 400

Prevalence of obesity among those consumed high caloric diet (P1 ) =
20 X 100 = 25%
80
Prevalence of obesity among those don’t consume high caloric diet
(P2) =
16 X 100 = 5%
320
The prevalence rate =
The total number of all cases (old and new) in certain area at a given time X 100
The total number of population in the same area and time

The uses of cross-sectional study:
 Estimation of prevalence rate of disease or
any health related phenomena.
It leads to formulation of hypothesis.
It is suitable for chronic diseases with long
latency.
 Quick & cheap, compared to prospective
cohort study.

The Limitations of the cross-sectional study:
 Can’t be used to test hypothesis (chicken egg
dilemma).
 Deals with survivals only but those who died,
cured or migrated are not included.
 Can’t be used in acute diseases of short duration.
 Not suitable for rare diseases (Compared with the
case control study)

Analytical epidemiology
(Finding the cause-effect)
Try to identify causal relationships between
some risk factors & occurrence of disease.
Try to answer why the disease occurs.

ANALYTICAL STUDIES
It is formed of 2 comparative groups.
Their types are:
1- Case-control
2- Cohort: -Prospective
-Retrospective
3- Comparative cross-sectional

Retrospective
(Case-Control(
a b
dc
DISEASEPresent Absent
E
X
P
O
S
U
R
E
Present
Absent
Case
s
Controls
Total Total
Prospective
(Cohort(
Exposed
Not exposed
A fourfold table
Mausner, 1985

Design of a Case-Control Study

The features of case control Study
The subjects are selected on the basis of whether they
have:
- The condition (e.g. cases with disease or any
health related events) or
- Free from the condition (the control).
Both are then compared with respect to the having
the history of exposure or certain characteristic.
It is used to test the hypothesis i.e. the causal
association between the exposure and the events
(disease).

Steps to conduct the case control study:
1-Selection of cases:
a. Establishment of diagnostic criteria (standard case def.(.
b. Sources of cases:
i) Hospitals or any health care facility ii) General population:
2-Selection of the control:
a. Matching.
b. Sources of the control.
i) Hospitals ii) Relatives. iii) Neighborhoods.
c. Size of the control
3. Assessment of the exposure:
4. Analysis and interpretation of the results.
a. Tabulation of data
b. Flow chart
c. Calculation & interpretation of the estimated risk (odds
ratio)

2-Selection of the control:
a. Matching: It is the process in which we select the control
in a way that they have the same confounding factors
affecting the cases (e.g. age) which are known to
influence the outcome of the disease.
b. Sources of the control:.
i) Hospitals or any health care facilities.
ii) Relatives: They are co-operative however they are unsuitable
control when genetic conditions are under study.
iii) Neighborhoods
vi) General population: it is expensive, time consuming, difficult
and the individuals may be uncooperative.
c. Size of the control:
If the number of the cases is >50 cases ,use one control for each case.
If the number of cases is < 50, use 2,3 or even 4 controls.

3. Assessment of the exposure: By interview, by
questionnaires, or by studying past records of cases
“hospital records, school or occupational records”
4. Analysis & interpretation of the results:
Tabulation of data:
Framework of case control Study
Exposure Cases Control
Exposed a b
Not Exposed c d
Total a+c b+d

The rate of exposure among the cases =
The number of those exposed among the cases X100 = a x 100
The total number of cases a + c
The rate of exposure among the controls =
The number of those exposed among the control X100 = b x 100
The total number of control b + d
Exposed a b
Not Exposed c d
Total a+c b+d
b. Exposure rate:

c. Estimation of risk associated with exposure: (Odds Ratio)
Measure of the strength of the association between the risk
factor & the disease.
 How to calculate the odds ratio?
What is the odds that a case is being exposed?
a ÷ c = a
a +c a+c c
 What is the odds that a control is being exposed?
b ÷ d = b
b+d b+d d
 What is the estimated risk (odds ratio)?
a ÷ b = a d
c d b c
The odds ratio = ad
bc
Exposed a b
Not Exposed c d
Total a+c b+d

1
No relation
between exposure
& disease
RiskProtective

Benefits of case control study:
1- Suitable :
to test the hypothesis that the disease of interest is caused by
an exposure.
for diseases with long latency period.
to study rare diseases
2- Easy, rapid, & cheap (compared withy prospective cohort)
3- Requires few subjects.
4-Can examine multiple exposure factors for a single disease.
5-Estimation of the risk (odds Ratio)
6-Minimal ethical problems.
7- No attrition problem.

