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

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BIOSTATISTICS

5.5

MEASURES OF FREQUENCY

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BIOSTATISTICS

• TERMINAL OBJECTIVE:

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

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• 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.

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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.

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Prevalence Rate

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

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Attack Rate

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

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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.

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Mortality Rate

– Case-fatality Rate

Number of cause-specific deaths among the incident cases divided by the number of new cases of that disease.

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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

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Relative Risk

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

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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

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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.

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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.

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Attributable Risk

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

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• 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.

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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

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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).

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– Numerator = 51– Denominator = 552,000– 51/552,000 x 105

= 0.0000924 x 100,000 – = 9.24 per 100,000

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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.

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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

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Calculate Prevalence Rate

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

x 100%

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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.

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Calculate Prevalence Rate

– Numerator = 80– Denominator = 500– Prevalence = 80/500 x 100%

= 16%

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Calculate Attack Rate

• Formula

# of new cases among the population during the

time period

Population at risk

x 100%

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• 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.

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• Crude attack rate– New cases = 46– Population at risk = 75

– AR = 46/75 x 100 = 61%

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• Female sex-specific attack rate– Cases = 12– Population at risk = 30

– AR = 12/30 x 100 = 40%

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

– AR = 34/45 x 100 = 75.5%

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x 10n

Calculate Mortality Rate • Crude mortality rate

Formula:

Deaths during calendar year

Population at mid-year

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Calculate Mortality Rate

• Example– In 1998 there were 21,000 deaths in a mid-year

population of 3,750,000. Calculate crude mortality.

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Calculate Mortality Rate

– Numerator = 21,000– Denominator = 3,750,000– CDR = 21,000/3,750,000 x 105

= 560 deaths per 100,000 pop.

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Calculate Mortality Rate• Case fatality rate

Formula:

number of cause-specific deaths among

incident cases

number of new cases of disease

x 100%

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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%

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Calculate Absolute Risk • Formula

Risk for group of interest (Ie)

Same as the incidence rate

Ie = incidence for exposed group

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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%

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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

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Calculate Relative Risk

• Two-by-two contingency tableTABLE 5.5A

NUMBER OF CASES FOR PELLAGRA BY SEX,SOUTH CAROLINA, 1920'S

Pellagra Yes No Total

Female a =46

b =1,438

1,484

Male

Total

c =18

d =1,401

1,419

2,903

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Calculate Relative Risk

Ie = a a+b = 46/1484 = .031

Io = c c+d = 18/1419 = .013

RR = Ie/Io

= .031/.013 = 2.4

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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.

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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. 91, No. 1, pp. 42-46.

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Attributable Risk

Ie = 37/126 = .29

Io = 13/130 = .10

AR = 0.29 – 0.10 = 0.19

19 per 100

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Attributable Risk

DEATH RATES FROM LUNG CANCER BY DAILYCIGARETTE USE

DOLL AND HILL PHYSICIAN FOLLOW-UP STUDY,1951-1961

Cigarettesper Day

Death Rates per 1000per Year

0 0.07

1-14 0.57

Using a rate table

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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)

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IAW PEF 5.5:

5.5.14 Prepare a Food Specific Attack Rate Table

Food Specific Attack Rate Table

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• Description:

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

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• Data is obtained by interviewing:

– Cases: the persons that became ill.

– Controls: The persons who did not become ill (well).

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– Compare the attack rates of those who ate the specific foods served, with the attack rates of those who did not eat the foods.

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– The food items which present the greatest difference in the attack rates are considered the prime suspects as the source of the illness.

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• Construction– The table has three major headings (columns):

• Food/Beverage

• Ate Specified Item

• Did Not Eat Specified Item

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• Subheadings under major headings– Food/Beverage

• Foods served during the implicated meal.

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– Ill• Total number of persons ill.

– Well• Total number of persons not ill.

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

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

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– 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.

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• 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.

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– 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.

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• 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.

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• 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..

Documents

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