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:
Incidence Rate.
Prevalence Rate.
Attack Rate.
Mortality Rate.
Relative Risk.
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:
Incidence Rate.
Prevalence Rate.
Attack Rate.
Case Fatality Rate.
Absolute Risk.
Relative Risk.
Attributable Risk.

18. Calculate Incidence Rate
Formula:
new cases occurring over a period of time
population at risk during same time
10n = base power of ten,
n = 2, 3, 4, 5
x 10n

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 (105).

20. Calculate Incidence Rate
Numerator = 51
Denominator = 552,000
51/552,000 x 105
= 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. # of new cases among the population during the time period
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 10n

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 105
= 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 (Ie)
Same as the incidence rate
Ie = 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 (Ie)
Risk for comparison group (I0)
Ie = incidence for exposed group
I0 = incidence for unexposed group
x 1
 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
Ie = a  a+b = 46/1484 = .031
Io = c  c+d = 18/1419 = .013
RR = Ie/Io
= .031/.013 = 2.4

41. Calculate Attributable Risk
Ie = Incidence rate of persons exposed to a disease or other event.
I0 = 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 Io = 13/130 = .10 AR = 0.29 – 0.10 = 0.19 19 per 100
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) Ie = 0.57
(2) I0 = 0.07
(3) AR = = per 1,000 per year (50 per 100,000)

47. Food Specific Attack Rate Table
IAW PEF 5.5:
Prepare a 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
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..