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Interpreting Rankings Data
A natural reaction of some
readers when looking at charts that rank their province's or territory's cancer rates is to seek explanations
as to why their province or territory has higher incidence rates for some cancers than other provinces or than
the national average. Some may be alarmed that exposure to environmental carcinogens may be
responsible when in fact there are several other more likely explanations. The following
points should be kept in mind when interpreting these rankings:
Variations in Populations and Health Behaviours: Some differences in cancer rates among
provinces may be explained by differences in known risk factors among the populations of
those provinces. For example, one finds higher rates of lung cancer and other tobacco-associated
cancers in provinces/territories with higher rates of smoking. Although environmental carcinogens are
responsible for some cancer cases, a majority of cases appear to be caused by lifestyle
factors such as smoking, and geographic variations in cancer incidence are thought
largely to reflect variations in these lifestyle factors.
Variations in Medical Care: Variations among provinces in medical care factors may
also result in differences in cancer incidence rates. In provinces or territories where higher percentages
of the population participate in cancer screening, such as PSA testing for detecting
prostate cancer, more cancers will be diagnosed. Screening leads to earlier detection
of tumours that have a better prognosis and may at times find tumours that grow so slowly
that they would not otherwise be recognized in a person's lifetime. Therefore, the cancer
rate without additional information only tells part of the story.
Influence of Aging on Cancer Rates: The likelihood of being diagnosed with cancer
increases steadily with age. These incidence rates have been adjusted for age so that
provinces can be compared without fear that differences in their rates result from differences
in the age distribution of their populations. However, this adjustment may be imperfect if
the relationship between age and cancer risk is not the same for all provinces.
Measuring Burden: The significance of a cancer as a public health problem for a province
is a function more of the absolute rate of that cancer than of its incidence in that province as
compared to others.
Completeness of Cancer Data: Provinces and territories contribute cancer data to these charts if their
registries collected 90% or more of the cancers diagnosed in 2005. Because provinces vary in
their completeness from 90-100%, rankings may vary to a minor extent because of differences
in reporting completeness.
Random Factors and Cancer Rates: Even if registries were able to collect 100% of
diagnosed cancer cases, there would still be some uncertainty in computed cancer rates
because many factors contribute to the incidence rate in any given year or province, and some
factors exhibit random behaviour. Chance plays a role in determining if and when cancer
develops in an individual, whether that cancer is detected, and whether the information
is entered into the cancer registry. For these reasons, the reported incidence rates are
expected to vary from year to year within a province or territory even in the absence of a general trend.
Caution is warranted, therefore, when examining cancer rates for a single year, and
especially when the rates are based on a relatively small number of cases
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