SELF-STUDY Course SS1978 Principles of Epidemiology in Public Health Practice Third Edition An Introduction to Applied Epidemiology and Biostatistics October 2006 Updated May 2012 U.S DEPARTMENT OF HEALTH AND HUMAN SERVICES Centers for Disease Control and Prevention (CDC) Office of Workforce and Career Development Atlanta, GA 30333 CONTENTS Acknowledgments v Introduction viii Lesson One: Introduction to Epidemiology Lesson Introduction 1-1 Lesson Objectives 1-1 Major Sections Definition of Epidemiology 1-2 Historical Evolution of Epidemiology 1-7 Uses 1-12 Core Epidemiologic Functions 1-15 The Epidemiologic Approach 1-21 Descriptive Epidemiology 1-31 Analytic Epidemiology 1-46 Concepts of Disease Occurrence 1-52 Natural History and Spectrum of Disease 1-59 Chain of Infection 1-62 Epidemic Disease Occurrence 1-72 Summary 1-80 Exercise Answers 1-81 Self-Assessment Quiz 1-85 Answers to Self-Assessment Quiz 1-90 References 1-93 Lesson Two: Summarizing Data Lesson Introduction 2-1 Lesson Objectives 2-1 Major Sections Organizing Data 2-2 Types of Variables 2-4 Frequency Distributions 2-7 Properties of Frequency Distributions 2-11 Methods for Summarizing Data 2-15 Measures of Central Location 2-16 Measures of Spread 2-36 Choosing the Right Measure of Central Location and Spread 2-53 Summary 2-58 Exercise Answers 2-59 Self-Assessment Quiz 2-64 Answers to Self-Assessment Quiz 2-69 References 2-71 Introduction Page ii Lesson Three: Measures of Risk Lesson Introduction 3-1 Lesson Objectives 3-1 Major Sections Frequency Measures 3-2 Morbidity Frequency Measures 3-10 Mortality Frequency Measures 3-20 Natality (Birth) Measures 3-38 Measures of Association 3-39 Measures of Public Health Impact 3-48 Summary 3-51 Exercise Answers 3-52 Self-Assessment Quiz 3-56 Answers to Self-Assessment Quiz 3-62 References 3-65 Lesson Four: Displaying Public Health Data Lesson Introduction 4-1 Lesson Objectives 4-1 Major Sections Introduction to Tables and Graphs 4-2 Tables 4-3 Graphs 4-22 Other Data Displays 4-42 Using Computer Technology 4-63 Summary 4-66 Exercise Answers 4-72 Self-Assessment Quiz 4-79 Answers to Self-Assessment Quiz 4-84 References 4-87 Lesson Five: Public Health Surveillance Lesson Introduction 5-1 Lesson Objectives 5-1 Major Sections: Introduction 5-2 Purpose and Characteristics of Public Health Surveillance 5-4 Identifying Health Problems for Surveillance 5-6 Identifying or Collecting Data for Surveillance 5-13 Analyzing and Interpreting Data 5-23 Disseminating Data and Interpretations 5-34 Evaluating and Improving Surveillance 5-38 Summary 5-42 Appendix A Characteristics of Well-Conducted Surveillance 5-43 Introduction Page iii Appendix B Appendix C Appendix D Appendix E CDC Fact Sheet on Chlamydia 5-45 Examples of Surveillance 5-48 Major Health Data Systems in the United States 5-52 Limitations of Notifiable Disease Surveillance and Recommendations for Improvement 5-53 Exercise Answers 5-57 Self-Assessment Quiz 5-63 Answers to Self-Assessment Quiz 5-68 References 5-73 Lesson Six: Investigating an Outbreak Lesson Introduction 6-1 Lesson Objectives 6-1 Major Sections: Introduction to Investigating an Outbreak 6-2 Steps of an Outbreak Investigation 6-8 Summary 6-57 Exercise Answers 6-59 Self-Assessment Quiz 6-64 Answers to Self-Assessment Quiz 6-71 References 6-75 Glossary Introduction Page iv ACKNOWLEDGMENTS Developed by U.S Department of Health and Human Services Centers for Disease Control and Prevention (CDC) Office of Workforce and Career Development (OWCD) Career Development Division (CDD) Atlanta, Georgia 30333 Technical Content Richard C Dicker, MD, MSc, Lead Author, CDC/OWCD/CDD (retired) Fátima Coronado, MD, MPH, CDC/OWCD/CDD Denise Koo, MD, MPH, CDC/OWCD/CDD Roy Gibson Parrish, II, MD Development Team Sonya D Arundar, MS, CDC (contractor) Ron Teske, MS, CDC (contractor) Susan Baker Toal, MPH, Public Health Consultant Nancy M Hunt, MPH, CDC (ORISE Fellow) Susan D Welch, MEd, Georgia Poison Center Cassie Edwards, CDC (contractor) Planning Committee Christopher K Allen, RPh, MPH, CDC W Randolph Daley, DVM, MPH, CDC Patricia Drehobl, RN, MPH Sharon Hall, RN, PhD, CDC Dennis Jarvis, MPH, CHES, CDC Denise Koo, MD, MPH, CDC Graphics/Illustrations Sonya D Arundar, MS, CDC (contractor) Lee Oakley, CDC (retired) Jim Walters, CDC Technical Reviewers Tomas Aragon, MD, DrPH, San Francisco Department of Public Health Diane Bennett, MD, MPH, CDC Danae Bixler, MD, MPH, West Virginia Bureau for Public Health R Elliot Churchill, MS, MA, CDC (retired) Roxanne Ereth, MPH, Arizona Department of Health Services Stephen Everett, MPH, Yavapai County Community Health Services, Arizona Michael Fraser, PhD, National Association of County and City Health Officials Introduction Page v Nancy C Gathany, MEd, CDC Marjorie A.Getz, MPHIL, Bradley University, Illinois John Mosely Hayes, DrPH, MBA, MSPH, Tribal Epidemiology Center United South and Eastern Tribes, Inc., Tennessee Richard Hopkins, MD, MSPH, Florida Department of Health John M Horan, MD, MPH, Georgia Division of Public Health