# lecture 5 - Lecture Goals Lecture 5 o Understand and...

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1 Lecture 5 Outcome Measures Lecture Goals o Understand and demonstrate the difference between ratios, rates, and proportions o Calculate and interpret measures of morbidity and mortality, including prevalence and incidence o Understand the relationship between prevalence, incidence, and duration o Distinguish between crude, specific, and adjusted rates o Calculate and interpret crude, specific and case-fatality rates o Calculate and interpret the proportional mortality ratio Outcome Measures Ratios are used to compare two groups or outcomes to each other Proportions and rates are used to measure the occurrence of disease in a population o Proportions describe what fraction of the population is affected by disease o Rates describe how fast the disease occurs Ratio A fraction where there is no specific relationship between the numerator and denominator: A / B Example: males females Proportion A fraction where the numerator is included in the denominator: A / (A+B) o May be expressed as a percentage Example: # infant deaths = 50 = 25% total # births 200 Rate A proportion that involves time o Numerator = frequency over time o Denominator = unit size of population o Time periods – involves start and stop points

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2 Calculating a Rate In New Jersey, 400 people died of tuberculosis between January 1, 2005 and December 1, 2005. In 2003, NJ had 1.2 million residents. Calculate the death rate from TB in 2005. Death rate = 400 TB deaths in 2005 1,200,000 people in 2005 Death rate = 0.0003 TB deaths/person in 2005 Calculating a Rate Death rate = 0.0003 deaths/person in 2005 Does this make sense??? Put the rate in perspective – multiply by a power of 10! Rate = 400 deaths in 2005 * 10,000 1,200,000 people Rate = 3 deaths per 10,000 people in 2005 Interpreting a Rate Rate: 3 deaths per 10,000 people in 2003 What does this mean? For every 10,000 people living in New Jersey in 2003, there were 3 deaths from TB. Rates Rates can be expressed in any form that is convenient o Per 100 o Per 1,000 o Per 10,000 o Per 100,000 o Per 1,000,000 In other words, the following are equivalent: o 0.03 per 100 o 0.3 per 1,000 o 30 per 10,000 o 300 per 100,000 Morbidity Describes illness in a population Two main measures of morbidity o Incidence o Prevalence Incidence Measures the number of new cases relative to the population at risk Incidence describes how quickly (or slowly) new cases of disease develop over a period of time Incidence = # new cases over time total population at risk* *Because populations are dynamic, use the midpoint population for the period of interest.
3 Calculating Incidence In March 2005, 324 cases of chicken pox were

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lecture 5 - Lecture Goals Lecture 5 o Understand and...

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