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  • 1.  
  • 2.  
  • 3. BIOSTATISTICS 5.5 MEASURES OF FREQUENCY
  • 4. BIOSTATISTICS
    • TERMINAL OBJECTIVE:
    • 5.5 Prepare a Food Specific Attack Rate Table IAW PEF 5.5.
  • 5.
    • DEFINE:
    • 5.5.1 Incidence Rate.
    • 5.5.2 Prevalence Rate.
    • 5.5.3 Attack Rate.
    • 5.5.4 Mortality Rate.
    • 5.5.5 Relative Risk.
    • 5.5.6 Attributable Risk.
  • 6. Incidence Rate
    • Expresses the probability of or risk of illness in a population over a period of time.
    • The frequency of new events (illness) for a specified population over a given period of time, multiplied by a population base.
  • 7. Prevalence Rate
    • Proportion of persons in a population who have a particular disease over a specified period of time.
  • 8. Attack Rate
    • Incidence rates which are calculated for narrowly defined populations, during limited periods of time, such as an epidemic
  • 9. Mortality Rate
    • Frequency of death in a population during a specified period of time
      • Crude mortality rate
        • The mortality rate from all causes of death for a population.
  • 10. Mortality Rate
      • Case-fatality Rate
    • Number of cause-specific deaths among the incident cases divided by the number of new cases of that disease.
  • 11. Relative Risk
    • The ratio of the incidence of the exposed group to the incidence of the unexposed group
    • Risk group of primary interest is in the numerator, while the comparison group is in the denominator
  • 12. Relative Risk
      • A value greater than 1 indicates a positive association, or an increased risk among the exposed.
  • 13. Relative Risk
    • Used as the measure of association for illness with the Food Specific Attack Rate Table.
    • Data derived from a two-by-two table
  • 14. Attributable Risk
    • Measures the difference between incidence rates in the exposed group and nonexposed group.
      • A measure of the excess risk of disease in those exposed compared with those nonexposed.
  • 15. Attributable Risk
    • With no association between incidence rates, the attributable risk = 0.
    • Positive values indicate the number of cases of disease that can be attributed to exposure.
  • 16. Attributable Risk
    • A useful measure of the public health impact of a particular exposure.
  • 17.
    • CALCULATE:
    • 5.5.7 Incidence Rate.
    • 5.5.8 Prevalence Rate.
    • 5.5.9 Attack Rate.
    • 5.5.10 Case Fatality Rate.
    • 5.5.11 Absolute Risk.
    • 5.5.12 Relative Risk.
    • 5.5.13 Attributable Risk.
  • 18. Calculate Incidence Rate
    • Formula:
    • new cases occurring over a period of time
    • population at risk during same time
      • 10 n = base power of ten,
      • n = 2, 3, 4, 5
    x 10 n
  • 19. Calculate Incidence Rate
    • Example
      • In CY 2000, 51 new cases of hepatitis B were reported in a county with a mid-year population of 552,000. Calculate the incidence rate using a base of 100,000 (10 5 ).
  • 20. Calculate Incidence Rate
      • Numerator = 51
      • Denominator = 552,000
      • 51/552,000 x 10 5
      • = 0.0000924 x 100,000
      • = 9.24 per 100,000
  • 21. Calculate Incidence Rate
    • A population of 200 experienced an Ebola outbreak between March and October. There were 17 cases, 9 which died. What is the six month incidence rate? Use base of 100.
  • 22. Incidence Rate
    • (1) Over time, 200 – 9 (deaths) = 191 people are included in the calculation
    • Numerator = 17
    • Denominator = 191
    • 17/191 x 100 = 8.9 cases per 100
    • population
  • 23. Calculate Prevalence Rate
    • Formula:
    • all new and existing cases during a given time period
    • population during same the time period
    • x 100%
  • 24. Calculate Prevalence Rate
    • Example
      • In a survey of records at a major medical center, 80 out of 500 patients were treated for one or more nosocomial infections in the past six months. Calculate prevalence.
  • 25. Calculate Prevalence Rate
      • Numerator = 80
      • Denominator = 500
      • Prevalence = 80/500 x 100%
      • = 16%
  • 26. Calculate Attack Rate
    • Formula
    • # of new cases among the population during the time period
    • Population at risk
    • x 100%
  • 27. Calculate Attack Rate
    • Example
      • A cohort of 75 persons (45 male, 30 female) attended a picnic. Of these, 46 developed gastroenteritis. Of the ill, 12 were females. Calculate the crude and sex-specific attack rates.
  • 28. Calculate Attack Rate
    • Crude attack rate
      • New cases = 46
      • Population at risk = 75
      • AR = 46/75 x 100 = 61%
  • 29. Calculate Attack Rate
    • Female sex-specific attack rate
      • Cases = 12
      • Population at risk = 30
      • AR = 12/30 x 100 = 40%
  • 30. Calculate Attack Rate
    • Male sex-specific attack rate
      • Cases = 46-12=34
      • Population at risk = 45
      • AR = 34/45 x 100 = 75.5%
  • 31. Calculate Mortality Rate
    • Crude mortality rate
    • Formula:
    • Deaths during calendar year
    • Population at mid-year
    x 10 n
  • 32. Calculate Mortality Rate
    • Example
      • In 1998 there were 21,000 deaths in a mid-year population of 3,750,000. Calculate crude mortality.