Limitations of case control study:
1- Incidence & Prevalence rates can not be calculated.
2- Not suitable for studying rare exposures.
3-The problem of bias.

What is Bias?
Bias is any systematic error in the determination of
the association between the exposure and the disease.
Types of Bias:
•Recall bias.
•Bias due confounding factors.
•Selection bias.

Selection bias: The cases may not represent
those in the general population.
Example:
The health awareness about the association
between CHD and smoking influences the
selection of cases. Smokers at the time of onset of
CHD are more likely to attend the health care
facilities than those with similar symptoms who
are non smokers. This results in an artificially high
proportion of cases of CHD among smokers.

Confounding factors:
Factors other than the studied one that disturb
relation between the studied exposure &
disease of interest.
For example: Association between average
family size & frequency of iron deficiency
anemia may be due to other factors such as
pattern of diet, infectious diseases,
socioeconomic conditions and parasitic
infections.

The Impacts of the Confounding Factors
Large Family size
(Exposure(
Iron deficiency anemia
(condition(
Parasitic Infection
Pattern of Diet
Mothers Awareness
Mothers Education
(Confounding factors(

Recall (Interview( Problems
Limitations in recall
Recall bias
One group (e.g., mothers with child with
birth defect) may clearly remember (recall)
an event (e.g., mild respiratory infection)
Other group (e.g., mothers with healthy
child) may not recall any such event

Matching
Concern that cases & controls may differ in
characteristics or exposures other than that
observed in the study
To overcome this problem, we can match cases in
controls in regard to potential factors of concern
Matching selects controls that are similar to cases
in characteristics such as age, race sex,
socioeconomic status, occupation, etc.

Matching
Group matching (frequency matching)
proportion of controls with a given
characteristic (variable) is identical to
proportion of cases with the same characteristic
Individual matching (matched pairs)
for each case, a control is selected who is
similar to the case for a given variable(s)

Advantages
1-Inexpensive & not time
consuming.
2-Suitable for rare diseases.
3-Suitable for diseases with
long latent periods.
4-Can examine multiple
etiologic factors for a
single disease at same
time.
5- No Drop-out problem.
Disadvantages
1-Relatioship between
exposure & disease
difficult to establish
2-Inefficient for rare
exposure.
3-Cannot calculate incidence.
4-Selective & recall bias.

The Case Control Study
Example:
An investigator selected 200 patients with basal cell carcinoma (BCC)
admitted to X hospital during the year 2004, and 200 subjects free from
the disease as a control from general population. Both groups were
interviewed to obtain information on history of exposure to sunrays
Those with history of exposure were 120 among cases and 40 among
the control .
1-Draw the flow chart
2-Tabulate the data.
3- Mention the dependent, independent & the confounding factors.
4-Estimate the risk of exposure to sunrays.

History of sun exposure (n=120(
History of no sun exposure (n=80(
History of sun exposure (n=40(
History of no sun exposure (n=160(
Past
Present
Patients with BCC
n=200
Control free from BCC
n=200
The Flow Chart

The Independent variable :
The Exposure to Sun Rays .
The Dependent variable :
The BCC
The Confounding factors :
Sex, Age, Local Chemicals,
Cosmetics or Chronic dermatitis,
Occupation.

Distribution of patients with BCC admitted to X hospital
& their controls during 2004 according to history of
exposure to sunrays.
History of exposure to sunrays BCC Cases Control
Yes 120 40
No 80 160
Total 200 200
Estimation of the risk:
Rate of exposure to sun rays among the cases= 120X100 = 60%
200
Rate of exposure to sun rays among control= 40X100 = 20%
200
Because 60% > 20 %
So there is an association between BCC and exposure to sun rays

History of exposure
to sunrays
Patients
with BCC
The control
Yes (a( 120 40(b(
No (c( 80 160(d(
Total 200 200
Calculation & interpretation of Odds ratio:
Odds Ratio = 120X160 = 6
40X80

Interpretation of the Odds ratio:
Those exposed to sun rays are 6 times at risk to have BCC
than those not exposed.
OR
Patients with BCC tended to be exposed to sun rays 6 times
greater than those without BCC.
OR
It is 6 times more likely to find prior exposure to sun rays
among patients with BCC than among those free from BCC.

