© 2006 Thomson-Wadsworth Chapter 4 Principles of Epidemiology

© 2006 Thomson-Wadsworth

Chapter 4

Principles of Epidemiology


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.


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.


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.


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.



• 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.



• 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.



• 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.



• 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.



• 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.


% of Pregnant Women 18-44 Reporting Alcohol Use (1991-1999)


Example of Targeted Media Campaign: Alcohol Use in Pregnancy


Basic Epidemiologic Concepts

• Basic concepts include:– Rates and risks– Epidemiologic method– Hypothesis testing– Explaining research observations



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.



• In epidemiology, a case is a particular instance of a disease or outcome of interest.



• 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.



• 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.


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


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



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.



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.


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



• Different types of bias include: – Selection bias (participants were self-

selected)– 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)



• 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.


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.



• 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.



• Cross-sectional or Prevalence Studies– Examine the relationships among

dietary intake, diseases, and other variables as they exist in populations at a particular time.



• Cohort Studies– Are like moving pictures of events

occurring within populations.



• 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.



• 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.



• 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.




• Controlled Trials– The most rigorous evaluation of a

dietary hypothesis is the randomized controlled trial conducted as a double-blind experiment.


Nutritional Epidemiology

• The epidemiologic method lends itself to the study of the relationship of diet to health and disease.




• 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.



• 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



• 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.



• The Nature of Dietary Variation (cont.)– Intake from vitamin supplements and other

sources must also be considered.– The primary factor of interest is the long-

term 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.



• 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.





• The Nature of Dietary Variation (cont.)– A variety of methods are available for

estimating dietary intake but none of the methods are perfect.



• 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.



• Food Consumption at the Household Level



• 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


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.



• The community nutritionist must be able to critically evaluate the scientific literature before formulating new nutrition policies or altering eating pattern messages.



• 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.




• Competence in this area is achieved by experience and determination.


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.



• 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



• Which Journals Should You Read?– Nutrition Journals– Nutrition Newsletters– Specialty Journals– Other Publications



• 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



• 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.



• 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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© 2006 Thomson-Wadsworth Chapter 4 Principles of Epidemiology