Transcript Chapter 01

Chapter 02
Concepts and Methods in
Physical Activity Epidemiology
 Epidemiologists – Use the application of scientific
method to the study of the distribution and dynamics of
disease in a population for the purposes of identifying
factors that affect this distribution and then seek to
modify the risks.
– These risks are called risk factors. (ex. Physical Inactivity is a
risk factor for CHD)
 Risk Factors – Increase the likelihood of morbidity
(sickness) and premature mortality (death). Thus, risk
factors increase the probability of disease in a group of
individuals who have that risk factor (characteristic) vs.
those who do not.
Concepts and Methods in
Physical Activity Epidemiology
 Epidemiology has three distinct goals:
– To describe the distribution of disease
– To identify risk factor associated with the disease
– To prevent disease occurrence by modifying risk factors
 (Small Group Activity) Read information on John Snow in
text, p. 18.
– How did Dr. Snow describe the distribution of disease
– How did he identify the risk factor associated with disease
– How did he prevent the disease occurrence by modifying the risk
factor?
Concepts and Methods in
Physical Activity Epidemiology
Web Resources
 www.cdc.gov/epiinfo/ - This site describes Epi
Info, a series of programs for Microsoft Windows
developed by the Centers for Disease Control and
Prevention for use by public health professionals
in conducting outbreak investigations and
managing databases and statistics for public
health surveillance (Dean 1999; Dean et al. 1996). Click
“Download” to see further instructions for
downloading Epi Info to a personal computer, or
browse the Epi Info pages to learn more about the
programs.
Concepts and Methods in
Physical Activity Epidemiology
 Physical Activity Epidemiology
– Physical activity epidemiology studies factors associated with
participation in a specific behavior— physical activity —and how
this behavior relates to the probability of disease or injury.
– Examples of this type of study include:
– description of the level of physical activity in a population,
– comparison of levels of physical activity among populations,
– determination of factors associated with participation in physical
activity,
– and investigation of the association between physical activity
and the risk for chronic diseases such as coronary heart disease
(CHD), stroke, diabetes, osteoporosis, and cancer.
Concepts and Methods in
Physical Activity Epidemiology
 Epidemiologic Measures
 A fundamental measurement in epidemiology is the frequency with
which an event under study occurs, usually an injury, disease, or cause
of death in a population.
– Incidence: Are the numbers of new cases of disease or injury
during the time of study (period of interest). How many new
cases of “X” occurred in the population from 2000-2010? Thus,
incidence infers that these are changes in health status.
– Prevalence: Are the numbers of persisting cases during the time
of study. Prevalence of disease is a function of both incidence
and duration. Thus, prevalence of disease can increase can
increase as a result of an increase in incidence or the duration of
time disease is present before recovery or death.
 If the incidence or prevalence of a condition is known, the incidence rates
and prevalence rates can be calculated. (see next slide)
Concepts and Methods in
Physical Activity Epidemiology
 Calculation of Incidence and Prevalence Rates
– Frequency (number of events) per unit time divided by the
average size of the population at risk (average size is
determined at the midpoint time of the study)
 RATE = Number of events
Average population size
If the incident rate was 115 new cases out of 10,000 persons, the
rate would also be 11.5 per 1000, and 1.15 per 100, or 1.15%.
– Incidence rates provide a measure of the rate at which people
contract disease over a specific time period.
– Prevalence rates indicate the rate of people who were diseased
over a specific time period.
– Rates can also be assessed for risk factors ( How many “new”
persons engaged in an “unhealthy” behavior)
Concepts and Methods in
Physical Activity Epidemiology
 Prevalence rates give a “snapshot” of the population at a
given time, and may be useful in determining proper
intervention and treatment protocols.
 Incidence rates give a “snapshot” of the changes in
disease state ( increase, decrease, unchanged).
 Prevalence rates are not useful when determining
factors that may increase the probability of disease,
because high prevalence does not necessarily equal
high risk. High prevalence may be due to increased
survival rates (decreased death rates, rapid cure).
