Glossary of Terms

Case-fatality rate: The proportion of cases of a specified condition that are fatal within a specified time.1

This definition can lead to paradox when more persons die of the disease than develop it during a given period. For instance, chemical poisoning that is slowly but inexorably fatal may cause many persons to develop the disease over a relatively short period of time, but the deaths may not occur until some years later and may be spread over a period of years during which there are no new cases. Thus, in calculating the case fatality rate, it is necessary to acknowledge that the time dimension varies: it may be brief (e.g., covering only the period of stay in a hospital); of finite duration (e.g., 1 year); or of longer duration still. The term case fatality rate is then better replaced by a term such as survival rate or by the use of a survivorship table.

Ecological study: In epidemiology, a study in which the units of analysis are populations or groups of people rather than individuals.1

Incidence: The number of instances of illness commencing, or of persons falling ill, during a given period in a specified population. More generally, the number of new health-related events in a defined population within a specified period of time. It may be measured as a frequency count, a rate, or a proportion.1

Incubation period: The time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease in question.1

Index case: The first case in a family or other defined group to come to the attention of the investigator.1

Infectivity: 1. The characteristic of the disease agent that embodies capability to enter, survive, and multiply in the host. A measure of infectivity is the secondary attack rate.1
2. The proportion of exposures, in defined circumstances, that results in infection.


Prospective cohort studies: “The analytic epidemiological study in which subsets of a defined population can be identified who are, have been, or in the future may be exposed or not exposed, or exposed in different degrees, to a factor or factors hypothesized to influence the occurrence of a given disease or other outcome. The main feature of cohort study is observation of large numbers over a long period (commonly years), with comparison of incidence rates in groups that differ in exposure levels.”1

Canadian Health Measures Survey (CHMS): A national survey that collects information from Canadians about their general health and lifestyles. Data is collected voluntarily by an interview at home and fully trained health specialists in a mobile clinic collect physical health measures.2


Latency period: The period of time between exposure and disease manifestation.

Survivor-effect: A distortion in the estimate of the effect due to the inclusion of only disease survivors as participants in a study; those who have succumbed to the disease are not included.1

Nosocomial: Relating to a hospital. Arising while a patient is in a hospital or as a result of being in a hospital. Denoting a new disorder (unrelated to the patient’s primary condition) associated with being in a hospital.1

Retrospective chart review: A retrospective study uses existing data that have been recorded for reasons other than research. In health care these are often called “chart reviews” because the data source is the medical record.3