Glossary

Glossary of Statistical Terms

Incidence: Incidence is the number of new cases arising in a given period in a specified population. This information is collected routinely by cancer registries. It can be expressed as an absolute number of cases per year or as a rate per 100,000 persons per year. The latter provides an approximation to the average risk of developing a cancer.
Mortality: Mortality is the number of deaths occurring in a given period in a specified population. It can be expressed as an absolute number of deaths per year or as a rate per 100,000 persons per year.
Survival: It is defined as the probability of survival, expressed as time elapsed since diagnosis (1,3 5-year survival). This observed survival probability is influenced by mortality both from cancer of interest and from other causes. For this reason, relative survival is usually calculated. It is defined as the ratio of the observed survival in the group of patients to the survival expected in a group of people in the general population, who are similar to the patients with respect to all possible factors affecting survival at the beginning of the follow-up period, except for the disease of interest.
Prevalence: The prevalence of a particular cancer can be defined as the number of persons in a defined population who have been diagnosed with that type of cancer, and who are still alive at a given point in time. Complete prevalence represents the proportion of people alive on a certain day who previously had a diagnosis of the disease, regardless of how long ago the diagnosis was, or if the patient is still under treatment or is considered cured. Partial prevalence, which limits the number of patients to those diagnosed during a fixed time in the past, is a particularly useful measure of cancer burden. Prevalence of cancers based on cases diagnosed within one, three and five years are presented in GLOBOCAN 2002, as they are likely to be of relevance to the different stages of cancer therapy, namely, initial treatment (one year), clinical follow-up (three years) and cure (five years). Patients who are still alive five years after diagnosis are usually considered cured since the death rates of such patients are similar to those in the general population. There are exceptions, particularly breast cancer.
Population at risk: The part of a population which is susceptible to have a specific cancer. It is defined on the basis of demographic data, such as place of residence, sex, age group and (where appropriate) ethnicity.
Crude rate: Data on incidence or mortality are often presented as rates. For a specific tumour and population, a crude rate is calculated simply by dividing the number of new cancers observed during a given time period by the corresponding number of people in the population at risk. For cancer, the result is usually expressed as an annual rate per 100,000 persons at risk.
ASR (age-standardized rate or age-adjusted rate): An age-standardized rate (ASR) is a summary measure of a rate that a population would have if it had a standard age structure. Standardization is necessary when comparing several populations that differ with respect to age because age has such a powerful influence on the risk of cancer. The most frequently used standard population is the World standard population. The calculated incidence rate is then called the World Standardized incidence rate. It is also expressed per 100,000.
Cumulative rate/Cumulative risk: Cumulative incidence is the probability or risk of individuals getting the disease during a specified period. For cancer, it is expressed as the number of new born children (out of 100, or 1000) who would be expected to develop a particular cancer before the age of 65 (or 70, or 75) if they had the rates of cancer currently observed. Like the age standardized rate, it permits comparisons between populations of different age structures.
Standard error: The standard error of a rate is a measure of the sampling variability of the rate.
Confidence interval: A range of values that has a specified probability of containing the rate or trend. The 95% (p-value = .05) and 99% (p-value = .01) confidence intervals are the most commonly used.
Annual percent change (APC): Annual Percent Change is used to measure trend or the change in rate over time. It is the 'average' annual rate of change over the time series selected.