Lecture measure of effect and disase causation

Somali International University(SIU)
Faculty Of Health Sciences
Department Of Public Health
Introduction to Basic Epidemiology(1)
Lecture 3 Measures of effect in epidemiology
Semester 3
Dr.Ahmed Ayna,MS.c PTH-UMST-Sudan
5/10/2021 1

Objectives
1. Define and calculate ratio measures of effect
2.Define and calculate difference measures of
effect
3.Explain how to select the proper measure of
effect
4.To define the assumptions behind the use of
these measures

Measures of effect
• If we suspect that a particular factor
may be causing a specific outcome, we
first look for an association between the
factor and the outcome

Examples
• A researcher wanted to study the influence of
exposure to organo-phospate chemicals on the
development of asthma in adults.
• First he measured the incidence risk of developing
asthma over a five-year period in a group of people
who work with organo-phosphates on a regular basis.
• Then he compared this data with the incidence risk
in a group of people not exposed to organo-
phosphates.

Examples
• A researcher wanted to investigate
whether coronary heart disease was more
common in men than in women. The
exposure of interest was 'male sex'.
• He compared the incidence rate of
coronary heart disease in a group of men
with the incidence rate in a group of
women.

• Incidence risk ratio
• Incidence rate ratio
• Odds ratio
• Prevalence ratio
Ratio
measures
• Risk difference
• Rate difference
Difference
measures
Measures of effect

Risk Ratio
• The risk ratio (RR) compares
the incidence risk of an outcome in two
populations.
• It can be used in health service planning
to compare the need for services in two
different populations.

Risk Ratio
• It is often used in analytical studies to
compare:
– the incidence risk of an outcome in a
population exposed to a suspected risk factor
with:
– the incidence risk of the outcome in a
population not exposed to the factor.

Risk Ratio (continued)
• RR = incidence risk in exposed population
incidence risk in unexposed population
• The risk ratio estimates the magnitude of the
effect of the exposure on the incidence.
• It is a measure of the strength of the
association between the exposure and the
outcome.

Measures of effect continued
Example 1:
• Incidence risk (per 100 000 per year) of malnutrition in
children in low-income families = 87.9 & high-income
families=23.2
Risk Ratio = 87.9 per 100 000 per year / 23.2 per 100 000
per year = 3.8
So, in this study the risk of malnutrition in children was
more than three and a half times higher in the low-income
families studied than in the high-income families.

Rate Ratio
• The rate ratio compares the incidence rate of
an outcome in two groups, one exposed to the
factor under study and one not exposed.
• It is the only measure that takes into account
the person-time at risk in each exposure group.

Rate Ratio (continued)
• It is the preferred measure in analytical
studies of common outcomes and in populations
with a large number of people entering or
leaving the different exposure groups.
• RR = incidence rate in exposed population
incidence rate in unexposed population

Measures of effect continued
• Example 2: The measured incidence rates (per 100
person-years) of tuberculosis for mineworkers = 3 & for
the unexposed group = 0.6
Rate Ratio =3 per 100 person-years / 0.6 per 100
person-years = 5
So in this study, the incidence rate of tuberculosis
was five times higher in the mineworkers than in the
unexposed group.

Odds Ratio
• The odds ratio of an outcome compares
the odds of having a particular outcome in
a population exposed to a suspected risk
factor with the odds in a population not
exposed.
• OR = odds of outcome in exposed individuals
odds of outcome in unexposed individuals

• Example: A researcher was investigating the influence of
parental smoking on the development of glue ear in childhood.
• Odds of developing glue ear in children with parent who
smokes = 0.39
• Odds of developing glue ear in children without parent who
smokes = 0.21
• Odds ratio of developing glue ear =0.39 / 0.21 = 1.86
• In this study, the children with a parent who smokes had
nearly twice the odds of developing glue ear as the children
whose parents do not smoke.

The prevalence ratio
• The prevalence ratio is sometimes used in health-
service planning to compare the burden of a condition
in different groups.
• It is neither an estimate of the magnitude of the
effect of the exposure on the prevalence nor a
measure of the strength of the association between
the exposure and the outcome.

The prevalence ratio continued
• There are a number of factors that can influence
the prevalence of a condition in any population so
we must interpret a prevalence ratio with caution
• The prevalence ratio is rarely used.
• PR = prevalence in population A
prevalence in population B

• Example:
The prevalence of HIV in population A
was 0.2% and in population B was 0.1%.
• Prevalence ratio =0.2 / 0.1 = 2
• The prevalence of HIV infection in
population A is twice as high as the
prevalence in population B.

