Measuring Association

MEASURING ASSOCIATION
PCAARRD Training, 23-27 May 2016, Central Luzon State University
Dr. Romeo S. Gundran
Professor
College of Veterinary Science and Medicine
Central Luzon State University

Topic Outline
1. Determinants of disease
2. The concept of risk and risk factors
3. The 2x2 Table
4. Investigation of risk factors in
epidemiological studies
5. Calculation and interpretation of Relative
Risk (RR) and Odds Ratio (OR)

EPIDEMIOLOGY
FREQUENCY DISTRIBUTION DETERMINANTS
Morbidity Rate
• Prevalence
• Incidence
Mortality Rate
• Crude Mortality, Case-fatality
• Cause-specific, Proportionate
Animal
Spatial
Temporal
Risk Factors
Frequency, distribution, and
determinants of health and disease in
populations (Martin et al, 1987)

Disease Determinant
• A determinant is any factor or variable that
can affect the frequency with which a
disease occurs in a population.
• Synonym: Risk factor

Measuring Association
Field Epidemiology Training Program for Veterinarians, 13-17 July 2015, Bangkok, Thailand

Agent, Host, Environment
AGENT
• Infectivity
• Pathogenicity
• Virulence
• Antigenic
variation
• Host range
• Reservoir
HOST
• Age, Species
• Breed, Sex
• Body Size
• Behavior
• Coat color
• Use
• Nutritional
status
• Existing
infections
ENVIRONMENT
• Climate
• Housing
• Water
• Location
• Pollution
• Overcrowding
• Vectors
• Floods
• Husbandry
practices

Necessary and Sufficient Causes
• Necessary cause: must be present for a
disease to occur
• Sufficient cause: a set of minimal conditions
and events inevitably producing disease
where all exposed individuals inevitably
become cases.

Sufficient cause and component causes
A sufficient cause
comprises a range
of component
causes which
makes the disease
multifactorial.

Component causes include factors that can be
classified as follows:
1. Predisposing factors
• increase the level of susceptibility, e.g. age, sex,
immune status, previous illness, environment
2. Enabling factors
• facilitate manifestation of a disease
e.g. housing, nutrition, climate
3. Precipitating factors
• associated with the definitive onset of disease
e.g. toxic and infectious agents, physical trauma
4. Reinforcing factors
• aggravate the presence of a disease
e.g. repeated exposure to an infectious
agent in the absence of immunity
Risk
Factor

RISK FACTOR
• An attribute or exposure that increases the
probability of occurrence of disease
• Any factor which on the basis of
epidemiologic evidence is known to be
associated with health-related condition(s)
considered important to prevent.

Why identify Risk Factors
• If we can identify factors that are associated
with (or causing) disease then we can help plan
a logical intervention or control measures.
• Managing the risk means managing the
disease
Surveillance Identify Risk
Factors
Intervention
Measures
Surveillance Identify Risk
Factors
Intervention
Measures

Objectives of Surveillance
1. Describe the level or distribution of disease
or risk factors for disease
2. Assess the progress of disease control or
eradication programs
3. Detect the incursion of new, emerging or exotic
diseases or pathogens, or their risk factors
4. Demonstrate freedom from disease or
pathogens.

Parts of a Questionnaire
1. Identifying Information
2. Farm Information
3. Clinical Information
4. Risk Factor Information
5. Source of the Information

Example of Risk Factor Information
1. Proximity to Other Farms
2. Proximity to Slaughterhouses
3. Selling of Animals to Middlemen
4. Sources of Stocks
5. Feed Source
6. Vaccination History
7. People Entering the Farm
8. Biosecurity Measures
9. Farmer’s Knowledge, Attitude and Practices

Can you Identify the Exposures and Outcomes
• PRRS
– are pigs vaccinated with live virus vaccine-Porcilis
likely to have active PRRSV infection compared
with pigs that received other vaccine?
• Hog Cholera
– are pigs on swill feeding more likely to get Hog
Cholera compared with pigs on commercial feeds?
• Surra
– are native buffaloes less likely to develop Surra
compared with purebred?

Can you identify the Exposures and Outcomes
• PRRS
• Hog Cholera
• Surra
Exposures

Can you identify the Exposures and Outcomes
• PRRS
• Hog Cholera
• Surra
Outcomes

Epidemiological Studies to Identify Causal Association

Cross-Sectional Study
Prevalence
Prevalence Ratio
Prevalence Difference
Odds Ratio
Cohort Study
Incidence Risk
Incidence Rate
Risk Ratio, Rate Ratio
Odds Ratio
Case-Control Study
Odds Ratio
Measures Used in Various Epidemiologic Studies

From association to cause-effect relationship
The primary objective in
epidemiology is to
evaluate whether an
association between
exposure and disease is,
in fact, causal.

