Download epidemiologic method

Chapter 4
Principles of Epidemiology
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Learning Objectives
• Define epidemiology.
• Describe various vital statistics used by
epidemiologists to monitor a
population’s health status.
• Explain prevalence rates and how they
differ from incidence rates.
• Describe the strengths and weaknesses
of various types of epidemiologic
studies.
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Learning Objectives
• Explain why the day-to-day
variation in an individual’s nutrient
intake can have important
implications for nutritional
epidemiologic studies.
• Discuss the advantages and
disadvantages of various dietary
assessment methods.
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Introduction
• Epidemiology is the study of
epidemics.
• The epidemiologic method was initially
used to investigate, control, and
prevent epidemics of infectious disease.
• Today, it is also applied to the study of
injuries, chronic disease, and social
problems.
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The Practice of Epidemiology
• The discipline of epidemiology is similar
to clinical medicine and laboratory
science in its concern with
understanding the processes of health
and disease in humans.
• It differs from these disciplines in its
focus on health problems of populations
rather than of individual patients.
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The Practice of Epidemiology
• Epidemiology - the study of the
distribution and determinants of
health-related states and events in
specified populations and the
application of this study to the
control of health problems.
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The Practice of Epidemiology
• Distribution refers to the relationship
between the health problem and the
population in which it exists and it
includes:
– The persons affected.
– The place and time of the occurrence.
– Patient parameters such as age, sex, race,
occupation, income and educational levels,
and social and environmental features.
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The Practice of Epidemiology
• Determinants refer to the causes and
factors that affect the risk of disease
and these are typically divided into two
groups:
– Host factors such as age, sex, race,
nutrition status, and physiologic state,
which determine an individual’s
susceptibility to disease.
– Environmental factors, such as living
conditions, occupation, geographical
location, and lifestyle, which determine the
host’s exposure to a specific agent.
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The Practice of Epidemiology
• Investigating Causes of Diseases
– Examining a Community’s Health
Status
• Epidemiology can be used to describe a
community’s particular health problems
and to determine where its overall health
is improving or getting worse.
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The Practice of Epidemiology
• Investigating Causes of Diseases
– Surveillance and Related Activities
• Public health data have also been used to develop
surveillance methods for identifying women at
high risk for giving birth to a child with fetal
alcohol syndrome and to design and implement
prevention activities.
• Based on vital statistics, such as age at death and
cause of death, recorded on death certificates,
the epidemiologic method can also be used to
calculate an individual’s risk of dying before a
certain age.
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% of Pregnant Women 18-44
Reporting Alcohol Use (1991-1999)
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Example of Targeted Media
Campaign: Alcohol Use in Pregnancy
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Basic Epidemiologic Concepts
• Basic concepts include:
– Rates and risks
– Epidemiologic method
– Hypothesis testing
– Explaining research observations
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Basic Epidemiologic Concepts
– Rates and Risks
• The basic operation of the
epidemiologist is to count cases and
measure the population in which they
arise in order to calculate rates of
occurrence of a health problem and
compare the rates in different groups of
people.
• The primary goal is to control and
prevent these health problems, typically
through the formulation of specific
health policies.
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Basic Epidemiologic Concepts
– Rates and Risks
• In epidemiology, a case is a
particular instance of a disease or
outcome of interest.
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Basic Epidemiologic Concepts
– Rates and Risks
• Risk refers to the likelihood that people
who are without a disease, but exposed
to certain risk factors, will acquire the
disease at some point in their lives.
– Risk factors may be found in the physical
environment or social environment, or they
may be inherited.
– Other risk factors may be behavioral, such
as smoking.
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Basic Epidemiologic Concepts
– Rates and Risks
• An expression of how frequently a
disease occurs in a population is called
incidence, defined as the proportion of
group initially free of a disease that
develops the disease over a period of
time.
• Another common method of frequency
of occurrence of an event is prevalence,
or the proportion of a group possessing
a disease at a specific time.