Cohort studies
Another type of analytical study
which is usually done to obtain evidence
to support the existence of an association
between suspected cause and a disease

Cohort study has 2 types:
Prospective cohort study: All data will be collected in
the future
 Retrospective prospective study: where part is carried
out retrospectively by collecting existing data then the
cohort is followed till the outcome under study is
developed.

Concept of a cohort
In epidemiology the word cohort is defined as a group of
people who share a common characteristic or experience
within a defined period of time (e.g. age, occupation,
exposure to drug, vaccine, pregnancy, birth or marriage
cohorts).
The comparison group may be the general population from
which the cohort is drawn or may be another cohort of
persons thought to have had little or no exposure to the
substance in question.

Known by a variety of names
Prospective study
Longitudinal study
Incidence study

Cause
RF
exposure
Effect
)Disease(
Cohort
Case control

Retrospective
)Case-Control(
a b
dc
DISEASEPresent Absent
E
X
P
O
S
U
R
E
Present
Absent
Case
s
Controls
Total Total
Prospective
)Cohort(
Exposed
Not exposed
A fourfold table
Mausner, 1985

Prospective cohort
The features of prospective cohort of the study:
 A group of individuals are defined on the basis of the presence
or absence of exposure to a suspected factor for a disease.
 At the time when the exposure status is defined, all individuals
must be free from the disease under investigation.
 They will be followed over a period of time to assess the
occurrence of that outcome.

Steps to carry out the prospective cohort study:
1- Selection of the cohorts: This depends on exposure:
2-Obtaining data on exposure:
a. Interviews or questionnaires from cohort members
b. Review of medical records: e.g., dose of radiation, kinds of
surgery, details of vaccination or medical treatment.
c. Medical examination or special test: blood pressure, cholesterol
d. Environmental survey: e.g. the level of air pollutants.
4-Follow up.
5-Analysis & interpretation.
Exposure Cohort Pattern of Pop. The comparison
Common
)Smoking(
General population
(smokers &non smokers)
Heterogeneous Internal
Rare
)Radiation(
Special group (Radiologists) Homogenous External

Elements of a cohort study
1- Selection of the study subjects:
General population (when exposure or the
cause of the disease is fairly frequent in the
population. The cohort residing in the same
geographical area as in (Framingham study)
Selected groups as professional group
Exposure group: cohorts selected with special
exposure to physical, chemical or other
disease agents.

2- Obtaining data on exposure from:
Cohort members, questionnaire through
personal interviews, or mailed questionnaire
in large cohorts.
Review of records: dose of radiation, number
of surgeries, details of medical treatment,
Medical examination or special tests e.g. BP
measurement, serum cholesterol……. etc.

3- Selection of comparison group:
Internal comparison: the same cohort that enters
the study may be classified into several comparison
groups according to the degree of exposure
(smoking, cholesterol) before the development of
the disease in question.
External comparison: if all of my cohort is
exposed to the risk factor (radiologist, so we
compare with ophthalmologist, this would make
external comparison).

Comparison with the rates of the general
population e.g. mortality experience of the
exposed group is compared with mortality
experience in the general population
(comparing the mortality rate of asbestos
workers with the mortality rate in the general
population).

4- Follow up:
- Periodic medical examination
- Reviewing physicians and hospital records
- Routine surveillance of death records
-Mailed questionnaires
-Telephone calls
-Home visits.

5- Analysis of cohort study:
The data obtained are analyzed in terms of:
A- Incidence rates of outcomes among
exposed and non exposed groups.
B- Estimation of RISK.

Benefits of cohort study:
-It is of value when the exposure is rare.
-Can examine multiple effects of single exposure .
-It estimates :
Incidence of disease among exposed & non exposed.
Relative & attributable risk.
Dose response relationship .
-It allows testing the hypothesis.
- No selection bias since the exposure is assessed prior to
the occurrence of the disease, the outcomes could not
influence the selection of the exposure.

