The three cancer statistics discussed in the following paragraphs are always adjusted for age. The main rationale for the adjustment is to make comparisons across years "fair" in the face of an aging population (to compare prostate cancer mortality in 1970, say, with prostate cancer mortality in 2003). Because older people are more likely to both get and die from virtually all adult cancers, without age adjustment incidence and mortality rates would continue to rise as the population ages. Health officials, however, want to distinguish changes in the frequency of cancer independent of the effects of an aging population.
Age adjustment helps do this. For convenience, I'll just talk in terms of mortality here, though the adjustment is exactly the same for incidence. First, mortality rates for the year of interest are calculated for each five-year age group (e.g., 30–34, 35–39). The result is 10 to 15 so-called age-specific rates (the exact number depends on how the very young and very old are grouped). To arrive at a single number, a summary rate is calculated: that is, a weighted average of the agespecific rates. The "weight" each age-specific rate gets depends on how many people were in the age group in 2000 (the so-called standard population).
So a mortality statistic from one year let's say 1970 is made comparable to another let's say 2003 by applying the same weights obtained for the 2000 population standard. Thus, if 5% of the 2000 population is age 30–34, then 5% of the 1970 mortality rate comes from 30–40-year-olds, as does 5% of the 2003 rate. An age-adjusted rate can thus be interpreted as what the 1970 (or 2003) death rate would have been if the population age distribution was exactly what it was in 2000.
Note that if cancer patients live longer, mortality will fall even if they still die of cancer. That is because the older age groups are smaller (i.e., there are fewer 80–84-year-olds than 75–79-year-olds), and they receive increasingly less weight in the summary rate. So delaying death to an older age has the effect of lowering the mortality rate (as it should).
In the case of five-year survival, cancer epidemiologists don't talk about age adjustment as such. Instead they talk about a "five-year relative survival rate." The fundamental motivation is identical, however. Consider the following: the typical age of a newly diagnosed prostate cancer patient is 70 years, that of a newly diagnosed testicular cancer patient is 33 years. Without knowing anything about the cancers, you can guess which cancer will probably have the higher five-year survival rate simply because younger people are more likely to live five more years than 70-year-olds are. The "relative" survival rate thus adjusts for the effect of age by comparing observed survival for patients with cancer to that expected in similar-aged people without cancer.
For example, the observed five-year survival rate for men with prostate cancer is 75%. But remember, many of these men are older and many die from other diseases, not prostate cancer. The expected five-year survival rate in men the same age without prostate cancer (the "general" population) is 78%. The five-year relative survival rate is the ratio of the two: 75%/78%, or 96 %. The five-year relative survival rate is thus the proportion alive five years later relative to what would be expected in the general population. Because fiveyear survival in an average joe is always lower than 100%, the fiveyear survival rate actually observed will always be lower than the relative survival that is reported. The effect of this is trivial in the young, but large in the elderly.
Our website is not responsible for the information contained by this article. Webworldarticles.com is a free articles resource thus practically any visitor can submit an article. However if you notice any copyrighted material, please contact us and we will remove the article(s) in discussion right away.
This article was sent to us by:
Fiona Landman at
08152010
1. Understanding the Mesothelioma Cause
All articles in this directory are property of their respective authors. Additionally, read our Privacy Policy
© 2010 WebWorldarticles.com - All Rights Reserved. Partners: Gunblade Saga