Epidemiology: Basic concepts and principles

1. Epidemiology: Basic concepts and principles ENV 415 21.9.12

2. Syllabus covered • Introduction • Definition • Some basic concepts and terminologies • Proportion, rate and ratio • Prevalence • Incidence • Population at risk • Role of the epidemiologist • Sources of Health data

3. What is epidemiology? • Epidemiology is the branch of science that deals with the study of the dynamics of health and ill health in populations

4. How do you define epidemiology? It is the study of the distribution and determinants of disease frequency in human population (Hennekens & Buring: Epidemiology In Medicine)

5. Another definition of Epidemiology__________________________ “The study of the DISTRIBUTION and DETERMINANTS of health related states and events in populations and the application of this study to the control of health problems” Last J M: A Dictionary of Epidemiology.

6. Underlying concepts • The health/ill health continuum in a population • Population at risk • Population dynamics • Multiple causality and multiple determinants

7. Population dynamics • Factors influencing the population dynamic • Population pyramid • Birth & death rates • In-migration & out-migration

8. Population at risk • A key feature of epidemiology is the measurement of disease outcomes in relation to a population at risk. • The population at risk is a defined population that shares similar experience or characteristics exposed to a risk of developing disease (outcome)

9. About epidemiology • Frequency count of disease (how many) • Distribution of disease (who, where, when) • The determinants of disease (what causes it, how did it happen, why did it happen)

10. Schematic diagram of Epidemiology Descriptive epidemiology Analytic epidemiology Disease frequency The determinants of disease Disease distribution Test hypothesis Measures of disease frequency Measures of association

11. Basic math in epidemiology • Ratio: a/b • Proportion: a/a+b • Rate: (a/a+b) x t

12. Measures of Disease Occurences • Counting cases (summarizing cases as ratio) • Prevalence (proportion) • Incidence (rate)

13. Epidemiologic measures • Prevalence • Incidence • Interpretation of prevalence and incidence • Its relationship with epidemiologic studies

14. Prevalence • Proportion: a/a+b • When expressed as a percentage: (a/a+b) x 100 i.e per 100 person • Prevalence of hepatitis is 3% means 3 cases of hepatitis per 100 person • For rare diseases multiply by 1,000 or 10,000

15. Ratios Ratio = A/B = A:B Example: In a classroom of 15 boys and 5 girls the male to female ratio is 15:5 or 3:1 In RATIO: numerator not part of denominator Odds ratio

16. Proportion Proportion is part of a whole Proportion = A/A+B Example: In a classroom of 15 boys and 5 girls, the proportion of female students is 5/5+15 = 25% In PROPORTION: numerator is part of the denominator Expressed per 100 = percentage Risk Ratio

17. Rate Rate = A/(A+B)t RATE:A proportion in which change over time is taken into account Comparison of Absolute Risk Relative Risk

18. Pseudo rate • When the true denominator or population at risk cannot be determined. • A related denominator is used as an indirect measure reflecting the population at risk Examples: Maternal mortality rate (MMR) Birth defect rate (BDR)

19. Pseudo rate BDR (Birth Defect Rate) • Number of babies born with birth defect /Total number of deliveries (or livebirths)

20. Prevalence The number of existing cases (new and old) of a disease over the total population at a given point in time. Prevalence is a proportion measure and not a rate - autopsy rate - birth defect rate

21. Prevalence measure • Surveys • Cross sectional study • Seroprevalence study • Screening

22. Uses of prevalence measures__________________________ • Planning for health care needs • Assess public health impact • Comparison of prevalence between exposed and non-exposed group is used for hypothesis generation (prevalence OR)

23. Incidence • The number of new cases during a given time period over the total population at risk • 2 types of incidence measures • Cumulative incidence • Incidence density • Examples of incidence measures • mortality and morbidity rates • cause-specific mortality rates • case-fatality rate • attack rate

24. Incidence Number of new cases developing over a given period of time CI = Population at risk Number of new cases developing over a given period of time ID = Total person-time of observation

25. Prevalence & Incidence Jan76 July76 Jan77 July77 Jan78 July78 TTR A ________________________ B ______________________x C __________________________________________________________ D _______________________x E ______________________x Period Point Prevalence prevalence

26. Interrelationship between incidence and prevalence________________________ _ P = I x D P is the prevalence I is the incidence and _ D = average duration of disease

27. Incidence Density (ID) • The denominator differs from the CI because it take into account : • Cases lost to follow-up • Cases died • Cases who enter locality registry later

28. Incidence - Quantifies number of new cases of disease that develop in a population at risk during a specified time period Three key concepts: • New disease events, or for diseases that can occur more than once, usually first occurrence of disease • Population at risk (candidate population) • Time must pass for a person to move from health to disease

29. Two Types of Incidence Measures (Continued) Cumulative incidence estimates the probability or risk that a person will develop disease DURING A SPECIFIED TIME. Note that the candidate population is comprised of people who are “at risk” of getting the disease Used mainly for fixed populations because its assumes that everyone is followed for the entire time period

30. Example: Cumulative incidence of Sudden Infant Death Syndrome during first year of life Population 1,000 livebirths Cases of SIDS 10 Cumulative Incidence 10/1,000 or 1% over one year • Note that all livebirths are ‘at risk’

31. Cumulative incidence calculation assumes that you have followed the entire population for the entire follow-up period. For example, it assumes you have followed all of the livebirths for one year or until SIDS occurred. • Often you can't follow everyone for entire time period • In a dynamic population, individuals enter population over time, become lost, etc. • So length of follow-up is not uniform for all • Incidence density do not make assumption of complete follow-up. It take into account the actual follow-up period

32. Incidence density (ID) = # new cases of disease person-time of observation in candidate population ID have more precision compared to CI because it directly integrates time into the denominator. However interpretation of the ID is not simple and may appear abstract.

33. Some Ways to Accrue 100PY • 100 people followed 1 year each = 100 py • 10 people followed 10 years each= 100 py • 50 people followed 1 year plus 25 people followed 2 years = 100 py Time unit for person-time = year, month or day Person-time = person-year, person-month, person-day

34. Attack rate: • # cases of disease that develop during ________defined period_______ • # in pop. at risk at start of period • (usually used for infectious disease outbreaks)

35. Further e.g of incidence rates • Infectivity rate = # infected / # susceptible x 100 • Pathogenicity rate = # clinical dis / # of infected x 100

36. Summary

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38. Summary

39. Issues in the calculation of incidence_________________________ • The denominator should only include persons at risk of developing disease • Exclude those who already have the disease or cannot develop the disease (age, immunized against, prior removal of an organ)

40. Measures of disease frequency Number of persons with disease Prevalence = at a point in time Number of person in population Number of new cases of disease in a given time period Cumulative = Incidence Total population at risk Number of new cases of disease in a given time period Incidence = Density Total person-time of observation

41. Food-specific Attack Rate (n=60) Eat Did not eat Food served ill not ill A.R ill not ill A.R Chicken 20 28 42% 5 7 42% Anchovies 12 17 41% 13 18 42% Meatballs 23 24 49% 2 11 15% Vegetables 15 24 38% 10 11 48% Potatoes 18 26 41% 7 9 44% Cake 22 34 39% 3 1 75%