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Basic Epidemiologic Concepts
– Epidemiologic Method
• The epidemiologic method uses
a variety of tools and incorporates
a rigorous, scientific approach that
includes the following steps:
– Observing
– Counting cases or events
– Relating cases or events to the
population at risk
– Making comparisons
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Basic Epidemiologic Concepts
– Epidemiologic Method
• Steps in epidemiologic method
(continued):
– Developing the hypothesis
– Testing the hypothesis
– Drawing scientific inferences
– Conducting experimental studies
– Intervening and evaluating
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Basic Epidemiologic Concepts
– Hypothesis Testing
• The importance of hypothesis testing in
the epidemiologic method cannot be
understated.
– The investigator identifies a cause-effect
comparison to be tested as the research
hypothesis.
– The statement of a clear, precise hypothesis
at the study outset ensures that the
appropriate data are collected to answer the
research question and avoids the pitfall of
drawing spurious conclusions from the data
set.
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Basic Epidemiologic Concepts
– Hypothesis Testing
• An important aspect of the
epidemiologic method is
determining whether the data are
valid, which is whether the data
represent the true state of affairs.
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Basic Epidemiologic Concepts –
Explaining Research Observations
• Research data can have three
possible explanations:
– The results of the study are incorrect
because they are biased
– The results are due simply to chance
and do not represent the true state of
affairs
– The study results represent the truth
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Basic Epidemiologic Concepts –
Explaining Research Observations
• Different types of bias include:
– Selection bias (participants were selfselected)
– Measurement bias (an error in measuring
one or more of the outcome variables)
– Confounding bias (confounding factors
such as age, gender, ethnicity, dietary or
lifestyle factors that make it difficult to
distinguish between a response to
treatment versus some other factor)
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Basic Epidemiologic Concepts –
Explaining Research Observations
• To say that the data are valid
means that they are neither biased
nor incorrect due to chance and
that they represent the true state
of affairs.
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Types of Epidemiologic
Studies
• Ecological or Correlational Studies
– Compare the frequency of events in
different populations with the per capita
consumption of certain dietary factors.
– Dietary data collected in this type of study
are usually disappearance data, which are
the figures for food produced for human
consumption minus the food that is
exported, fed to animals, wasted, or
otherwise not available for human
consumption.
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Types of Epidemiologic
Studies
• Ecological or Correlational Studies
(continued)
– Data from ecological studies cannot
be used to draw conclusions about
the role of foods or nutrients in the
development of specific diseases, but
they can be used to generate
hypotheses which can then be tested
with a more rigorous study design.
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Types of Epidemiologic
Studies
• Cross-sectional or Prevalence
Studies
– Examine the relationships among
dietary intake, diseases, and other
variables as they exist in populations
at a particular time.
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Types of Epidemiologic
Studies
• Cohort Studies
– Are like moving pictures of events
occurring within populations.
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Types of Epidemiologic
Studies
• Cohort Studies (continued)
– A group of people, called a cohort,
free from the disease is identified and
examined, and then followed for
months or even years
– Group members are examined
periodically to determine which
individuals develop the characteristics
of interest and which do not.
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Types of Epidemiologic
Studies
• Cohort studies may be either:
– Retrospective, those that look back
in time to reconstruct exposures and
health outcomes, or
– Prospective, those that follow a
group into the future.
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Types of Epidemiologic
Studies
• Case-Control Studies
– A group of persons with the disease is
compared with a group of persons
without the disease to compare
characteristics, such as previous
exposure to a factor, between cases
and controls.
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Types of Epidemiologic
Studies
• Controlled Trials
– The most rigorous evaluation of a
dietary hypothesis is the randomized
controlled trial conducted as a
double-blind experiment.
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Nutritional Epidemiology
• The epidemiologic method lends
itself to the study of the
relationship of diet to health and
disease.
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Nutritional Epidemiology
• The method can also be used to:
– Describe the nutrition status of
populations or specific subgroups of a
population and develop specific
programs or services for members of
the group whose nutrition status
appears to be compromised.
– Evaluate nutrition interventions.
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Nutritional Epidemiology
• Behavioral Risk Factor Surveillance
System (BRFSS)
– Source of information on behaviors that
increase the risk for chronic disease
• Youth Risk Behavior Surveillance
System (YRBSS)
– Source of information on the prevalence of
health risk behaviors among young people
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Nutritional Epidemiology
• The Nature of Dietary Variation
– One challenge to the study of the
relationship of diet to disease is the nature
of dietary variation and the complexity of
our diets.
– Foods we consume each day are complex
mixtures of chemicals, some of which are
known to be important to human health
while others have not even been identified
or measured.
