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 (see age-specific rate and ASR below). The rate provides an approximation of 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.

Relative survival 
Relative survival 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 sex, age and calendar time at the time of diagnosis. It can be interpreted as the probability of patient survival in the absence of other causes of death. NORDCAN generally use cohort methods, following op patients for death for 1 and 5 years after diagnosis, and also 10 years for breast and prostate cancers. For the later periods not all patients can be followed up for 5 or 10 years, and calculations supplement with survival experience for patients diagnosed earlier years, called hybrid methods. Age-standardisation uses the International Cancer Survival Standards (ICSS).

International Cancer Survival Standards (ICSS). Three standard weight functions (W) for age-standardisation of relative survival as used in Nordic cancer survival study 1964-2003.
Standard ICSS 3 ICSS 2 ICSS 1
Characteristics Young adults Little age dependency Elderly
% of cancers 2.5 10.2 87.3
Cancer sites Testis, Hodgkin lymphoma Acute lymphatic leukaemia Bone Melanoma, cervix, brain, thyroid, soft tissue Other sites and summary groups
Age groups and weights (W) in % Ages W Ages W Ages W Ages W Ages W
0-29 31 0-29 31 0-29 7
30-39 21 30-39 13
40-49 13 30-49 34 40-49 16 0-49 36 0-49 12
50-59 19 50-59 17
50-69 20 50-69 20 50-69 41 60-69 22 60-69 27
70-79 10 70-79 16 70-79 29
70-89 15 80-89 5 70-89 23 80-89 7 80-89 15
90+ 0 90+ 0 90+ 0 90+ 0 90+ 0


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 the end of a given year, the survivors. For persons with more than one cancer the person is included as a prevalent cancer in each cancer site from the time of diagnosis of that cancer and in the 'All cancer but non-melanoma skin' group from the first diagnosis.
Total prevalence represents the number of previously diagnosed persons alive at the end of a year 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, five, ten and all years are presented in NORDCAN, 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. Prevalence is available both as numbers and as proportions per 100.000 persons. Age-standardised prevalence proportions are available for comparing countries, offering a choice of World(W), European(E) and Nordic(N) age-standards as for incidence and mortality.

Population at risk
The part of a population which is susceptible to develop a specific cancer. It is defined on the basis of demographic data, such as place of residence, sex, age group, etc. Years of risk duration are counted in person-years. 

Age-specific rate
The age-specific rate is calculated simply by dividing the number of new cancers or cancer deaths observed in a given age category during a given time period by the corresponding number of person years in the population at risk in the same age category and time period. For cancer, the result is usually expressed as an annual rate per 100,000 person-years. In NORDCAN, five-year age categories are used (highest group 85+).

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 or cancer deaths observed during a given time period by the corresponding number of person years in the population at risk. For cancer, the result is usually expressed as an annual rate per 100,000 persons at risk.

ASR (age-standardised rate)
An age-standardised rate (ASR) is a summary measure of the rate that a population would have if it had a standard age structure. Standardisation 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 ASR is a weighted mean of the age-specific rates; the weights are taken from population distribution of the standard population. The most frequently used standard population is the World Standard Population. The calculated incidence or mortality rate is then called age-standardised incidence or mortality rate (world). It is also expressed per 100000. The European standard is often used and NORDCAN gives the possibility of also choosing a Nordic standard where the age-distribution is from the NORDCAN population in 2000.

Age distributions of the standard populations used for age standardisation in NORDCAN (per 100,000)
Age group World
ASR (W)
European
ASR (E)
Nordic
ASR (N)
0-4 12000 8000 5900
5-9 10000 7000 6600
10-14 9000 7000 6200
15-19 9000 7000 5800
20-24 8000 7000 6100
25-29 8000 7000 6800
30-34 6000 7000 7300
35-39 6000 7000 7300
40-44 6000 7000 7000
45-49 6000 7000 6900
50-54 5000 7000 7400
55-59 4000 6000 6100
60-64 4000 5000 4800
65-69 3000 4000 4100
70-74 2000 3000 3900
75-79 1000 2000 3500
80-84 500 1000 2400
85+ 500 1000 1900
Total 100000 100000 100000

Cumulative risk
Cumulative incidence/mortality is the probability or risk of individuals getting/dying from the disease within a specified age interval within a time period. For cancer and from age 0, it is expressed as the number of new born children (out of 100) who would be expected to develop/die from a particular cancer before the age of 75 or (80 or 85) if they had the rates of cancer observed in the period in the absence of competing causes. Like the age standardised 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 unknown true rate or trend. The 95% (p-value = .05) and 99% (p-value = .01) confidence intervals are the most commonly used.

Estimated annual percentage change (EAPC)
The estimated annual percentage change is used to describe the magnitude of change in the trend on fitting a simple regression model to the log of the ASR. It is the average annual rate of change in the ASR over the time period selected.

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