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Chapter 1

Chapter 1. Approach and Evolution of Epidemiology. What is Epidemiology?. Epidemiology is based on two fundamental assumptions… 1) Human disease does not occur at random; there are factors or determinants which can increase or decrease the likelihood of disease.

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Chapter 1

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  1. Chapter 1 Approach and Evolution of Epidemiology

  2. What is Epidemiology? Epidemiology is based on two fundamental assumptions… 1) Human disease does not occur at random; there are factors or determinants which can increase or decrease the likelihood of disease. 2) The factors or determinants (either causal or preventive) can be identified by systematic investigation of populations or subgroups within populations.

  3. What is Epidemiology?

  4. What is Epidemiology? • The study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems • How is disease distributed in the population? • Person: age, sex, race • Place: urban, US states, near power plant • Time: June, annual, decade

  5. What is Epidemiology? • The study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems • Determinants – factors that cause some people to get disease and that prevent some people from getting disease. • We also call these factors exposures

  6. What is Epidemiology? • The study of the distribution and determinants of disease frequency in human populationsand the application of this study to control health problems • disease frequency – counting the number of cases of a disease in a population over a specific time period • human populations – a group of people with a common characteristic such as residence, age, gender

  7. What is Epidemiology? • The study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems • There is no point to collecting and analyzing data if it cannot be used to help control health problems.

  8. Epidemiology Helps Answer Questions • Does smoking cause lung cancer? • Does football cause head (brain) injuries? • What was the cause of the salmonella outbreak? • Who is most likely to get the flu? • Do cell phones cause brain tumors? • Does screen time increase the risk of ADHD? • Does vaping increase the risk of smoking in teenagers? • Do certain genes put people at higher risk for diabetes?

  9. Sequence of Epidemiologic Investigations Suspect that exposure influences disease occurrence Form specific hypotheses about exposure-disease association Conduct epidemiologic studies to measure relationship between exposure and disease Evaluate preventions and treatment Judge whether association is valid and causal

  10. What is a Hypothesis? • A testable statement that tries to explain a set of observations and can be tentatively accepted or rejected through scientific research

  11. Two Criteria for Good Hypotheses • Hypotheses are statements that project the expected association between two or more measurable variables • Hypotheses carry clear implications for testing stated relations

  12. Hypotheses are Important for • Advancing epidemiologic research • Guiding study design, sample selection, data analyses, and interpretation of results.

  13. Hypothesis Formation • Bench science  biomarkers • Clinical observation • Observation of the population • Descriptive studies • Literature review

  14. Descriptive Epidemiology • Describe patterns of disease by person, place, and time • Person: Who is getting the disease? (for example, what is their age, sex, religion, race, educational level etc?)

  15. Ex. U. S. mortality rates per 100,000 from diseases of the heart by age and sex (2004)What hypotheses can you generate from these data?

  16. Place • Where are the rates of disease the highest and lowest? What hypotheses can you generate from this map? Malignant Melanoma of Skin

  17. Place What hypotheses can you generate from this map? Cancer of the Trachea, Bronchus and Lung

  18. Place What hypotheses can you generate from this map? (Ref Karp DN. Using Small Area Spatial Statistics to Identify Clusters of High Stroke Mortality in the United States. Stroke 2016;47:1939-1942.

  19. Variation on Place: Migrant StudiesMortality rates (per 100,000) due to stomach cancer. What hypotheses can you generate from these data?

  20. Time: • Is the present frequency of disease different from the past? • What hypotheses can you generate from these data?

  21. The results of an epidemiologic study, regardless of type, is an association between the disease under study and an exposure. Evaluating Validity and Causality Two questions must be answered: • Is the observed association valid? • Random Error (chance) • Bias (systematic error) • Confounding (mixing of effects) • Is the association causal? What we are trying to do is evaluate whether that relationship is causal. How is this done?

  22. Valid statistical association does not imply cause and effect. You must use your judgment.

  23. The Routine Physical is a Routine Killer Physical exams cause dangerous long-term increases in blood pressure and cholesterol levels. Survey respondents who had been given a physical during the previous three years were twice as likely as other respondents to report problems with high blood pressure and cholesterol.

  24. Some of Hill’s Guidelines • Time sequence established: There is evidence that exposure preceded disease. • Strength of association: Stronger associations are more likely to be causal. • Consistency: If other investigations using different populations, different study designs show similar results, there is strong support for causality. • Biologic credibility: Does association make sense biologically? • Dose‑response: Does disease risk increase as exposure level increases?

  25. Historical Development of Epidemiology • Spans about 400 years • Progress slow and unsteady • Key figures: • John Graunt summarized the pattern of mortality in 17th century London • James Lind used an experimental study to discover the cause of prevention of scurvy in the 18th century • John Snow showed that cholera was transmitted by fecal contamination of drinking water in the 19th century

  26. Cholera • Swept into Europe in early 1800s • Epidemics in London and Paris • Symptoms included nausea, dizziness, violent vomiting, diarrhea, extreme muscle cramps, insatiable thirst • Could lead to cardiovascular collapse and death

  27. Beliefs About Transmission of Cholera • Person-to-person • “Miasmas” - mysterious vapors from swamps, cemeteries, cesspools

  28. Cholera: John Snow • London physician to the poor • Studied cholera outbreaks in 1830s • Symptoms were gastrointestinal so he reasoned it might be transmitted by water or food. • Snow conducted a landmark series of studies which tested his hypothesis about mode of transmission and led to an intervention

  29. London Water Supply • Some families carried water from community pumps, but others got water from companies that pumped water from Thames River via pipes.

  30. Original Map by John Snow: Clusters of Cholera cases, London 1854 epidemic, drawn and lithographed by Charles Cheffins

  31. Snow compared death rates fromcholera according to the water source

  32. Significance of John Snow’s Work • He proposed a new hypothesis for how cholera was transmitted based on observation & reason. • He tested the hypothesis by collecting data systematically & comparing groups of people. • He established an association between certain (contaminated) drinking water and getting cholera. • He argued for an intervention to prevent more cases.

  33. The Cause of Cholera • We know today that the cause is a bacterium, Vibrio cholera, which is transmitted by ingestion of water or food contaminated with sewage.

  34. Solve the Mystery Epidemic • The following data relate to an unusual episode that actually occurred. • Describe the epidemiologic features of this episode. For example, the overall mortality ‘rate’ was 68.2% • Based on the descriptive characteristics, formulate a hypothesis concerning the etiology of this episode.

  35. Solve the Mystery Epidemic

  36. Selected Epidemiologic Milestones since the Cholera Investigation

  37. Modern Practice of Epidemiology: Selected Sub-disciplines

  38. Future and Current Challenges • The 21st century poses challenging problems for epidemiologists such as • air, water and soil pollution • global warming • poverty and social inequality • emerging infectious diseases • gun violence – mental health • opioid epidemic

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