2 x 2 array representing disease status and the
presence or absence of risk factors
Diseased (D+) No Disease (D-)
Risk Factor
Present (F+)
Diseased with
Risk Factor
No Disease
with Risk Factor
Total with Risk
Factor
Risk Factor
Absent (F-)
Diseased
without Risk
Factor
No Disease
without Risk
Factor
Total without
Risk Factor
Total Diseased Total Non-
Diseased
Total All
Animals
The 2x2 Table

Relative Risk
• Relative Risk (RR; risk ratio, cumulative
incidence ratio) is used if the following
question is asked:
• How many times more (or less) likely are
exposed individuals to get the disease
relative to nonexposed individuals?

• It is calculated as the ratio of
incidence risk between exposed
and non-exposed individuals.
• RR = Ie/Iu
Calculation of Relative Risk

INTERPRETATION
• The disease is RR times more likely to
occur among those exposed to the
suspected risk factor than among those
with no such exposure.
Interpretation of Relative Risk

• RR = 1: no association exists between occurrence of
disease and exposure to the factor
• RR > 1: the exposure is positively associated to the
disease (risk-factor)
• RR < 1: the exposure is negatively associated to the
disease (preventive/sparing/protective factor)
Interpretation of Relative Risk

• The odds ratio (OR; Crossproduct
Ratio) is calculated as the ratio
between the odds of disease in
exposed individuals and the odds of
disease in non-exposed individuals.
Odds Ratio

• OR = 1 no association exists between
occurrence of disease and exposure to the
factor
• OR > 1 the factor is positively associated with
the disease (risk-factor)
• OR < 1 the factor is negatively associated with
the disease (preventive/sparing/protective
factor)
Interpretation of Odds Ratio

What is the incidence in the chickens that ate the
suspect feed?
What is the incidence in the chickens that did not eat
the suspect feed?
What is the relative risk for the chickens that ate the
suspect feed?
Interpretation: The risk of becoming ill was 36 times
higher for those chickens that ate the suspect feed than
for those that did not.

PRRS+ PRRS-
With Mycotoxin
in Feed
100 11 111
Without
Mycotoxin in
Feed
30 57 87
130 68 198
RR = (a/a+b) / (c/c+d)
OR = (a*d) / (b*c)
a b
c d
Risk Factor Identification in PRRS Surveillance

PRRS+ PRRS-
With Mycotoxin
in Feed
100 11 111
Without
Mycotoxin in
Feed
30 57 87
130 68 198
OR = (a*d) / (b*c)
a b
c d
OR = (100* 57) / (11*30)
= 17.27

OR = (a*d) / (b*c)
OR = (100* 57) / (11*30)
= 17.27

PRRS+ PRRS-
With Mycotoxin
in Feed
50 61 111
Without
Mycotoxin in
Feed
30 57 87
130 68 198
OR = (a*d) / (b*c)
a b
c d

Field Epidemiology Training Program for Veterinarians, 13-17 July 2015, Bangkok, Thailand
Is there a significant
association between
mycotoxin in feed
and PRRS?

PRRS + PRRS - Total
Vaccinated 29 11
Unvaccinated 17 6
Total
• Calculate the OR
• OR = (a*d) / (b*c)
Sparing/Protective/Preventive Factor

PRRS + PRRS - Total
Vaccinated 29 11 40
Unvaccinated 17 6 23
Total 46 17 63
• Calculate the RR and OR
OR = (29* 6) / (11* 17)
= 0.93
Is vaccination a preventive factor in this case?

Province Positives N Seroprevalence OR
Batangas 221 500 44.20 3.333
Bulacan 515 750 68.67 9.222
Cavite 201 418 48.09 3.898
Laguna 205 426 48.12 3.904
Nueva Ecija 169 332 50.90 4.363
Pampanga 147 210 70.00 9.819
Pangasinan 96 500 19.20 1
Quezon 46 104 44.23 3.338
Rizal 248 423 58.63 5.964
Tarlac 151 187 80.75 17.650
1999 3850 51.92
Seroprevalence and Odds Ratio for PRRS in 10 Provinces

PRRS+ PRRS- N
Pampanga 147 a b 63 210
Pangasinan 96 c d 404 500
OR = ad/bc = 59,388 = 9.819
6,048
2x2 table showing the computation of
Odds Ratio of PRRS in Pampanga using
Pangasinan as reference

Tools to Use: 2x2 Table Using Excel
• Calculations using Excel
• Ready to use – just enter
data in the highlighted
area

Tools to Use: Epi Tools (Online Software)
PCAARRD Training, 4-8 May 2015, Central Luzon State UniversityPCAARRD Training, 23-27 May 2016, Central Luzon State University

Tools to Use: Epi-Info Statcalc Function

Tools to Use: Epi-Info Logistic Regression Function

52
Thank You
for Your
Attention!

Measuring Association

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