III) The limitations of the prospective cohort study:
 Not suitable for studying rare diseases.
 Loss of experienced staff, loss of funds.
 Change in the environmental factors.
 Change in standard diagnostic methods or diagnostic
criteria of diseases.
 The study itself may alter the participants behavior.
 Attrition problem:Drop-outs.
Ethical problems.
Expensive.
Time consuming (20-30 years in cancer studies).

Cohort studies
Advantages
1-Time sequence of
Relationship between
exposure & disease
can be established
2-Suitable for rare
exposures.
3-Can calculate
incidence.
4-Selective & Recall
bias are absent.
Disadvantages
1-Expensive & time
consuming
2-Not feasible for rare
diseases
3-Drop-outs.

Example:
A group of individuals are classified according to exposure
to sunrays into exposed (n= 400) and not exposed (n= 400).
The two groups are similar in all other aspects as age, sex,
and social class. They are followed up for ten-year period.
Among those exposed, 40 BCCs are detected and among
those not exposed, 4 cases of BCC were detected.
1-Mention the type of the epidemiologic study.
2- Draw the flow the flow chart
3- Tabulate the data.
4- Calculate the risk of exposure to sunrays.

Persons developed
BCC )n= 40(
Persons didn’t develop
BCC )n= 360(
Persons developed
BCC )n=4(
Persons don't develop
BCC )n=396(
Persons exposed to
Sunrays
n=400
Persons not exposed to
Sunrays
n=400
Present Future
Direction of the study
The flow Chart:

Tabulation of data:
Distribution of the cohort groups (exposed & not exposed to
sun rays) according to the detected BCC after a 10 years
follow up period.
Exposure
to sunrays
Persons
with
BCC
Persons
without BCC Total
Yes
)Ee( 40 360 400
No

Calculation of the rate of occurrence of BCC:
The incidence of BCC among exposed = 40 X100 = 10%
)Ie( 400
The incidence of BCC among not exposed = 4 X100 = 1%
)I0( 400
Calculation of Risk:
1- Relative Risk )RR(.
2-Attributable risk percent )ARP(.

Estimation of risks:
1- The relative risk: (Risk Ratio) (RR)
It is the ratio of the incidence among exposed to that of none
exposed.
RR = Incidence among exposed = (Ie) = 10 = 10
Incidence among none-exposed (I0) 1

Measure of the strength of
association between the suspected
cause & the effects based on
prospective studies )cohort
studies(.

In the previous example:
RR=10 indicates that those exposed to sunrays
are 10 times at greater risk to develop BCC
than those not exposed to sunrays.

Amount of disease that can be
attributed to a certain exposure.

2- Attributable risk percent (ARP):
ARP = ( Ie - I0 )X100
(Ie)
ARP in previous example= (10 -1)X100 = 90%
10
This indicates that 90% of the BCC is attributed to
exposure to sunrays i.e. 90 % of BCC could be
prevented if persons avoid exposure to sunrays.

Rates in cohort study
Cigarette smoking +ve
lung cancer
-ve
lung cancer
Total
Yes 70 6930 7000
No 3 2997 3000
Incidence rate of lung cancer among exposed (smokers) =
70/7000 = 10 per thousand
Incidence rate of lung cancer among non exposed (non
smokers) = 3/3000 = 1 per thousand

Estimation of risk
Relative Risk )Risk ratio(: Ratio of the incidence of the
disease among exposed to the incidence of disease among
non exposed
RR = IR among exposed/ IR among non exposed
= 10/1 =10
RR for development of lung cancer = 10
This indicates that the risk of developing lung cancer is 10
times higher in exposed compared to non exposed group.

Cohort Study
(Prospective Design(
Passive smoking & respiratory infections in children
Is passive exposure to tobacco smoke associated with
increased respiratory infections in children ?
Design:
Children exposed and not exposed tobacco smoke in their
homes Follow them in time for disease
occurrence.

