# BIOSTATISTICS

Document Sample

```					1
2
BIOSTATISTICS

5.5
MEASURES OF FREQUENCY

3
BIOSTATISTICS
• TERMINAL OBJECTIVE:

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

4
• 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.
5
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.

6
Prevalence Rate
• Proportion of persons in a population who
have a particular disease over a specified
period of time.

7
Attack Rate
• Incidence rates which are calculated for
narrowly defined populations, during
limited periods of time, such as an epidemic

8
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.

9
Mortality Rate
– Case-fatality Rate
Number of cause-specific deaths among the
incident cases divided by the number of
new cases of that disease.

10
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

11
Relative Risk
– A value greater than 1 indicates a positive
association, or an increased risk among the
exposed.

12
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

13
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.

14
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.

15
Attributable Risk

• A useful measure of the public health
impact of a particular exposure.

16
• 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.
17
Calculate Incidence Rate
• Formula:

new cases occurring over a period of time
x 10n
population at risk during same time

– 10n = base power of ten,
n = 2, 3, 4, 5

18
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).

19
Calculate Incidence Rate
– Numerator = 51
– Denominator = 552,000
– 51/552,000 x 105
= 0.0000924 x 100,000
– = 9.24 per 100,000

20
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.

21
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

22
Calculate Prevalence Rate
• Formula:

all new and existing cases during a given time period
population during same the time period

x 100%

23
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.

24
Calculate Prevalence Rate
– Numerator = 80
– Denominator = 500
– Prevalence = 80/500 x 100%
= 16%

25
Calculate Attack Rate
• Formula

# of new cases among the population during the
time period
Population at risk

x 100%

26
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.

27
Calculate Attack Rate
• Crude attack rate
– New cases = 46
– Population at risk = 75

– AR = 46/75 x 100 = 61%

28
Calculate Attack Rate
• Female sex-specific attack rate
– Cases = 12
– Population at risk = 30

– AR = 12/30 x 100 = 40%

29
Calculate Attack Rate
• Male sex-specific attack rate
– Cases = 46-12=34
– Population at risk = 45

– AR = 34/45 x 100 = 75.5%

30
Calculate Mortality Rate
• Crude mortality rate
Formula:

Deaths during calendar year
x 10n
Population at mid-year

31
Calculate Mortality Rate
• Example
– In 1998 there were 21,000 deaths in a mid-year
population of 3,750,000. Calculate crude
mortality.

32
Calculate Mortality Rate
– Numerator = 21,000
– Denominator = 3,750,000
– CDR = 21,000/3,750,000 x 105
= 560 deaths per 100,000 pop.

33
Calculate Mortality Rate
• Case fatality rate
Formula:

number of cause-specific deaths among
incident cases
number of new cases of disease

x 100%

34
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%
35
Calculate Absolute Risk
• Formula
Risk for group of interest (Ie)

Same as the incidence rate         1

Ie = incidence for exposed group

36
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%

37
Calculate Relative Risk
• Formula

Risk for group of interest (Ie)
Risk for comparison group (I0)       x 1 1

Ie = incidence for exposed group
I0 = incidence for unexposed group

38
Calculate Relative Risk
• Two-by-two contingency table
TABLE 5.5A
NUMBER OF CASES FOR PELLAGRA BY SEX,
SOUTH CAROLINA, 1920'S

Pellagra
Yes    No      Total
Female   a =         b =   1,484
46        1,438
Male    c =         d =   1,419
18        1,401
Total                      2,903
39
Calculate Relative Risk
Ie = a  a+b = 46/1484 = .031

Io = c  c+d    = 18/1419 = .013

RR = Ie/Io

= .031/.013 = 2.4
40
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.

41
Attributable Risk
• Using a 2x2 contingency table
INCIDENCE OF HEPATITIS C IN SYRINGE SHARING
N = 256*

HCV
Shared               Yes               No
Syringe                                           Total

Yes                  37               89          126

No                  13               117          130_
Total                                             256_
*Hagan, H. et al., Sharing of Drug Preparation Equipment as a
Risk Factor for Hepatitis C, Am J Public Health, Jan 01, Vol.   42
91, No. 1, pp. 42-46.
Attributable Risk
Ie = 37/126 = .29

Io = 13/130 = .10

AR = 0.29 – 0.10 = 0.19

19 per 100
43
Attributable Risk
Using a rate table
DEATH RATES FROM LUNG CANCER BY DAILY
CIGARETTE USE
DOLL AND HILL PHYSICIAN FOLLOW-UP STUDY,
1951-1961

Cigarettes   Death Rates per 1000
per Day          per Year
0              0.07
1-14             0.57

44
Calculate Attributable Risk

(1) Ie = 0.57

(2) I0 = 0.07

(3) AR = 0.57 - 0.07 = 0.50
0.50 per 1,000 per year
(50 per 100,000)

45
46
Food Specific Attack Rate Table

IAW PEF 5.5:

5.5.14 Prepare a Food Specific Attack Rate
Table

47
Food Specific Attack Rate Table

• Description:

– Used to ascertain which food or foods caused
the illness when the incriminating meal is
known

48
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).

49
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.

50
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.

51
Food Specific Attack Rate Table

• Construction
– The table has three major headings (columns):
• Food/Beverage
• Ate Specified Item
• Did Not Eat Specified Item

52
Food Specific Attack Rate Table

– Food/Beverage

• Foods served during the implicated meal.

53
Food Specific Attack Rate Table

– Ill
• Total number of persons ill.
– Well
• Total number of persons not ill.

54
Food Specific Attack Rate Table

– Total
• Total number of persons Ill and the number Well.

55
Food Specific Attack Rate Table

– Attack Rate %
• Ill divided by Total x (100)%

56
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.

57
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.

58
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.

59
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.

60
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..

61

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 13 posted: 4/23/2011 language: English pages: 61