Interpreting Rankings Data
A natural reaction of some readers when looking at charts that rank their state's cancer rates is to seek explanations as to why their state has higher incidence rates for some cancers than other states 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:
- Differences Among Racial and Ethnic Populations: Some cancers have different incidence rates for different racial and ethnic populations. For example, breast cancer rates are usually higher in white women than in women of other racial and ethnic populations, and prostate cancer rates are higher in black men. Therefore, when comparing incidence rates across states, the racial makeup of the state's population should be taken into account. Although adjustment of rates by race and ethnicity can take such differences into account, presentation of rates for specific racial and ethnic populations (as opposed to statistical adjustment) may be preferable and is more easily understood by a lay audience.
- Variations in Populations and Health Behaviors: Some differences in cancer rates among states may be explained by differences in known risk factors among the populations of those states. For example, one finds higher rates of lung cancer and other tobacco-associated cancers in states 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 states in medical care factors may also result in differences in cancer incidence rates. In states 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 tumors that have a better prognosis and may at times find tumors 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 states 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 states.
- Measuring Burden: The importance of a cancer as a public health problem in a state is more a function of the absolute rate of cancer rather than the state's relative ranking in incidence. For example, Utah has proportionately fewer smokers than other states and also has the lowest lung cancer incidence of any state. Nevertheless, in Utah lung cancer kills more people than any other cancer, a fact that might be overlooked if one focused only on its low ranking in incidence relative to other states. Also, the true burden of cancer on the health care system and economy of a state is determined by the number of people diagnosed with cancer and not by the cancer rate. The rate is the number of cases divided by the population. Therefore, the observation that a cancer rate in a state appears high relative to other states may obscure the fact that the absolute number of cases is not large.
- Completeness of Cancer Data: States contribute cancer data to these charts if their registries collected 90% or more of the cancers diagnosed. Because states 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 state, and some factors exhibit random behavior. 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 state 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.