Christina M Bruton Kwon, MSPH, Science Applications International Corporation, Atlanta Edmond F Maes, PhD, CDC Sharon McDonnell, MD, MPH, Darmouth Medical School William S Paul, MD, MPH, Chicago Department of Public Health James Ransom, MPH, National Association of County and City Health Officials Lynn Steele, MS, CDC Donna Stroup, PhD, MSc, American Cancer Society Douglas A Thoroughman, PhD, MS CDC Kirsten T Weiser, MD, Darmouth Hitchcock Medical School Celia Woodfill, PhD, California Department of Health Services Field Test Participants Sean Altekruse, DVM, MPH, PhD, U.S Public Health Service Gwen A Barnett, MPH, CHES, CDC Jason Bell, MD, MPH Lisa Benaise, MD, Med Immune, Inc., Maryland Amy Binggeli, DrPH, RD, CHES, CLE, Imperial County Public Health Department, California Kim M Blindauer, DVM, MPH, Agency for Toxic Substances and Disease Registry R Bong, RN, BSN, Federal Bureau of Prisons Johnna L Burton, BS, CHES, Tennessee Department of Health Catherine C Chow, MD, MPH, Hawaii Department of Health Janet Cliatt, MT, CLS(NCA), National Institutes of Health Catherine Dentinger, FNP, MS, New York City Department of Health and Mental Hygiene Veronica Gordon, BSN, MS, Indian Health Service, New Mexico Susan E Gorman, PharmD, DABAT, CDC Deborah Gould, PhD, CDC Juliana Grant, MD, MPH, CDC Lori Evans Hall, PharmD, CDC Nazmul Hassan, MS, Food and Drug Administration Daniel L Holcomb, BS, Agency for Toxic Substances and Disease Registry Asim A Jani, MD, MPH FACP, CDC Charletta L Lewis, BSN, Wellpinit Indian Health Service, Washington Sheila F Mahoney, CNM, MPH, National Institutes of Health Cassandra Martin, MPH, CHES, Georgia Department of Human Resources Joan Marie McFarland, AS, BSN, MS, Winslow Indian Health Care Center, Arizona Rosemarie McIntyre, RN, MS, CHES, CDC Introduction Page vi Gayle L Miller, DVM, Jefferson County Department of Health and Environment, Colorado Long S Nguyen, MPH, CHES, NIH Paras M Patel, RPh, Food and Drug Administration Rossanne M Philen, MD, MS, CDC Alyson Richmond, MPH, CHES, CDC (contractor) Glenna A Schindler, MPH, RN, CHES, Healthcare Services Group, Missouri Sandra K Schumacher, MD, MPH, CDC Julie R Sinclair, MA, DVM, MPH, CDC Nita Sood, RPh, PharmD, U.S Public Health Service P Lynne Stockton, VMD, MS, ELS(D), CDC Jill B Surrency, MPH, CHES, CDC (contractor) Joyce K Witt, RN, CDC Introduction Page vii INTRODUCTION This course was developed by the Centers for Disease Control and Prevention (CDC) as a self-study course Continuing education is available for certified public health educators, nurses, physicians, pharmacists, veterinarians, and public health professionals CE credit is available only through the CDC/ATSDR Training and Continuing Education Online system at http://www.cdc.gov/TCEOnline To receive CE credit, you must register for the course (SS1978) and complete the evaluation and examination online You must achieve a score of 70% or higher to pass the examination If you not pass the first time, you can take the exam a second time For more information about continuing education, call 1-800-41-TRAIN (1-800-4187246) or by e-mail at ce@cdc.gov Course Design This course covers basic epidemiology principles, concepts, and procedures useful in the surveillance and investigation of health-related states or events It is designed for federal, state, and local government health professionals and private sector health professionals who are responsible for disease surveillance or investigation A basic understanding of the practices of public health and biostatistics is recommended Course Materials The course materials consist of six lessons Each lesson presents instructional text interspersed with relevant exercises that apply and test knowledge and skills gained Lesson One: Introduction to Epidemiology Key features and applications of descriptive and analytic epidemiology Lesson Two: Summarizing Data Calculation and interpretation of mean, median, mode, ranges, variance, standard deviation, and confidence interval Lesson Three: Measures of Risk Calculation and interpretation of ratios, proportions, incidence rates, mortality rates, prevalence, and years of potential life lost Lesson Four: Displaying Public Health Data Preparation and application of tables, graphs, and charts such as arithmetic-scale line, histograms, pie chart, and box plot Lesson Five: Public Health Surveillance Processes, uses, and evaluation of public health surveillance in the United States Introduction Page viii Lesson Six: Investigating an Outbreak Steps of an outbreak investigation A Glossary that defines the major terms used in the course is also provided at the end of Lesson Six Supplementary Materials In addition to the course materials, students may want to use the following: • A calculator with square root and logarithmic functions for some of the exercises • A copy of Heymann, DL, ed Control of Communicable Diseases Manual, 18th edition, 2004, for reference Available from the American Public Health Association (202) 777-2742 Objectives Students who successfully