Children
)>12yrs(
1000
Family smoker
500 children
Exposed
Family non-
smoker
500 children
Not exposed
1year
Diseased
300
Not diseased
200
Diseased
120
Not diseased
380
OutcomeStart

Rate: Incidence rate
•Incidence of Resp. Infection among exposed
children: 300
500 = 60%
•Incidence of Resp. Infect. Among non exposed
children: 120
500 = 24%

Cohort Study (cont.(
Relative Risk: Incidence rate among exposed
Risk Ratio Incidence rate in non exposed.
60
24 = 2.5
Relative Risk is a direct measure of risk (to assess the etiologic
role of a factor in disease occurrence).
300 x 500
500 120

Cohort Study (cont.(
Relative Risk:
Smoking
- Lung Cancer mortality: RR=18.57
- Myocardial infarction mortality: RR=1.35
It measures the strength of
association

Examples from the literature
Framingham Heart Study
initiated in 1948 by US Public Health Services: to study the
relationship of a variety of factors to the subsequent
development of heart disease
Group of persons
30 – 62yrs
6,500
Both sexes
20years follow up
Information:
S. cholest.level
Bl.pressure , weight
Cig. Smoking
outcome

Occupation Based Studies to study
effect of exposures
•Benzene workers & Leukemia
• Coke-oven workers & lung cancer
•Asbestos workers & lung cancer
•Radium dial painters & oral cancer

OBJECTIVE: To identify risk factors for breast cancer
among female survivors of childhood cancer.
Exposure: Survivors of childhood cancer are at risk for
secondary breast cancer.
DESIGN: Retrospective cohort study.
SETTING: The Childhood Cancer Survivor Study (CCSS), a
multicenter study of persons who survived more than 5 years
after childhood cancer diagnosed from 1970 to 1986.

PARTICIPANTS: Among 6068 women in the CCSS, 95
women had 111 confirmed cases of breast cancer.
MEASUREMENTS: Standardized incidence ratios for breast
cancer were calculated by using age-specific incidence rates
in the general population.
Breast cancer incidence was evaluated with respect to
primary cancer diagnosis and therapy, age at and time since
primary diagnosis, menstrual and reproductive history, and
family history of cancer.

RESULTS:
Breast cancer risk was increased in survivors who
were treated with chest radiation therapy
(standardized incidence ratio, 24.7 [95% CI, 19.3 to
31.0]) and survivors of bone and soft-tissue sarcoma
who were not treated with chest radiation therapy
(standardized incidence ratios, 6.7 and 7.6,
respectively).

Survivors of childhood sarcomas and those who
received chest radiation therapy are at risk for
secondary breast cancer. When assessing a survivor's
risk, clinicians should consider primary diagnosis,
previous radiation therapy, family cancer history, and
history of thyroid disease.

Case-control or Cohort.
How to choose?
When the outcome is rare  start with it.
So case-control study.
Search for possible incriminated exposures
retrospectively
When the exposure is rare  start with it.
So cohort study.
Follow them up compared with those unexposed
 When the exposure is new  follow it up.

CanCer lung
&Smoking
Case-control Cohort
- One group already have
ca.lung “cases”
- 2nd
healthy group “controls”
- Comparing smoking status
“smoker or not & duration of
smoking in past history of
both groups”
- Start by a cohort selected
from population living in a
locality.
- Individuals in this cohort
divided into exposed
“smoker” & non-exposed
“non-smoker”
- Then these 2 groups followed
for some period of time to
find out who among both
groups will develop ca.lung.

B. Experimental )Intervention( studies:
)Proving cause-effect relationship(
Active trial to change disease determinant
by the investigator who allocates the
exposure & follows the subjects.
Can be viewed as a type of prospective
cohort study.

Ethical points must be considered:
it should have beneficial effect to patients,
not to harm anyone by intervention
 participants should know what the
experiment is and have the right to refuse
 if any unplanned complications occur to
any participant he should be excluded from
the trial and treated.