Interpretation of the effect measures
Is there an association?
If the relative risk
is 1
The risk in exposed and
unexposed persons is the
same.
No evidence of an
association between the
exposure and the outcome.
If a relative risk is
greater than 1
The risk in exposed persons is
greater than the risk in
unexposed persons.
This is evidence of a positive
association between the exposure
and the outcome.
The exposure is a risk factor for
the outcome.
The higher the number is above
1, the stronger the positive
association.
If a relative risk is
less than 1
The risk in exposed persons is
less than the risk in unexposed
persons.
This is evidence of a negative
association between the exposure
and the outcome.
The exposure is a protective factor
for the outcome.
The lower the number is below
1, the stronger the negative
association.

What does this association mean?
Finding an association between an
exposure and an outcome is only the
first step in an analytical study. An
association between an exposure and an
outcome does not necessarily mean that
the exposure causes the outcome.

Explanation of the observed association
between exposure and outcome
Bias
Confounders
Random error
True association

Assignment
• Please, classify between these terms:-
• Bias
• Confounders
• Random error

• Lecture 4 concepts Disease Causation

Objectives
• Explain concepts of disease causation
• Define and differentiate
– Association vs. causation
– Necessary vs. sufficient cause
– Direct vs. indirect cause
• Describe the different models of disease
causation (triangle, wheel, lever)
• Define cause of disease
• Discuss the different risk factors for disease

Concept of Cause
• Event/condition/characteristic that plays a
role in producing occurrence of a disease
Disease
Factor

• The causes of disease can be classified in to
two:
1. Primary causes – these are the factors which
are necessary for a disease to occur, in whose
absence the disease will not occur. The
term”Etiologic Agent” can be used instead of
primary cause for Infectious causes of
diseases. For example “Mycobacterium
tuberculosis” is the primary cause (etiologic
agent) of pulmonary tuberculosis.

• 2. Risk factors (contributing, predisposing, or
aggravating factors):-These are not the
necessary causes of disease but they are
important for a disease to occur. A factor
associated with an increased occurrence of a
disease is risk factor for the exposed group; and
a factor associated with a decreased
occurrence of a disease is a risk factor for the
non exposed group. Risk factors could be
related to the agent, the host and the
environment.

• The etiology of a disease is the sum total of all
the factors (primary causes and risk factors)
which contribute to the occurrence of the
disease.
• It is the interaction of the agent, the host, and
the environment which determines whether or
not a disease develops, and this can be
illustrated using the epidemiologic triangle.

Association Vs. Causation
• Association – identifiable relationship
between an exposure and disease
 Association does not always indicate that there is cause-effect relationship
• Causation – presence of a mechanism that
leads from exposure to disease

Statistical Association
• Statistical dependence between two variables,
that is, the degree to which the rate of disease
in persons with a specific exposure is either
higher or lower than the rate of disease
among those without that exposure.
Exposed: higher rate of disease OR Exposed: lower rate of
disease
Unexposed: lower rate of disease Unexposed: higher rate of disease

• Epidemiology does not determine the cause
of a disease in a given individual. Instead, it
determines the association between a given
exposure and frequency of disease in
population.
 Studies are undertaken to demonstrate a link (association) between F and
D.
• We infer causation based upon the association
and several other factors

• The epidemiologic triangle:-The epidemiologic
triangle, depicts the relationship among three
key factors in the occurrence of disease or
injury: agent, environment, and host. An
agent is a factor whose presence or absence,
excess or deficit is necessary for a particular
disease or injury to occur.
• The environment includes all external factors,
other than the agent, that can influence health.

• These factors are further categorized
according to whether they belong in the
social, physical, or biological environments.
• The social environment encompasses a broad
range of factors, including education,
unemployment, culture regarding diet; and
many other factors about to political, legal,
economic, communications, transportation,
and health care systems.

• Physical environmental factors are factors like
climate, land, and pollution.
• Biological environmental influences include
vectors, humans and plants serving as
reservoirs of infection. From the perspective of
epidemiologic triangle, the Host, Agent, and
Environment can coexist harmoniously.
Disease and injury occur only when there is
altered equilibrium between them.