complete this course should be able to correctly: • Describe key features and applications of descriptive and analytic epidemiology • Calculate and interpret ratios, proportions, incidence rates, mortality rates, prevalence, and years of potential life lost • Calculate and interpret mean, median, mode, ranges, variance, standard deviation, and confidence interval • Prepare and apply tables, graphs, and charts such as arithmetic-scale line, scatter diagram, pie chart, and box plot • Describe the processes, uses, and evaluation of public health surveillance • Describe the steps of an outbreak investigation General Instructions Self-study courses are “self-paced.” We recommend that a lesson be completed within two weeks To get the most out of this course, establish a regular time and method of study Research has shown that these factors greatly influence learning ability Each lesson in the course consists of reading, exercises, and a self-assessment quiz Reading Assignments Complete the assigned reading before attempting to answer the self-assessment questions Read thoroughly and re-read for understanding as necessary A casual reading may result in missing useful information which supports main themes Assignments are designed to cover one or two major subject areas However, as you progress, it is often necessary to combine previous learning to accomplish new skills A review of previous lessons may be necessary Frequent visits to the Glossary may also be useful Exercises Exercises are included within each lesson to help you apply the lesson content Some exercises may be more applicable to your workplace and background than others You should review the answers to all exercises since the answers are very detailed Answers to Introduction Page ix the exercises can be found at the end of each lesson Your answers to these exercises are valuable study guides for the final examination Self-Assessment Quizzes After completing the reading assignment, answer the self-assessment quizzes before continuing to the next lesson Answers to the quizzes can be found at the end of the lesson After passing all six lesson quizzes, you should be prepared for the final examination • Self-assessment quizzes are open book • Unless otherwise noted, choose ALL CORRECT answers • Do not guess at the answer • You should score at least 70% correct before continuing to the next lesson Tips for Answering Questions • • Carefully read the question Note that it may ask, “Which is CORRECT?” as well as “Which is NOT CORRECT?” or “Which is the EXCEPTION?” Read all the choices given One choice may be a correct statement, but another choice may be more nearly correct or complete for the question that is asked Final Examination and Course Evaluation The final examination and course evaluation are available only on-line The final requirement for the course is an open-book examination We recommend that you thoroughly review the questions included with each lesson before completing the exam It is our sincere hope that you will find this undertaking to be a profitable and satisfying experience We solicit your constructive criticism at all times and ask that you let us know whenever you have problems or need assistance Introduction Page x experimental study see study, experimental exposed group a group whose members have had contact with a suspected cause of, or possess a characteristic that is a suspected determinant of, a particular health problem exposure having come into contact with a cause of, or possessing a characteristic that is a determinant of, a particular health problem F false-negative a negative test result for a person who actually has the condition similarly, a person who has the disease (perhaps mild or variant) but who does not fit the case definition, or a patient or outbreak not detected by a surveillance system false-positive a positive test result for a person who actually does not have the condition Similarly, a person who does not have the disease but who nonetheless fits the case definition, or a patient or outbreak erroneously identified by a surveillance system field epidemiology see epidemiology, field follow-up study see study, cohort fomite an inanimate object that can be the vehicle for transmission of an infectious agent (e.g., bedding, towels, or surgical instruments) forest plot a graph that displays the point estimates and confidence intervals of individual studies included in a meta-analysis or systematic review as a series of parallel lines frequency the amount or number of occurrences of an attribute or health outcome among a population frequency distribution a complete summary of the frequencies of the values or categories of a variable, often displayed in a two-column table with the individual values or categories in the left column and the number of observations in each category in the right column frequency polygon a graph of a frequency distribution in which values of the variable are plotted on the horizontal axis, and the number of observations are plotted on