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Nutritional Epidemiology
• The Nature of Dietary Variation (cont.)
– Intake from vitamin supplements and other
sources must also be considered.
– The primary factor of interest is the longterm dietary intake of foods, which is more
important than short-term dietary intake in
the development of many diseases, which
take years or a lifetime to develop.
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Nutritional Epidemiology
• The Nature of Dietary Variation (cont.)
– People do not eat the same foods every day
and nutrient intake varies from day to day,
yet it is difficult to determine the number of
days of food intake records needed to
estimate the true average intake of a small
number of adults.
– If only one day’s intake is determined, then
the true long-term nutrient intake may be
misrepresented.
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Nutritional Epidemiology
• The Nature of Dietary Variation
(cont.)
– A variety of methods are available for
estimating dietary intake but none of
the methods are perfect.
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Nutritional Epidemiology
• Food Consumption at the National Level
– The primary method of assessing the
available food supply at the national level is
based on food balance sheets and these
results in a per capita figure.
• Food balance sheets are affected by errors that
arise in calculating production, waste, and
consumption.
• They are not used to describe nutritional
inadequacies but are used to formulate
agricultural policies.
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Nutritional Epidemiology
• Food Consumption at the
Household Level
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Nutritional Epidemiology
• Food Consumption by Individuals
– Food consumption by individuals can
be measured by using:
• A food consumption survey
• Diet history
• 24-hour recall
• Food record
• Food frequency questionnaire
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Epidemiology and the
Community Nutritionist
• Epidemiology is essential to the
delivery of effective nutrition
programs and services.
• The key roles of the community
nutritionist include identifying
nutritional problems within the
community and interpreting the
scientific literature.
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Epidemiology and the
Community Nutritionist
• The community nutritionist must
be able to critically evaluate the
scientific literature before
formulating new nutrition policies
or altering eating pattern
messages.
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Epidemiology and the
Community Nutritionist
• Certain elements can be used in judging
the strength of epidemiologic
association, and interpreting
epidemiologic data basically involves
two steps:
– Evaluate the criterion for a causal
association carefully.
– Assess the causal association critically for
the presence of bias and the contribution of
chance.
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Epidemiology and the
Community Nutritionist
• Competence in this area is
achieved by experience and
determination.
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The Well-Read Community
Nutritionist
• Ten Good Arguments for Reading
Journals*
–
–
–
–
To impress others
To keep abreast of professional news
To understand pathophysiology
To find out how a seasoned health
practitioner handles a particular problem
– To find out whether to use a new or an
existing diagnostic test, survey instrument,
or educational tool with your patients or
clients
*D. L. Sackett,How to read clinical journals. I. Why to read them and how to start reading them
critically, CMA Journal 124 (1981): 555–58. © 2006 Thomson-Wadsworth
The Well-Read Community
Nutritionist
• Ten Good Arguments for Reading
Journals
– To learn the clinical features and course of a
disorder
– To determine etiology or causation
– To distinguish useful from useless or even
harmful therapy
– To sort out claims concerning the need for
and the use, quality, and cost-effectiveness
of clinical and other health care
– To be titillated by the letters to the editor
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The Well-Read Community
Nutritionist
• Which Journals Should You Read?
– Nutrition Journals
– Nutrition Newsletters
– Specialty Journals
– Other Publications
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The Well-Read Community
Nutritionist
• How to Get the Most Out of a
Journal
– Scan the table of contents
– Check the professional updates and
news features
– In choosing articles for in-depth
reading, be selective and
discriminating
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The Well-Read Community
Nutritionist
• How to Tease Apart an Article
– Abstract or summary - Provides an
overview of the study, highlights the
results, and indicates the study’s
significance.
– Introduction - Presents background
information.
– Methods - Describes the study design,
selection of subjects, methods of
measurement, specific hypotheses to be
tested, and analytical techniques.
– Results - Details the study’s outcomes.
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The Well-Read Community
Nutritionist
• How to Tease Apart an Article (cont.)
– Discussion - Provides an analysis of the
meaning of the findings and compares the
study’s findings with those of other
researchers.
– Conclusions/implications - A short
section that summarizes the findings or
considers how the study results can be
applied to practice.
– References/bibliography
• What Else Should You Read?
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