Epidemiology Lab 2 OFFICIAL.docx - Epidemiology Lab 2 Frequency measures used in Epidemiology Objectives After studying this lesson and answering the

# Epidemiology Lab 2 OFFICIAL.docx - Epidemiology Lab 2...

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Epidemiology Lab 2: Frequency measures used in Epidemiology Objectives: After studying this lesson and answering the questions in the exercises, you will be able to calculate and interpret the following epidemiologic measures: 1. Ratio 2. Proportion 3. Incidence proportion (attack rate) 4. Incidence rate 5. Prevalence Background : Frequency Measures A measure of central location provides a single value that summarizes an entire distribution of data. In contrast, a frequency measure characterizes only part of the distribution. Frequency measures compare one part of the distribution to another part of the distribution, or to the entire distribution. Common frequency measures are ratios , proportions , and rate s . All three frequency measures have the same basic form: (Numerator / denominator) x 10 n Recall that: 10 0 = 1 (anything raised to the 0 power equals 1) 10 1 = 10 (anything raised to the 1st power is the value itself) 10 2 = 10 x 10 = 100 10 3 = 10 x 10 x 10 = 1,000 So the fraction of (numerator/denominator) can be multiplied by 1, 10, 100, 1000, and so on. This multiplier varies by measure and will be addressed in each section. Ratio Definition of ratio A ratio is the relative magnitude of two quantities or a comparison of any two values. It is calculated by dividing one interval- or ratio-scale variable by the other. The numerator and denominator need not be related. Therefore, one could compare apples with oranges or apples with number of physician visits.
Method for calculating a ratio Number rateof events,items , persons ,etc agroup Number rateof events,items, persons ,etc another group After the numerator is divided by the denominator, the result is often expressed as the result “to one” or written as the result “:1.” Note that in certain ratios, the numerator and denominator are different categories of the same variable, such as males and females, or persons 20–29 years and 30–39 years of age. In other ratios, the numerator and denominator are completely different variables, such as the number of hospitals in a city and the size of the population living in that city. Proportion Definition of proportion A proportion is the comparison of a part to the whole. It is a type of ratio in which the numerator is included in the denominator. You might use a proportion to describe what fraction of clinic patients tested positive for HIV, or what percentage of the population is younger than 25 years of age. A proportion may be expressed as a decimal, a fraction, or a percentage. Method for calculating a proportion Totalnumber of persons events,of whichthe numeratoris a ¿ x 10 n Number of persons oneventswith a particular characteristic ¿ For a proportion, 10 n is usually 100 (or n=2) and is often expressed as a percentage.

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