Accordingly, low prevalence rates may be due to
higher death rates.
Concepts and Methods in
Physical Activity Epidemiology
Think about it…
It is particularly important to be sure that the information you use to make
comparisons among groups is based on actual rates:
For example, a sports medicine physician reports that he has seen 100
cases of ruptured patellar tendons in runners over the past year. Does
this indicate that running is the cause of this problem and that indeed it is
a large problem that needs to be dealt with? The answer is that, with
information only on the number of cases (numerator) and no information
regarding the number of people at risk (denominator), it is impossible to
tell. To make these assessments, you need to know how many runners
visited the clinic over the course of the year. If 100 runners were seen and
100 cases of ruptured patellar tendons were diagnosed, then the
incidence would be 100%, a potentially serious problem! On the other
hand, if 1000 runners were seen, the rate would be only 10%, requiring a
completely different interpretation.
Concepts and Methods in
Physical Activity Epidemiology
 Crude, Specific, and Standardized Rates

Standardizing rates between populations allows for comparisons in the
rates of incidence and prevalence between the two populations.
– Crude Rates: Rates that are based on a total population without
consideration of any of the population characteristics.
– Specific Rates: Rates that are calculated separately for
population subgroups.
– Standardized Rates: Crude rates that are adjusted for some
population characteristic to allow valid comparisons of rates
among populations where the distribution of the characteristic
(disease) may be different. Standardized rates are crude rates
that have been adjusted to control for the effect of some
population characteristic.
TO MAKE COMPARISONS OF RATES BETWEEN TWO POPULATIONS WITH
UNEQUAL DISTRIBUTIONS OF RISK FACTORS, STANDARDIZED RATES
SHOULD BE USED.
 Example – Table 2.1, p.20. (Direct Standardization)
– Population age ranges are unequal in the eldest and youngest, where
the rate is the greatest, and least, thus effecting the crude rate
– The age categories are adjusted for number, and allows us to make
comparisons using the standardized rates in the lower table.
– These are adjusted rates are used for comparison only. “All things being
equal…the death rate in population “B” would be twice that of “A”.” (note
this conclusion is the opposite we would attain if using only crude rates)
– To make a valid comparison of the death rates in these two populations,
it is necessary to adjust death rates to account for the difference in age
distribution.
Concepts and Methods in
Physical Activity Epidemiology
A note about Adjusted Rates…
 Though standardized rates are useful for making
valid comparisons across populations, it must be
remembered that they are fictional rates. The
adjusted rates can vary, depending on the
standard population that is used in the adjustment
process. Therefore, the adjusted rate can be
misleading and should be used only for
comparison purposes.
Concepts and Methods in
Physical Activity Epidemiology
 Research Design in Epidemiology
– Design: the way that participants are grouped and compared
according to behavior or attributes (e.g., physical activity or
fitness), the health-related events being studied, time, and
factors other than physical activity or fitness that could explain
the occurrence of health-related events.
 The goal of a design in physical activity epidemiologic research is to
make sure that comparisons of groups based on differences in
physical activity or fitness are not biased by other factors.
 In other words, the research design used determines whether it is
reasonable to infer that physical inactivity was a direct, or the only,
explanation for the occurrence of an injury, disease, or death.
Concepts and Methods in
Physical Activity Epidemiology
 Research Design in Epidemiology
– Variables:
 Independent – What is being manipulated (changed) ?
 Dependent – What is being measured ?
– When change in the independent variable is manipulated by the
investigator, the design is experimental.
– When the independent and dependent variables are observed or
manipulated across a period of time, the design is longitudinal or
prospective.
– When the study looks back in time after the occurrence of injury,
disease, or death in an attempt to reconstruct an influencing
factor, such as physical activity habits, the design is
retrospective.
– Observational design is when the change in the dependent
variable occurs as a result of natural history ( self-initiated by the
people being studied).
