1 2 BIOSTATISTICS 5 5 MEASURES OF FREQUENCY

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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 population at risk during same time x 10 n – 10 n = 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 Population at mid-year x 10 n 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 2 x 2 observed table • Formula: a / H 1 x 100 – Example: a = 46; H 1 = 1484 Risk = 46/1484 x 100 = 3. 1% 37

Calculate Relative Risk • Formula Risk for group of interest (Ie) Risk for comparison group (I 0) x 1 1 Ie = incidence for exposed group I 0 = incidence for unexposed group 38

Calculate Relative Risk • Two-by-two contingency table TABLE 5. 5 A NUMBER OF CASES FOR PELLAGRA BY SEX, SOUTH CAROLINA, 1920'S 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. – I 0 = Incidence rate of persons not exposed to the disease or event. – Expressed as a rate per base population. 41

Attributable Risk • Using a 2 x 2 contingency table 42

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 44

Calculate Attributable Risk (1) Ie = 0. 57 (2) I 0 = 0. 07 (3) AR = 0. 57 - 0. 07 = 0. 50 per 1, 000 per year per 100, 000) 0. 50 (50 45

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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 • Subheadings under major headings – 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