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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.

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

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

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.

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

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

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

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

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.

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.

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

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 x10n

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).

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

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.

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

Calculate Prevalence Rate • Formula: all new and existing cases during a given time period population during same the time period x 100%

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.

Calculate Attack Rate • Formula # of new cases among the population during the time period Population at risk x 100%

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.

Calculate Attack Rate • Crude attack rate • New cases = 46 • Population at risk = 75 • AR = 46/75 x 100 = 61%

Calculate Attack Rate • Female sex-specific attack rate • Cases = 12 • Population at risk = 30 • AR = 12/30 x 100 = 40%

Calculate Attack Rate • Male sex-specific attack rate • Cases = 46-12=34 • Population at risk = 45 • AR = 34/45 x 100 = 75.5%

Calculate Mortality Rate • Crude mortality rate Formula: Deaths during calendar year Population at mid-year x 10n

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

Calculate Mortality Rate • Case fatality rate Formula: number of cause-specific deaths among incident cases number of new cases of disease x 100%

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%

Calculate Absolute Risk • Formula Risk for group of interest (Ie) Same as the incidence rate Ie = incidence for exposed group 1

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%

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

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

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.

Food Specific Attack Rate Table • Description: • Used to ascertain which food or foods caused the illness when the incriminating meal is known

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).

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.