Concepts and Methods in
Physical Activity Epidemiology
Research Design in Epidemiology
Retrospective Design: is when the study looks back in time after
the occurrence of injury or disease in an attempt to “reconstruct”
the risk factor(s) that were responsible for the injury or disease.
– Several types of research designs are commonly used in
epidemiologic research: cross-sectional surveys, case–control
studies, cohort studies, and randomized controlled trials ( See
Table 2.2, p. 21).
 The design employed in any particular study depends on the questions to be
answered, the time and financial resources available, and the availability of
data.
– The major advantages and disadvantages of the commonly used
epidemiologic study designs are summarized in Table 2.3, p.22.
Concepts and Methods in
Physical Activity Epidemiology
 Common Types of Research Design:
– The most common types of epidemiology research designs are
the cross-sectional surveys, case-control, prospective cohort,
and randomized trail studies.
– Cross Sectional Surveys
 Measure both risk factors and the presence (or absence) of disease
at the same point in time
 It is quick and easy to conduct
 Can assess risk factors and disease outcome ( prevalence )
 Cannot associate (compare) risk factors and the presence or
absence of disease (Read example in text, “Health and Religion
Project , p. 22)
 Can be useful in generating hypotheses on risk and disease
Concepts and Methods in
Physical Activity Epidemiology
– Case Control Studies
 Subjects are selected on the presence of disease of interest and
matched with controls who do not have the disease
 Appropriate for study of rare events or diseases
 Is quick and inexpensive (involves interview process that
determines risk factors, possible multiple risk factors)
 Can only study one event / disease per study
 Cannot be used to determine absolute risk and is subject to recall
bias, making temporal relationships uncertain.
 Case control studies may be used to determine whether a more
time consuming study may be needed (Cohort Study)
– Prospective Cohort Studies
 A subject group (cohort) is selected at random from a defined
population.
 Baseline risk factor analysis is conducted on the cohort and the
cohort is tracked over a period of time ( longitudinal study) for the
incidence of those exposed to the risk factor and those who are not
for the incidence of disease
Concepts and Methods in
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 Thus, prospective cohort studies are also called longitudinal studies
and/or incidence studies
 Provides an absolute measure of risk because baseline measures
were recorded; however it can only assess the factors measured at
baseline
 Allows for the study of multiple disease outcomes, and allows the
researcher to assess the changes in risk over time.
 Is expensive and time consuming, and can suffer from subject dropout and lack of follow-up.
– Randomized Control Trial (gold standard of research designs for testing a research
hypothesis)
 Participants are randomly selected and assigned to a control or
experimental group
 Can directly compare the effects of the independent variable on the
dependent variable, thus sit is considered the gold standard for
intervention evaluation
 Expensive and time consuming
 Results are often cannot be generalized to the population
 Lack of compliance and drop-outs may occur
Summary on Study Designs
 The ultimate goal of a research design is to
assess the degree to which change in an
independent variable (e.g., physical activity or fitness) is
causally associated with change in a dependent
variable (e.g., injury, disease, or death). It is important to
remember the inherent strengths and weaknesses
of the various study design options as we discuss
the evidence for physical activity in reducing the
risk of chronic disease.
Concepts and Methods in
Physical Activity Epidemiology
 Evaluating Associations in Epidemiologic Studies
– Regardless of how complex the issues under study become,
most epidemiologic research can be conceptually framed in a
standard 2 × 2 table (See table 2.4).
– Prospective Cohort Studies (Review Table 2.5 / 2.6)
 Incidence Rate = Those who developed CHD. Incidence is the
number of those individuals who developed CHD out of the entire
population in the category (sedentary or active). In Table 2.6, the
incidence of disease is equal to the number who developed CHD
out of the Sedentary, and out of the Active Groups.
– Sedentary = 400 /(400+ 5600) = 0.6666 or 6.7%
– Active = 100 / (100+3900) = 0.025 or 2.5%
 Risk Difference: Subtract the incidence rate of the non-risk group
from the at risk group (4.2%). If the risk difference is zero ( 0% ),
then the risk factor makes no difference in CHD. If the risk ratio is
greater than zero, then the risk factor is harmful. If the risk factor
was actually protective, the risk ratio is less than zero.