the vertical axis Data points are plotted at the midpoints of the intervals and are connected with straight lines G geometric mean see mean, geometric graph a visual display of quantitative data arranged on a system of coordinates H health a state of complete physical, mental, and social well-being and not merely the absence of Glossary Page disease or other infirmity health indicator any of a variety of measures (e.g., mortality rate) that indicate the state of health of a given population health information system a combination of health statistics from different sources Data from these systems are used to learn about health status, health care, provision and use of services, and the impact of services and programs on health healthy worker effect the observation that employed persons generally have lower mortality rates than the general population, because persons with severe, disabling disease (who have higher mortality rates) tend to be excluded from the workforce herd immunity see immunity, herd high-risk group a group of persons whose risk for a particular disease, injury, or other health condition is greater than that of the rest of their community or population HIPAA the Health Insurance Portability and Accountability Act, enacted in 1996, which addresses the privacy of a person’s medical information as well as postemployment insurance and other health-related concerns histogram a visual representation of the frequency distribution of a continuous variable The class intervals of the variable are grouped on a linear scale on the horizontal axis, and the class frequencies are grouped on the vertical axis Columns are drawn so that their bases equal the class intervals (i.e., so that columns of adjacent intervals touch), and their heights correspond to the class frequencies host a person or other living organism that is susceptible to or harbors an infectious agent under natural conditions host factor an intrinsic factor (e.g., age, race/ethnicity, sex, or behaviors) that influences a person's exposure, susceptibility, or response to an agent hyperendemic the constant presence at high incidence and prevalence of an agent or health condition within a given geographic area or population hypothesis a supposition, arrived at from observation or reflection, that leads to refutable predictions; any conjecture cast in a form that will allow it to be tested and refuted hypothesis, alternative the supposition that an exposure is associated with the health condition under study The alternative is adopted if the null hypothesis (see also hypothesis, null) proves implausible hypothesis, null the supposition that two (or more) groups not differ in the measure of interest (e.g., incidence or proportion exposed); the supposition that an exposure is not associated with the health condition under study, so that the risk ratio or odds ratio equals The null hypothesis is used in conjunction with statistical testing Glossary Page I immunity, active resistance developed in response to an antigen (i.e., an infecting agent or vaccine), usually characterized by the presence of antibody produced by the host immunity, herd the resistance to an infectious agent of an entire group or community (and, in particular, protection of susceptible persons) as a result of a substantial proportion of the population being immune to the agent Herd immunity is based on having a substantial number of immune persons, thereby reducing the likelihood that an infected person will come in contact with a susceptible one among human populations, also called community immunity immunity, passive immunity conferred by an antibody produced in another host This type of immunity can be acquired naturally by an infant from its mother or artificially by administration of an antibody-containing preparation (e.g., antiserum or immune globulin) incidence a measure of the frequency with which new cases of illness, injury, or other health condition occurs among a population during a specified period incidence proportion the fraction of persons with new cases of illness, injury, or other health condition during a specified period, calculated as the number of new cases divided by the size of the population at the start of the study period (see also attack rate) incidence rate a measure of the frequency with which new cases of illness, injury, or other health condition occur, expressed explicitly per a time frame Incidence rate is calculated as the number of new cases over a specified period divided either by the average population (usually mid-period) or by the cumulative person-time the population was at risk incubation period the time interval from exposure to an infectious agent to the onset of symptoms of an infectious disease independent variable see variable, independent index case see case, index indirect transmission see transmission, indirect individual data values or observations from each record (also called raw data) infant mortality rate see mortality rate, infant infection invasion of the body tissues of