Part 1 - Measures of Disease Frequency
Counting Cases of Disease
In order to estimate the frequency of a particular disease, we need to know:
- The number of cases of disease (the numerator)
- The size of the source population
- The relevant time period
The expression "frequency of disease" should be interpreted broadly to include any health outcome of interest (e.g., diseases, congenital defects, injuries, deaths, mental health problems)
There are many potential ways to collect information on health outcomes in a population:
- Medical records and reports of diagnostic tests
- Health insurance data bases
- Surveillance data from local, state, or federal health data bases
- Disease registries
- Death certificates
A well-defined case definition ((i.e., criteria defining an individual with the health outcome of interest) is important for surveillance, investigation of acute disease outbreaks, and research on chronic diseases. Ill-defined definitions of outcomes can introduce errors in classification of health outcome status that can bias the results of these studies, (We will address misclassification bias later in the course).
Simple Ratios, Proportions, and Rates
A simple ratio is just a number that indicates the relative size of one measurement to another without implying any relationship between the numerator and the denominator. For example, if there were 100 women in a class and 20 men, the ratio of women to men would be 100/20 or 5 women for each man. This is just a simple ratio that indicates how many times larger one quantity is compared to the other.
A proportion is a type of ratio that relates a part to a whole; expressed either as a fraction (e.g., 0.92) or as a percentage (e.g., 92%). For example, if there are 120 women in a class of 130 students, then the proportion of women is 120/130 = 0.92 = 92%.
A rate is a type of ratio in which the denominator also takes into account time. For example, speed is measured in miles per hour; it can be calculated by dividing the number of miles traveled by the number of hours that it took. Water flow can be quantified in gallons per minute; one might measure the number of gallons released during a period of time and divide by the number of minutes it took in order to calculate the average rate. An example of a rate that does not involve time is motor vehicle deaths, which are often reported as deaths per vehicle-miles traveled. This is one way in which the relative safety of different types of transportation (automobiles, buses, trains, airplanes) can be compared.
The term "rate" is used very broadly among the general population (birth malformation rate, autopsy rate, smoking rate, smoking rate, tax rate), but in reality all these measures are proportions. For example, the smoking "rate" among adults is actually the number of adults in a population who smoke divided by the total number of adults in the population, i.e., a proportion, because the numerator is a subset of the whole. One way to tell a proportion from a true rate is that a rate can never be expressed as a percentage, while a proportion can be expressed as a percentage.
Division of one quantity by another to express relative magnitude.
Example: The number of women in a class compared to the number of men:
46 women / 23 men = 2 to 1 ratio of women to men.
A ratio that relates a part to the whole. The numerator must be a subset of the denominator.
Example: A class with 69 students consists of 46 women and 23 men. The proportion of woment is 46/69 = 2/3 = 0.667 = 66.7%
A special ratio in which the numerator and denominator are in different units. Most frequently the denominator incorporates some measure of time.
Example: 60 gallons of water were collected over 3 hours. The rate of collection was 60 gallons/3 hours= 20 gallons per hour on average.
Measures of Disease Frequency
There are three fundamental measures of disease frequency:
- Prevalence (the proportion of a population or group that has a specified condition)
- Cumulative Incidence (the proportion of a population or group at risk that develops a new occurrence of a health outcome over a specified interval of time
- Incidence Rate (the rate at which new cases of disease are occurring in a poulation at risk)
All three of these basic measures of disease frequency take into account:
- Definition of the population of interest (the eligibility criteria)
- The number of cases (the numerator)
- The size of the source population:
- For prevalence the source population includes all people living in the population at the point in time when the cases are counted
- For incidence the source population includes all people at risk of developing the health outcomes of interest at the beginning of the observation period
- The specified time during which outcomes were being counted