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Assessing Disease Frequency

South Asian Cardiovascular Research Methodology Workshop. Basic Epidemiology. Assessing Disease Frequency. Thomas Songer, PhD. Why should we be concerned with monitoring disease(s)?. Disease Control and Prevention. Good monitoring does not necessarily ensure the making of right decisions,

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Assessing Disease Frequency

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  1. South Asian Cardiovascular Research Methodology Workshop Basic Epidemiology Assessing Disease Frequency Thomas Songer, PhD

  2. Why should we be concerned with monitoring disease(s)?

  3. Disease Control and Prevention

  4. Good monitoring does not necessarily ensure the making of right decisions, but it reduces the risk of wrong ones. Languimer, 1963

  5. There are several decisions regarding disease monitoring • What level of resources should be allocated to disease monitoring? • What outcomes do we want to achieve? • What benefits are obtained from these items and to whom do the benefits accrue? • Can the counting methods be readily accepted into the community?

  6. Approaches Towards Monitoring Disease and Injury Death Certificates Population Surveys Surveillance Registries Screening

  7. Surveillance: Systematic, regular ascertainment of incidence using methods distinguished by their practicality, uniformity, and frequently their rapidity, rather than by complete accuracy. Last, 1990

  8. Types of Surveillance • Active • Passive

  9. Active Surveillance • the collection of data on a disease by regular outreach. Designated medical personnel are called at regular intervals to collect information on the new cases of disease. monitoring domestic violence in emergency departments

  10. Active Surveillance Health Dept.

  11. Passive Surveillance • data generated without contact by the agency carrying out the surveillance. Reportable diseases fall under this type of surveillance.

  12. Passive Surveillance Health Dept.

  13. Comparison of Disease Counting Approaches Population Communicable Registries Screening Disease Surveillance Source Academia Academia Health Depts Speed Slow Slow Fast Cost/Case High High Low Ascertain >90 % 65-75% Low and -ment Variable

  14. So you have identified the frequency of disease in a given area…. What do you do with it?

  15. death rates disability rates hospitalization rates incidence rates prevalence rates Epidemiology is a Science of Rates

  16. Rates • Rates are the basic tool of epidemiologic practice • Why are rates important? • because they provide more complete information to describe or assess the impact of disease in a community or population

  17. Rate: a measure of the occurrence of a health event in a population group at a specified time period Number of events in time period numerator : denominator Number at risk for the event

  18. Why are rates useful? • Can help to identify groups with an elevated risk of disease • can target interventions to these groups • these groups can be studied to identify risk factors Page, Cole 1995

  19. Rates • Relate health events to a population base • This provides a basis for making valid comparisons of health events by considering the number at risk in each population Page, Cole 1995

  20. Injury Deaths, Australia, 1992 Harrison, 1995

  21. Injury Death Rates, Australia, 1992 Harrison, 1995

  22. Mortality: is one of the major measures of disease in the population • information available from death certificates (required by law) • Death rate: Number of deaths in time period = Number at risk of dying

  23. Three common types of rates • Crude rates • Specific rates • Adjusted rates Page, Cole 1995

  24. Three common types of rates • Crude rates • consider the entire population • Specific rates • consider differences among subgroups of the population • Adjusted rates • adjust for differences in population composition Page, Cole 1995

  25. number of deaths in time period • Crude rates • Specific rates Crude death rate = total population number of deaths in age group in time period Age-specific death rate = population in age group

  26. Adjusted Rates • Use statistical procedures to adjust for differences in characteristics between populations • Age is the most frequent factor adjusted for because age is related to both death and disease • Adjusted rates do not describe actual occurrence, but are hypothetical given certain assumptions

  27. Crude and Age-Adjusted Death RatesUnited States, 1940-1992 crude death rate per 100,000 population age-adjusted death rate

  28. Morbidity • any departure from health • i.e. the extent of illness, injury or disability in a defined population • morbidity rates are used as indicators of health • in epidemiology, the main measures of morbidity are incidence and prevalence

  29. Measures of disease frequency • Incidence rates • crude incidence • cumulative incidence • Prevalence rates • crude prevalence • period prevalence • point prevalence Paneth

  30. Incidence: is one of the major measures of disease in the population • information available from surveys, registries, or investigations • Incidence rate: Number of new cases of disease in population in time period = Number at risk of developing disease in same time period x 1000

  31. The numerator has to come from the population at risk for developing disease The denominator may change over time as people develop disease The denominator does not include persons with the disease numerator Incidence Rate denominator

  32. in large studies, the denominator is often the mid-year population in small studies, the denominator does not include persons with the disease numerator Incidence Rate denominator However, in practice

  33. What is the incidence rate from October 1, 1990 to Sep 30, 1991?

  34. What is the incidence rate from October 1, 1990 to Sep 30, 1991? 4 4 / 14

  35. Cumulative incidence • Number of new cases of disease occurring over a specified period of time in a population at risk (at the beginning of the interval) Number of new cases of disease identified over a given time interval Cumulative incidence rate = Estimated population at beginning of interval

  36. Prevalence

  37. Prevalence: is another major measure of disease in the population • information available from surveys, registries, or investigations Number of existing cases of disease in population in time period Prevalence Rate = Population at risk in same time period x 1000

  38. Numerator: New cases occurring during a given time period Denominator: Number at risk of developing disease Numerator: All cases present (new and existing) during a given time period Denominator: Number in population Difference between incidence rates and prevalence rates Incidence Prevalence

  39. Several factors may affectprevalence • Incidence • Duration of disease • Disease treatments

  40. Incidence rate x average duration of disease Prevalence rate =

  41. Point Prevalence • Number of individuals in a specified population at risk who have the disease of interest at a given point in time Number of cases of disease at a given point in time Point prevalence rate = Estimated population at the same point in time

  42. What is the point prevalence on April 1?

  43. What is the point prevalence on April 1? 7 7 / 18

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