  • 33. Calculate Mortality Rate
      • Numerator = 21,000
      • Denominator = 3,750,000
      • CDR = 21,000/3,750,000 x 10 5
      • = 560 deaths per 100,000 pop .
  • 34. Calculate Mortality Rate
    • Case fatality rate
    • Formula:
    • number of cause-specific deaths among
    • incident cases
    • number of new cases of disease
    x 100%
  • 35. Calculate Case Fatality Rate
    • Example
    • A hospital treated 80 individuals for
    • malaria. There were 39 deaths. Numerator = 39
    • Denominator = 80
    • 39  80 x 100%
    • = 49%
  • 36. Calculate Absolute Risk
    • Formula
    • Risk for group of interest (I e )
    • Same as the incidence rate
    • I e = incidence for exposed group
     1
  • 37. Calculate Absolute Risk
    • Use 2x2 observed table
    • Formula: a / H 1 x 100
      • Example: a = 46; H1 = 1484
      • Risk = 46/1484 x 100
      • = 3.1%
  • 38. Calculate Relative Risk
    • Formula
    • Risk for group of interest (I e )
    • Risk for comparison group (I 0 )
    • I e = incidence for exposed group
    • I 0 = incidence for unexposed group
     1 x 1
  • 39. Calculate Relative Risk
    • Two-by-two contingency table
    • TABLE 5.5A
    • NUMBER OF CASES FOR PELLAGRA BY SEX,
    • SOUTH CAROLINA, 1920'S
  • 40. Calculate Relative Risk
    • I e = a  a+b = 46/1484 = .031
    I o = c  c+d = 18/1419 = .013 RR = I e /I o = .031/.013 = 2.4
  • 41. Calculate Attributable Risk
      • I e = Incidence rate of persons exposed to a disease or other event.
      • I 0 = Incidence rate of persons not exposed to the disease or event.
      • Expressed as a rate per base population.
  • 42. Attributable Risk
    • Using a 2x2 contingency table
  • 43. Attributable Risk
        • Ie = 37/126 = .29
    • Io = 13/130 = .10
    • AR = 0.29 – 0.10 = 0.19
    • 19 per 100
  • 44. Attributable Risk Using a rate table
  • 45. Calculate Attributable Risk
    • (1) I e = 0.57
    • (2) I 0 = 0.07
    • (3) AR = 0.57 - 0.07 = 0.50 0.50 per 1,000 per year (50 per 100,000)
  • 46.  
  • 47.
    • IAW PEF 5.5:
    • 5.5.14 Prepare a Food Specific Attack Rate Table
    Food Specific Attack Rate Table
  • 48. Food Specific Attack Rate Table
    • Description:
      • Used to ascertain which food or foods caused the illness when the incriminating meal is known
  • 49. Food Specific Attack Rate Table
    • Data is obtained by interviewing:
      • Cases: the persons that became ill.
      • Controls: The persons who did not become ill (well).
  • 50. Food Specific Attack Rate Table
      • Compare the attack rates of those who ate the specific foods served, with the attack rates of those who did not eat the foods.
  • 51. Food Specific Attack Rate Table
      • The food items which present the greatest difference in the attack rates are considered the prime suspects as the source of the illness.
  • 52. Food Specific Attack Rate Table
    • Construction
      • The table has three major headings (columns):
        • Food/Beverage
        • Ate Specified Item
        • Did Not Eat Specified Item
  • 53. Food Specific Attack Rate Table
    • Subheadings under major headings
      • Food/Beverage
        • Foods served during the implicated meal.
  • 54. Food Specific Attack Rate Table
      • Ill
        • Total number of persons ill.
      • Well
        • Total number of persons not ill.
  • 55. Food Specific Attack Rate Table
      • Total
        • Total number of persons Ill and the number Well.
  • 56. Food Specific Attack Rate Table
      • Attack Rate %
        • Ill divided by Total x (100)%
  • 57. Food Specific Attack Rate Table
      • Difference (%)
        • The last column under Did Not Eat Specified Item
        • The difference between the attack rates (%) from those who ate the specified food and those who did not eat it.
  • 58. Food Specific Attack Rate Table
        • This value is obtained by subtracting the attack rate of those who did not eat the food from those who did eat.
        • Negative values can occur.
  • 59. Food Specific Attack Rate Table
      • Fill in the columns with the appropriate data from food histories obtained during the outbreak investigation.
      • The total columns for each food item should equal the number of persons (ill and well) in the investigation.
  • 60. Food Specific Attack Rate Table
    • Identify the food item(s) with the largest positive difference in attack rates (Attributable Risk). This is the food(s) which is suspected as the source of the illness.
  • 61. Food Specific Attack Rate Table
    • Statistical analysis is performed on the values of the suspected food(s) to determine if there is a statistical association between its consumption and the occurrence of the illness..