Types of experimental studies:
a( Clinical trials:
It is usually used to assess efficacy of a new line of ttt (a
new drug for example) or to compare 2 types of ttts:
surgical or medical.
Diseased subjects are randomly allocated into 2 groups,
"ttt” group (who are given the new drug) and "control
group" (who are given the usual ttt or no ttt in placebo).
Results are assessed by comparing health improvement
of the 2 groups at end of trial.
Example: surgical or medical treatment of peptic ulcer

EXPERIMENTAL STUDY
Random Allocation ?
Yes No
Randomized Non-Randomized
Controlled trial Controlled trial
)RCT(

Randomization: assigned to ttt & control group.
Matching: matched pair design to arrange ttt &
control groups similar for the main variables such
as age, sex. Matching determine data analysis.
Cross–over design: In a clinical trial of short term
benefits it may be appropriate to use participants as
their self-controls.
Single & double–blind designs: single blind when
the participants don’t know the preparation while in
double blind method, both investigator &
participants do not know, only (designer) knows.
“Triple blind: subjects & investigators &
statisticians”

b( Community trials:
Involve people who are not diseased (but presumed likely
to be at risk) and the sample is drawn from the community.
Data collection takes place in the field.
 For example: in studies carried out to assess the efficacy
of new vaccines. The participants are divided into 2
groups: one who is the experimental group (will take the
new vaccine) and the 2nd
is the control group (will not take
the vaccine).
The participant will be followed to compare the level of
occurrence of the disease in both groups. Therefore, these
groups should be alike as much as possible in all aspects
other than ttt /intervention received.

Hierarchy of major study designs
Systematic review of RCTs
RCT
Cohort
Case control
Cross sectional
Interventional
Observational
Validity

Exercise1:-
Description of 35 patients with thyroid cancer
are regarding past history of exposure to
radiation and response to surgical treatment
Feedback:-
Case series

Exercise2:-
Patients admitted for uterine prolapse were age
and social class-matched with fellow patients
without prolapse and surveyed as to chronic
constipation history to assess the possible
association of chronic constipation and
uterine prolapse.
Feedback 2:-
Case-control study

Exercise3:-
A 39-year old man who presents with mild sore
throat, fever, malaise and headache was treated
with penicillin for presumed streptococcal
infection.
He returned after a week with hypotension, fever and
abdominal pain .
A diagnosis of Rocky Mountain spotted fever was
made and he responded good to chloramphenicol.
Feedback 3:-
Case report

Exercise4:-
A total of 298 who have minor operations during
March 1980 in one hospital, half of them are
known and recorded to be exposed to hepatitis B
contaminated vials discovered and half of them to
vials free of this pollution are followed up starting
from July 2000 till 2010 to diagnose liver cancer.
Feedback 4:-
Retrospective cohort

Exercise 5:-
500 patients were classified according to their
body mass index (obese or not) and
simultaneously according to having knee
osteoarthrosis
Feedback of Exercise5:-
Cross sectional study

Exercise 6:-
47 men between 40 and 64years of age who had
major ECG abnormalities at initial examination
and 144 men of the same age group with no ECG
abnormalities were followed up for 20 years and
deaths from CHD were recorded.
Feedback of Exercise 6:-
Prospective cohort
study

Exercise 7:-
An oncologist determined that 75 out of 100
randomly selected leukemia patients had
experienced exposure to ionizing radiation while
60 out of 100 randomly selected healthy
individuals who did not differ from patient with
respect to age or sex had experienced exposure to
ionizing radiation .
Feedback of Exercise7:-
Case-control study

Exercise 8:-
In one of two capital cities of two adjacent
governorates, health education & strict application
of helmets use for motorcycle drivers were done
& in the other city no application of such
awareness or law & then the incidence of head
injury among motorcycle drivers was found for a
year
Feedback 8:-
Community trial

Exercise 9:-
A team of clinical researchers decide to investigate
if ovarian cancer responds better to Taxol than to
conventional chemotherapy. They choose suitable
patients & randomize to Taxol & control groups
(subjects are alike, apart from the exposure to
which therapy).
The researchers measure % of tumors responding in
both groups blindly.

In this study we started with patients and
randomize to study & control group to test
an exposure (therapeutic modality) which is
assigned by researchers.
Hence, this is a
Randomized controlled trial )RCT(
)an experimental= interventional study(

Exercise 10:-
Framingham study is a large scale study that was
initiated in 1949 to investigate putative risk factors
for coronary heart disease (CHD). Study
participants underwent a complete physical
examination at beginning of study & every 2 years
thereafter
What is the type of this study?
Prospective cohort study

Epidemiological studies

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