Causal Vs. Non-Causal Association
• Causal – association between categories of
events/characteristics (F and D) in which an
alteration in the frequency or quality of one
category is followed by a change in the other
• Non-causal – association of both categories of
events (F and D) with a third category

A
C
B
A = mother’s age
B = low birth
weight
C = father’s age

A
C
B
If: Alter A, expected change in B
Alter C, no expected change
in B

Direct Vs. Indirect Cause
Direct Cause: immediate
A B
Tubercle bacilli TB
Indirect Cause (intermediate stage) predisposing cause
A V
Smoking asbestosis
B
LC

Factor and Disease
association
causal non-causal
indirect
no association
direct
F ---- D
F D
F  V  D
F  D
F D
V
F
D

Henle-Koch’s Postulates (1882)
(postulates for determining than an infectious agent is the cause
of a disease)
• The organism must be present in every case of
the disease
• The organism must be able to be isolated and
grown in pure culture
• The organism must, when inoculated into a
susceptible animal, cause the specific disease
• The organism must then be recovered from
the animal and identified

Koch’s Postulates (assumption)
• A particular disease has one cause
• A particular cause results in one disease
– BUT not true for many diseases (Ex. Smoking
causes many diseases; CAD has multiple causes)
• Cause cannot be established by means of
Koch’s postulates since many factors act
together to cause diseases

Knowledge of Cause
• Develop intervention methods
• Formulate prevention strategies

Determination of Cause
• Biomedical scientists – elucidate pathogenic
causes/mechanisms (cellular/subcellular
processes)
• Epidemiologists – investigate other less
specific factors (env, behavioral, etc.) --- risk
factors that are associated with an increased
risk of becoming diseased

Cause
• Risk factor
• Preventive factor
F D
F D
(+)
(-)

Knowledge of Risk Factors
(even in the absence of known pathogenic cause
• Effective Rx and Preventions
– Epi research provided info that has formed the
basis for public health decisions long before the
mechanism of a particular disease was
understood
• Examples:
– Cig smoke – lung cancer (health education)
– Use of PPE – Asbestosis-LC
– Washing of hands – infections (OB should wash
their hands)

Necessary Vs. Sufficient Cause
• Necessary cause
– A cause is termed necessary if a disease cannot
develop in its absence
– If A is a necessary cause of B, then B will only
occur if preceded by A (absolutely necessary)
– A necessary condition is one that must be satisfied
for the result to happen

• The presence of A does not ensure that B will
occur, but the presence of B ensures that A
must have occurred (all cases are exposed)
• If A is absent, B cannot occur

• Sufficient Cause (satisfying condition)
– A cause is termed sufficient when it inevitably
produces or initiates a disease
– If A is a sufficient cause of B, the presence of A
guarantees B. However, other events may also
cause B, and thus B’s presence does not ensure
the presence of A
– If A is present, B will always occur

Cause Effect
Necessary: 18 years old Driver’s license
Necessary cause: tubercle
bacilli
Tuberculosis
Sufficient cause: jumping Feet off the ground
Sufficient cause: vaccination Immunity
Sufficient cause: ____________ ______________________

• Necessary: sexual intercourse (doesn’t guarantees
preg) effect: pregnancy (result of intercourse)
Sufficient cause: absenteeism cause: failing grades
(failing grades maybe caused by a poor performance)

• Sufficient Cause (satisfying condition)
– A cause is termed sufficient when it inevitably
produces or initiates a disease
– If A is a sufficient cause of B, the presence of A
guarantees B. However, other events may also cause
B, and thus B’s presence does not ensure the presence
of A
– If A is present, B will always occur
– A set of minimum conditions that produce disease

Confounding
• A third factor which is related to both
exposure and outcome, and which accounts
for some/all of the observed relationship
between the two
• Confounder not a result of the exposure
– e.g., association between child’s birth rank
(exposure) and Down syndrome (outcome);
mother’s age a confounder?
– e.g., association between mother’s age (exposure)
and Down syndrome (outcome); birth rank a
confounder?

Confounding
• To be a confounding factor, two conditions
must be met:
Exposure of interest
(Drinking of coffee)
Confounder
(Smoking)
Outcome of interest
(Cancer of the pancreas)

Birth Order Down Syndrome
Maternal Age
Confounding
Maternal age is correlated with birth order and a risk factor
even if birth order is low

Birth Order
Down Syndrome
Maternal Age
Confounding ?
Birth order is correlated with maternal age
but not a risk factor in younger mothers

Levels of Prevention
• Natural history of disease
• The “natural history of disease” refers to the
progression of disease process in an individual
over time, in the absence of intervention.
• There are four stages in the natural history of a
disease. These are:
1. Stage of susceptibility
2. Stage of pre-symptomatic (sub-clinical) disease
3. Stage of clinical disease
4. Stage of disability or death

1. Stage of susceptibility:-In this stage, disease
has not yet developed, but the groundwork has
been laid by the presence of factors that favor
its occurrence. Example: unvaccinated child is
susceptible to measles.
2. Stage of Pre-symptomatic (sub-clinical)
disease:-In this stage there are no
manifestations of the disease but pathologic
changes (damages) have started to occur in the
body. The disease can only be detected
through special tests since the signs and
symptoms of the disease are not present.

