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In this report, Indiana cancer incidence numbers and age-adjusted Indiana cancer incidence rates for the year 1996 are presented. Rates and numbers are reported for the state as a whole, and for each of the 92 counties, individually. Rates are also given for both sexes combined, and for males and females individually. In addition, the state and county cancer incidence rates and the black and white population rates for selected cancers are compared. The data utilized in calculating these rates was that available to the Indiana State Cancer Registry as of January 24, 2001. These cases represent 88.6% of the estimated number of cases to be diagnosed in Indiana in 1996, as calculated using the method of estimating completeness of case ascertainment specified by the North American Association of Central Cancer Registries (NAACCR).
By convention, cancer incidence rates do not include carcinoma in situ (with the exception of bladder cancer in situ), nor do they include basal and squamous cell carcinomas of the skin. The numbers and rates of reported cancers that appear in all Table Is, all Table IIIs, Table V and Table VI follow this convention.
In contrast, in situ and skin cancers are included in the numbers given in Table II and all Table IVs since these tables concern cancers diagnosed by stage. Thus, the total numbers in the two types of tables will not "match."
The cancer incidence rate is the number of new cancers of a specific site or type occurring in a specified population during a year, expressed as the number of cancers per 100,000 people. It should be noted that the numerator of the rate can include multiple primary cancers occurring in one individual. This rate can be computed for each type of cancer, as well as for all cancers combined. These rates are age-standardized to the U.S. 1970 standard million population to allow for comparisons between groups (geographic or demographic) that have different age distributions.
When comparing rates over time or across different populations, crude rates (the number of newly-diagnosed cancer cases per 100,000 persons) can be misleading because differences in the age distributions of the various populations are not considered. Since cancer is age-dependent, the comparison of crude incidence rates from cancer can be especially deceptive.
Age-adjusted rates take into account the diverse age distributions of the populations. Valid comparisons between age-adjusted rates can be made, provided the same standard population and age groups have been used in the calculation of the rates. The direct method of adjustment was used to produce the age-adjusted rates for this report. In this method, the population is first divided into reasonably homogeneous age ranges and the age-specific rate is calculated for each age range; then each age-specific rate is weighted by multiplying it by the proportion of the standard population in the respective age group. The age-adjusted rate is the sum of the weighted age-specific rates.
Comparison of Rates
Rates based on small numbers of events over a given period of time or for sparsely populated geographic areas should be viewed with caution. These rates show considerable random variation and are considered "unstable," which limits their usefulness in comparisons and estimation of rare occurrences.
In this report, by convention, whenever the number of cases of any type of cancer is less than 5 at the county level, the actual number is not reported to protect the privacy of these individuals. An asterisk (*) will denote this in all tables. If the number of cases of any type of cancer is less than twenty, the rate generated is considered "unstable" and is either marked with a double asterisk (**) when given in the tables or suppressed (in Table IIIs).
"Higher" or "Lower" in the Table IIIs denote a county rate that was significantly higher or lower, respectively, than the rate for Indiana as a whole. A comparison between rates was considered statistically significant if the difference between the rates would have occurred by chance less than five times out of 100 (i.e., p < 0.05). Statistical tests were conducted using the Z-score method, described in all basic statistics texts.
While the form of the Z test is similar, the calculation of the standard error for an age-adjusted rate is not straightforward. The following formula is used to calculate the standard error of an age-adjusted rate:1
where, in this case, SEAArate is the standard error for the age-adjusted rate, stdmili is the standard million for the ith age group, counti is the number of cases for the ith age group, and populationi is the relevant population for the ith age group. Subsequently, the Z test for the difference between the state and county rates takes the form:
Z = (State rate - County rate) / SE(diff)
where the SE(diff) is defined as:
SE(diff) = [(SE(s))2 + (SE(c))2 ]1/2
where SE(s) is the standard error of the state rate and SE(c) is the standard error of the county rate.
When testing the differences between the Indiana rate and the rate for each of the 92 counties for a given cancer, the large number of comparisons increases the chance that a rate would be statistically significant due to chance alone. To account for multiple comparisons, based on Bonferroni's theory of inequalities,2 the significance level for each individual comparison was set equal to 0.05/92, where 92 is the number of comparisons made for each type of cancer. Thus, any individual comparison with an associated "p" value less than 0.0002 (Z=3.48) was considered to be statistically significant.
1. A Guide to Using SEER*Stat, Version 3.0, National Cancer Institute, Cancer Statistics Branch, DCCPS.
2. Snedecor, G.W. Statistical Methods, Seventh Edition, The Iowa State University Press, 1980.
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