– Attributable Risk (a.k.a. risk difference): the estimate of the
amount of risk attributed to the risk factor. If the risk difference is
4.2%, thus 4.2% of the risk in this population is attributable to the
exposure of the risk factor of sedentary behavior.
– Relative Risk (RR): is the ratio of the risk (incidence) in the
exposed group to the risk of the unexposed group.
 RR = 0.067 / 0.025 = 2.68 (see formula, p.28, text)
 Thus, if the incidence of disease was the same in both the
sedentary and active groups, the RR would be 1.0
 In this case, the sedentary group is 2.68 times higher than the
active group
 Recall that the absolute risk for each group is 6.7% for the
sedentary, and 2.5 % for the active group
 in some cases the relative risk can be extremely high even while the
absolute risk in both groups is rather low.
Concepts and Methods in
Physical Activity Epidemiology
 Relative risks can also be used to judge the
efficacy of clinical outcomes in randomized
controlled trials.
– Example : If 20% of heart patients in a control group who get usual
care or no treatment die from a second heart attack, but the rate is
only 10% in an exercise group, the absolute risk difference between
the groups is 10%. Inverting that difference (i.e., 1.0/0.10 = 10)
yields what is known as the number needed to treat (NNT). In this
example, an additional life would be saved for every 10 patients
treated with exercise.
Concepts and Methods in
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– Odds Ratio (OR):
– compares the likelihood of an event between two groups.
– is calculated by dividing the odds of exposure to the risk factor in
the diseased group by the odds of exposure in the non-diseased
group. Thus, comparing THE CHANCES of developing CHD in
the group that was sedentary vs. active.
 OR = Disease / No Disease (a/b)/(c/d)
 OR = (ad/bc)
 Comparing if you were active and did not develop disease or
sedentary and did develop disease TO if you were active and
developed disease or sedentary and did not develop disease
(ad/bc)
Concepts and Methods in
Physical Activity Epidemiology
 Case Control Study
– A case–control study doesn’t permit computation of a relative
risk directly since the people studied are selected because they
already have a disease rather because they have been exposed
to a risk factor. In a case–control study, participants are selected
based on disease status (i.e., whether the disease is present or
absent). Table 2.7 is a 2 × 2 table illustrating the organization of
data from a case–control study.
– If exposure to the risk factor is positively related to the disease,
then the proportion of cases exposed to the risk factor should be
greater than the proportion of controls exposed to the risk factor.
– The only measure of strength of the risk of the association
between risk factor and disease is the Odds Ratio
– In most instances the ORs from well- conducted case–control
studies are reasonable estimates of the relative risk that would
have been derived from a prospective cohort study, provided that
the overall risk of disease (i.e., prevalence or incidence rates) in
the population is low (i.e., less than 5%).
 Additional Epidemiologic Research
Estimates
– Attributable Risk (AR): Determines the “impact” of the exposure
to the risk factor. It is the “estimate” of the disease burden (See
values, page 28, text)
– Formulas for determining the total risk of disease (CHD) that
results form the exposure to the risk factor (inactivity) among
those that are exposed to that risk factor:
 AR % = (Risk of those exposed to risk factor – unexposed )/
exposed
 AR% = (RR-1)/RR
 The AR% can also be calculated in a case-control study by
substituting the odds ration for the RR when AR% = (RR-1) / RR
– Population Attributable Risk: (PAR) is the percentage of the risk
of a disease that is attributable to a particular risk factor in the
entire population studied. What is the percentage of the risk of
CHD in this population is attributable to the risk factor
(inactivity). (see text, p, 28)
 Determine the total risk of the population:
– Approx 500 cases of disease / 10,000 persons
 Determine the risk of those without the risk factor
 Use the following formula:
 PAR% = (total risk – risk in those without the risk factor) / total risk
The results of the formula determine the risk for disease (CHD) in
the population that is attributable to the risk factor (inactivity)
Concepts and Methods in
Physical Activity Epidemiology
Diagnostic Test Terms
The sensitivity and specificity don’t alone determine the predictive value of
a test. The prevalence of the outcome in a population must also be
considered. Thus, a test’s predictive value depends on the frequency of
the trait that underlies the test and the prevalence of the disease, relative
to the decision point on the diagnostic test.
