1. 2 3 biostatistics 5.5 measures of frequency 4 biostatistics terminal objective: 5.5 prepare a...
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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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Calculate Incidence Rate
– Numerator = 51– Denominator = 552,000– 51/552,000 x 105
= 0.0000924 x 100,000 – = 9.24 per 100,000
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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.
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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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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.
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Calculate Attack Rate
• Crude attack rate– New cases = 46– Population at risk = 75
– AR = 46/75 x 100 = 61%
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Calculate Attack Rate
• Female sex-specific attack rate– Cases = 12– Population at risk = 30
– AR = 12/30 x 100 = 40%
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Calculate Attack Rate
• 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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Food Specific Attack Rate Table
• Description:
– Used to ascertain which food or foods caused the illness when the incriminating meal is known
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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).
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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.
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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.
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Food Specific Attack Rate Table
• Construction– The table has three major headings (columns):
• Food/Beverage
• Ate Specified Item
• Did Not Eat Specified Item
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Food Specific Attack Rate Table
• Subheadings under major headings– Food/Beverage
• Foods served during the implicated meal.
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Food Specific Attack Rate Table
– Ill• Total number of persons ill.
– Well• Total number of persons not ill.
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Food Specific Attack Rate Table
– Total• Total number of persons Ill and the number Well.
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Food Specific Attack Rate Table
– Attack Rate %• Ill divided by Total x (100)%
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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.
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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.
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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.
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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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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..