Principles of Epidemiology

Chapter 4 Principles of Epidemiology

Lecture Launcher

Have students volunteer to answer the question: In your own words, what does epidemiology mean to you?

Chapter Outline

I. Introduction

A. Epidemiology is the study of epidemics and the epidemiologic method was initially used to investigate, control, and prevent epidemics of infectious disease. B. Today, it is also applied to the study of injuries, chronic disease, and social problems.

II. 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, but it differs from these disciplines in its focus on health problems of populations rather than of individual patients.  Epidemiology is defined as 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.

A. Investigating Causes of Diseases 1. 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. 2. Surveillance and Related Activities a. 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.

IM for Community Nutrition in Action 4e, by Melanie Burns of Eastern Illinois University b. 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.

III. Basic Epidemiologic Concepts

A. Rates and Risks 1. 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. 2. The primary goal is to control and prevent these health problems, typically through the formulation of specific health policies. 3. In epidemiology, a case is a particular instance of a disease or outcome of interest. 4. 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. a. Risk factors may be found in the physical environment or social environment, or they may be inherited. b. Other risk factors may be behavioral, such as smoking. 5. 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. 6. 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.

B. The Epidemiologic Method  The epidemiologic method uses a variety of tools and incorporates a rigorous, scientific approach that includes the following steps:

1. Observing 2. Counting cases or events 3. Relating cases or events to the population at risk 4. Making comparisons 5. Developing the hypothesis 6. Testing the hypothesis 7. Drawing scientific inferences 8. Conducting experimental studies 9. Intervening and evaluating

C. Hypothesis Testing 1. The importance of hypothesis testing in the epidemiologic method cannot be understated. a. The investigator identifies a cause-effect comparison to be tested as the research hypothesis. b. 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. 2. 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.

IM for Community Nutrition in Action 4e, by Melanie Burns of Eastern Illinois University D. Explaining Research Observations 1. Research data can have three possible explanations: a. 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, or the study results represent the truth. b. Different types of bias include: selection bias (participants were self-selected), measurement bias (an error in measuring one or more of the outcome variables), and 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). 2. 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.

IV. Types of Epidemiologic Studies

A. Ecological or Correlational Studies. Ecological or correlational studies compare the frequency of events in different populations with the per capita consumption of certain dietary factors. 1. 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. 2. 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.

B. Cross-sectional or Prevalence 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.

C. Cohort Studies. Cohort studies are like moving pictures of events occurring within populations. 1. A group of people, called a cohort, free from the disease is identified and examined, and then followed for months or even years during which the group members are examined periodically to determine which individuals develop the characteristics of interest and which do not. 2. 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.

D. Case-Control Studies. In a case-control study, 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. E. Controlled Trials. The most rigorous evaluation of a dietary hypothesis is the randomized controlled trial conducted as a double-blind experiment.

V. 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:

IM for Community Nutrition in Action 4e, by Melanie Burns of Eastern Illinois University  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.  The Centers for Disease Control and Prevention’s (CDC) Behavioral Risk Factor Surveillance System (BRFSS) and Youth Risk Behavior Surveillance System (YRBSS) can be used as sources of information on behaviors that increase the risk for chronic disease and the prevalence of health risk behaviors among young people, respectively.

A. 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.  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.  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.  A variety of methods are available for estimating dietary intake but none of the methods are perfect.

1. 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. a. Food balance sheets are affected by errors that arise in calculating production, waste, and consumption. b. They are not used to describe nutritional inadequacies but are used to formulate agricultural policies. 2. Food Consumption at the Household Level 3. Food Consumption by Individuals. Food consumption by individuals can be measured by using a food consumption survey, diet history, 24-hour recall, food record, and food frequency questionnaire.

VI. Epidemiology and the Community Nutritionist

A. Epidemiology is essential to the delivery of effective nutrition programs and services. B. The key roles of the community nutritionist include identifying nutritional problems within the community and interpreting the scientific literature. 1. The community nutritionist must be able to critically evaluate the scientific literature before formulating new nutrition policies or altering eating pattern messages. 2. Certain elements can be used in judging the strength of epidemiologic association, and interpreting epidemiologic data basically involves two steps:

IM for Community Nutrition in Action 4e, by Melanie Burns of Eastern Illinois University a. Evaluate the criterion for a causal association carefully. b. Assess the causal association critically for the presence of bias and the contribution of chance. 3. Competence in this area is achieved by experience and determination.

IM for Community Nutrition in Action 4e, by Melanie Burns of Eastern Illinois University