 Models In Epidemiology
– Epidemiology models help us to understand the causation
(Etiology), interaction, and progression of disease.
– Model Types (p.31)
 Epidemiolgic Triagle
– Simple association between the host (person) the agent (pathogen),
and the environment.
Concepts and Methods in
Physical Activity Epidemiology
 Models In Epidemiology
– Epidemiology models help us to understand the causation
(Etiology), interaction, and progression of disease.
– Model Types (p.31)
 The Web of Causation
– Accounts for multiple etiologies
– Considers that the multiple etiologies may interact to promote disease,
but this makes the etiology more complex, and it is harder to predict
specific health outcomes.
 Models In Epidemiology
– Epidemiology models help us to understand the causation
(Etiology), interaction, and progression of disease.
– Model Types (p.31)
 The Wheel
– Considered the most valid model of Epidemiologic Study because the
model recognizes that the host develops from a genetic core that is
modifiable to varying degrees by the environment to which the host is
exposed.
Thus, ones genetic make-up
predisposes how one will be affected
by coming in contact with a potential
agent
Also, the environment predisposes
how one will be affected by the
agent.
 Determining Cause in Epidemiologic Studies
– Casual Association is defined as an association
between categories of events or characteristics in which
the alteration in one is followed by change in another
– Associations may be noncausal, even though a
significant statistical association exists. Two issues,
Confounding and Effect Modification, can result in
“false” causal associations.
Concepts and Methods in
Physical Activity Epidemiology
– Confounding: A stated variable being studied is believed to be the
causal variable, but it is not because of other extraneous variables
 Ex. Male baldness is associated with myocardial infarction, and
baldness could be identified as the causal variable; however the
extraneous variable not under consideration is age. Age is associated
with male baldness, thus age has “confounded” the findings of the
study.
 It is important to have multiple studies that consider different samples
and measure different “confounding” variables to add weight to a
proposed causal variable.
– Effect Modification: A stated variable interacts with another variable
and modifies the treatment effect of the first variable. Thus, effect
modification means that the effect of the exposure on the outcome
depends on the other variable. (See next slide for example)
Concepts and Methods in
Physical Activity Epidemiology
Example of Effect Modification

Mortality decreases linearly with higher
levels of physical activity. However, the
slope of the decrease is much steeper
among older people. The least-active
older person has a much higher mortality
rate than does the least-active young
person. But the mortality rates for older
people become increasingly similar to
those of young people at higher levels of
physical activity. Said another way, low
activity is a bigger problem for older people,
but high activity protects against mortality
regardless of age. Hence, age modifies the Determining the degree to which
certain factors act as effect modifiers
effect of physical activity for reducing
can provide important information for
mortality risk.
the development of preventive
strategies
Criterion for Causation (Mill’s Canons) – p35, text
-If the following Criterion are met, it is likely that the association is
causal:
1. Temporal sequence: exposure to the risk factor must precede
development of the disease with sufficient time to account for
disease progression.
2. Strength of association: there is a large and clinically meaningful
difference in disease risk between those exposed and those not
exposed to the risk factor.
3. Consistency: the observed association is always observed if the
risk factor is present (e.g., regardless of sex, race, age, or methods of measurement).
4. Dose response: the risk of disease associated with the risk factor
is greater with stronger exposure to the risk factor.
5. Biological plausibility: the observed association is explainable by
existing knowledge about possible biological mechanisms of the
disease, which may be alterable (e.g., by physical activity).
END OF PRESENTATION
Concepts and Methods in
Physical Activity Epidemiology