Definitions

Health-related state or event: A particular disease, injury, other health condition or attribute. ‘Health-related state or event’ refers to the fact that epidemiology involves more than just the study of disease states, but includes the study of events, behaviours, and conditions associated with health.

Population (specified): Specified populations are specific parts of the overall community, which share predefined characteristics that are of interest of a certain study. This may be a group of persons who have been exposed to a specific risk or a group of persons of a certain age. These may also be the total number of persons living in a particular place (e.g., a town or country), or the total number of persons in a particular group (e.g., with the same job or educational or socioeconomic background).

Prevalence, sometimes referred to as prevalence rate: the proportion of persons in a (specified) population who have a particular disease or health condition, all cases, both new and pre-existing, at a particular timepoint or over a specified period of time.

Ratio: A ratio is just a number that is obtained by dividing one number (Numerator) by another (Denominator). A ratio doesn't necessarily imply any particular relationship between the numerator and the denominator. For example, if there were 100 women in this 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.

Proportion: A type of ratio that relates a part to a whole; for instance, the ratio between health events occurring and population, often shown as a fraction or percentage. For example, if there are 30 students having a cold in a class of 120, then the proportion of sick students is 30/120 = 0,25 or, multiplied: 0.25 x 100 = 25%.

Rate: A type of ratio in which the denominator also takes into account another dimension, usually time. For example, water flow might be quantified in litres/minute; one might measure the number of litres released and divide by the number of minutes it took in order to calculate the average rate per minute.

While the term "rate" is used very broadly among the general population (birth malformation rate, autopsy rate, smoking rate), in reality all these measures are proportions. For example, the smoking "rate" among adults is the number of adults in a population who smoke divided by the total number of adults in the population—in other words, 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 should always be able to be expressed as a percentage.

Person-year

Person years and person months are types of measurement that consider both the number of participants in a study and the amount of time each person spends in the study. For example, a study that followed 1000 people for 1 year would contain 1000 person years of data. Likewise,
10 persons followed for one year equals one person followed for 10 years, each would amount to 10 person years.

The calculation with examples and connection to incidence rate is given later in the text.

Incidence: The number of new cases of a specific disease or health event over a specified period of time (such as a month or year) in a specified population.