a host by an infectious agent, whether or not it causes disease infectivity the ability of an infectious agent to cause infection, measured as the proportion of persons exposed to an infectious agent who become infected information bias see bias, information interquartile range a measure of spread representing the middle 50% of the observations, calculated as the difference between the third quartile (75th percentile) and the first quartile (25th Glossary Page 10 percentile) isolation the separation of infected persons to prevent transmission to susceptible ones Isolation refers to separation of ill persons; quarantine refers to separation of potentially exposed but well persons L latency period the time from exposure to a causal agent to onset of symptoms of a (usually noninfectious) disease (see also incubation period) life expectancy a statistical projection of the average number of years a person of a given age is expected to live, if current mortality rates continue to apply line graph, arithmetic-scale a graph that displays patterns or trends by plotting the frequency (e.g., number, proportion, or rate) of a characteristic or event during some variable, usually time The y-axis, measuring frequency, uses an arithmetic scale line graph, semilogarithmic-scale a graph that displays patterns or trends by plotting the frequency (e.g., number, proportion, or rate) of a characteristic or event during some variable, usually time The y-axis, measuring frequency, uses a logarithmic scale line listing a type of epidemiologic database, organized similar to a spreadsheet with rows and columns in which information from cases or patients are listed each column represents a variable, and each row represents an individual case or patient logarithmic transformation conversion of nominal or ordinal data to logarithmic data The purpose is to examine rate of change instead of amount of change only M map, area (shaded, choropleth) a visual display of the geographic pattern of a health problem, in which a marker is placed on a map to indicate where each affected person lives, works, or might have been exposed mean (or average) commonly called the average; it is the most common measure of central tendency mean, arithmetic the measure of central location, commonly called the average, calculated by adding all the values in a group of measurements and dividing by the number of values in the group mean, geometric the mean, or average, of a set of data measured on a logarithmic scale measure of association a quantified relationship between exposure and a particular health problem (e.g., risk ratio, rate ratio, and odds ratio) measure of central location a central value that best represents a distribution of data Common measures of central location are the mean, median, and mode also called the measure of central Glossary Page 11 tendency measure of dispersion see measure of spread measure of spread a measure of the distribution of observations out from its central value Measures of spread used in epidemiology include the interquartile range, variance, and the standard deviation measurement scale the complete range of possible values for a measurement mechanical transmission see transmission, mechanical median the measure of central location that divides a set of data into two equal parts, above and below which lie an equal number of values (see also measure of central location) medical surveillance see surveillance, medical midrange the halfway point, or midpoint, in a set of observations For the majority of data, the midrange is calculated by adding the smallest observation and the largest observation and dividing by two The midrange is usually calculated as an intermediate step in determining other measures mode the most frequently occurring value in a set of observations (see also measure of central location) mode of transmission the manner in which an agent is transmitted from its reservoir to a susceptible host (see also transmission) morbidity disease; any departure, subjective or objective, from a state of physiological or psychological health and well-being mortality death mortality rate a measure of the frequency of occurrence of death among a defined population during a specified time interval mortality rate, age-adjusted a mortality rate that has been statistically modified to eliminate the effect of different age distributions among different populations mortality rate, age-specific a mortality rate limited to a particular age group, calculated as the number of deaths among the age group divided by the number of persons in that age group, usually expressed per 100,000 mortality rate, cause-specific the mortality rate from a specified cause, calculated as the number of deaths attributed to a specific cause during a specified time interval among a population divided by the size of the midinterval population mortality rate, crude a mortality rate from all causes of death for an entire population, without adjustment mortality rate, infant the mortality rate for children aged