• Examples:
• Detection of antibodies against HIV in an
apparently healthy person.
• Ova of intestinal parasite in the stool of apparently
healthy children.
• The pre-symptomatic (sub-clinical) stage may lead
to the clinical stage, or may sometimes end in
recovery without development of any signs or
symptoms.
• The Clinical stage:-At this stage the person has
developed signs and symptoms of the disease. The
clinical stage of different diseases differs in
duration, severity and outcome. The outcomes of
this stage may be recovery, disability or death.

• Examples:
• Common cold has a short and mild clinical stage
and almost everyone recovers quickly.
• Polio has a severe clinical stage and many
patients develop paralysis becoming disabled
for the rest of their lives.
• Rabies has a relatively short but severe clinical
stage and almost always results in death.
• Diabetes Mellitus has a relatively longer clinical
stage and eventually results in death if the
patient is not properly treated.

• Stage of disability or death:-Some diseases run
their course and then resolve completely either
spontaneously or by treatment. In others the
disease may result in a residual defect, leaving the
person disabled for a short or longer duration.
Still, other diseases will end in death.
• Disability is limitation of a person's activities
including his role as a parent, wage earner, etc
• Examples:
• Trachoma may cause blindness
• Meningitis may result in blindness or deafness.
Meningitis may also result in death.

• Levels of Disease Prevention
• The major purpose in investigating the
epidemiology of diseases is to learn how to
prevent and control them. Disease prevention
means to interrupt or slow the progression of
disease. Epidemiology plays acentral role in
disease prevention by identifying those
modifiable causes.

• There are three levels of prevention
• Primary Prevention for Health Promotion:
• Health Education
• Adequate Nutrition
• Individual Development
• Adequate working environment and recreation,
resting
• Regular Physical Examination
• Comply with immunization
• Individual hygiene
• Improve environment sanitary
•

• Occupational Safety
• Prevent Accident in all ages
• Adequate nutrition
• Avoid cancer cause agents
• Avoid allergic

• Primary prevention:-The main objectives of
primary prevention are promoting health,
preventing exposure and preventing disease.
Primary prevention keeps the disease process
from becoming established by eliminating
causes of disease or increasing resistance to
disease.

• Primary prevention has THREE components.
These are: Health promotion, prevention of
exposure, and prevention of disease.
• A. Health promotion:- consists of general non-
specific interventions that enhance health and
the body's ability to resist disease.
Improvement of socioeconomic status,
provision of adequate food, housing, clothing,
and education are examples of health
promotion.

• B. Prevention of exposure:- is the avoidance of
factors which may cause disease if an individual
is exposed to them.
• Examples can be provision of safe and adequate
water, proper excreta disposal, and vector
control.
• C. Prevention of disease:- is the prevention of
disease development after the individual has
become exposed to the disease causing factors.
Immunization is an example of prevention of
disease. Immunization acts after exposure has
taken place.

• Immunization does not prevent an infectious
organism from invading the immunized host,
but does prevent it from establishing an
infection. If we take measles vaccine, it will not
prevent the virus from entering to the body but
it prevents the development of
infection/disease.

• Secondary prevention:-
• Secondary Prevention:
• Identify cases in the community
• Screen and test
• Conduct special physical examination
• Treat and prevent the progress of illness
• Avoid the spreading of disease
• Reduce the disability time

• The objective of secondary prevention is to stop or
slow the progression of disease so as to prevent
or limit permanent damage. Secondary prevention
can be achieved through detecting people who
already have the disease as early as possible and
treat them. It is carried out before the person is
permanently damaged.
• Examples:
• • Prevention of blindness from Trachoma
• • Early detection and treatment of breast cancer
to prevent its progression to the invasive stage,
which is the severe form of the disease.

• Tertiary prevention:–
• Treat and stop the disease progress and avoid
complication and side effect
• Limit the chance for disability
• Provide rehabilitation for physical and
psychological well-being, occupational therapy,
availability of long-term care is targeted
towards people with permanent damage or
disability. Tertiary prevention is needed in
some diseases because primary and

• Secondary preventions have failed, and in others
because primary and secondary prevention are not
effective. It has two objectives:
• Treatment to prevent further disability or death and
• To limit the physical, psychological, social, and financial
impact of disability, thereby improving the quality of
life.
• This can be done through rehabilitation, which is the
retraining of the remaining functions for maximal
effectiveness.
• Example: When a person becomes blind due to vitamin
A deficiency, tertiary prevention (rehabilitation) can
help the blind or partly blind person learn to do gainful
work and be economically self supporting.

Lecture measure of effect and disase causation

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