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How Changes in U.S. Census Counts Af fect NCI Cancer Rates [Annual SEER's Update]
I received this through another list server, and
thought it would be of interest so some on this list.
> -----Original Message-----
> From: Grissom, Mike [mailto:mikeg@SLAC.Stanford.EDU]
> Sent: Monday, April 21, 2003 4:16 PM
> To: 'Medhp-Sec (E-mail)'
> Subject: MEDHP-SEC: US NIH FS re: How Changes in
> U.S. Census Counts
> Affect NCI Cancer Rates [Annual SEER's Update]
. . .
> The following NCI Fact Sheet was provided on the
> National Institutes of Health's NCI Web site:
>
> NOTE:Inaccurate census data can be a confounder
> in radioepidemiology studies and disease
> incidence calculations.
>
>
-----------------------
NCI FACT SHEET: How Changes in U.S. Census Counts
Affect NCI Cancer Rates
Posted Date:
Tuesday, April 15, 2003
Print Version
Key Points
• Because NCI cancer rates are calculated by
dividing the number of cancer cases (numerator) by a
census-generated denominator, the rates can be heavily
influenced by changes or uncertainties in census
counts. (Question 1)
• Revisions to population estimates will affect
the denominator in all SEER cancer rate calculations
for 1990-2000. (Question 3)
• Because calculations are based on 2000 Census
counts, the majority of rates will have a greater
degree of certainty, not less. The potential for
variability or uncertainty will be greatest for the
smallest population groups -- particularly specific
race or age groups and county-level residents.
(Question 4)
• Even with updated census figures, overall cancer
rates for major cancers will not change significantly.
(Question 6)
The National Cancer Institute (NCI) today released its
annual update of national cancer rates on its Web
site, http://seer.cancer.gov/csr/1975_2000/index.html.
The Cancer Statistics Review includes incidence,
mortality, and survival rates for all cancers for the
period 1975-2000 using new population estimates from
the U.S. Census Bureau. These data are also used to
track cancer trends over time.
1. How does NCI use census data to calculate cancer
rates?
NCI's Surveillance, Epidemiology, and End Results
(SEER) Program calculates cancer rates using
incidence, survival, and mortality data gathered
through two sources: incidence data from SEER, and
death data from the National Center for Health
Statistics (NCHS).
To determine a cancer rate, NCI divides the number of
cancer cases or deaths in a given geographic area
(numerator), by the total number of people in that
area as reported by the Census Bureau (denominator).
The resulting cancer rate is the proportion of people
in that area affected by cancer. Because SEER rates
are calculated by dividing the numerator by the
census-generated denominator, the rates can be heavily
influenced by changes or uncertainties in census
counts.
2. How often does the Census Bureau update and revise
population estimates?
The Census Bureau routinely updates and revises
population estimates every year. The bureau calculates
'intercensal' estimates after a new census is
completed -- for example, using information from both
the 1990 and 2000 Censuses, the bureau obtains better
estimates for the 1990s. These revisions are based on
the most recent census information and on the best
available demographic data reflecting components of
population change (namely, births, deaths, net
internal migration, and net international
immigration). Recalculating disease rates to reflect
updated population estimates is standard practice.
3. How did the Census Bureau's procedures change in
2000?
In the 1990 Census, respondents were asked to "select
one" racial classification (White, Black, Asian or
Pacific Islander, American Indian or Alaska Native.).
The 2000 Census asked respondents to "select one or
more" race groups and separated the Native Hawaiian
and other Pacific Islanders group from the Asian
group, resulting in a total of 31 different
classifications. For agencies such as the NCI and NCHS
to continue reporting long-term trends in disease
rates for single-race groups, a method is needed to
"bridge" these multi-race classifications into a
single race category. Such a method was developed by
the NCHS using information collected as part of their
National Health Interview Surveys. In collaboration
with NCHS, the Census Bureau produced a set of year
2000 population estimates that assigned everyone to
one race group only. The resulting 2000 estimates were
then used to produce an improved set of 1990-2000
population estimates. NCI and NCHS are making these
bridged population estimates available on their
respective Web sites. These revisions to the
population estimates will affect the denominator in
all SEER cancer rate calculations for 1990-2000.
The Census Bureau has other studies and research in
progress and expects to make final intercensal bridged
estimates available sometime in 2004.
4. Is there still uncertainty in the cancer rates and
if so, where?
Although efforts were made to use the best available
data and methods to produce the bridged estimates, the
calculations themselves could introduce a small amount
of uncertainty. However, because these calculations
are based on 2000 Census counts, the majority of rates
will have a greater degree of certainty, not less. The
potential for variability or uncertainty will be
greatest for the smallest population groups --
particularly specific race or age groups and
county-level residents.
In less populated areas such as rural counties, or in
adjacent urban and suburban areas where there was
substantial migration of residents, a change in the
denominator can affect the county rate by as much as
20 percent -- unlike large counties, where a small
change in a large denominator will not affect rates
nearly as much.
If a new census population estimate is larger than an
earlier one and the number of cancer cases remains the
same, the new rate will be smaller. Similarly, the
rate will increase if the census population decreases.
5. Can you give a specific example of an area where
rates will be affected by Census 2000?
Projecting population shifts in specific geographic
areas is one of the greatest sources of uncertainty in
trying to produce accurate census counts. For example,
the population counts for Blacks in the Atlanta
metropolitan area are higher than previously estimated
due to suburban migration that the Census Bureau had
not fully captured during the 1990s (before the 2000
Census information became available). This means that
cancer rates are actually lower for Blacks in
metropolitan Atlanta than originally calculated by
SEER's use of the Census Bureau's earlier population
estimates. In general, differences between projected
populations and actual census counts are more likely
to occur in small areas such as counties and
metropolitan areas than in entire states or regions of
the country.
It is important to note that these population changes
will not affect actual counts in the number of cancer
cases or deaths, just the cancer rates calculated for
these populations. Furthermore, interpreting these new
cancer rates fully will take time since there are many
ways in which population changes have occurred.
6. Will these updated census counts significantly
affect cancer rates?
No. Even with updated census figures, overall cancer
rates for major cancers will not change significantly.
Some rates for less common cancers, cancers in
minority populations, smaller geographic areas (such
as counties), or specific age groups may be affected.
For instance, updates to American Indian and Hispanic
populations will result in slight increases in their
cancer rates. Although it is too soon to tell the
extent to which these rates may change reported cancer
rates, overall changes are relatively small -- for
example, 1 percent or 2 percent.
7. Are there other factors that could cause cancer
statistics to be adjusted?
Normal reporting delays (how long it takes to collect
statistics on new cases of cancer from SEER regional
offices) and other adjustments, such as more
information on race, ethnicity, or tumor
characteristics, could affect cancer statistics. A
study of these factors by NCI researchers was reported
in the Oct. 16, 2002 issue of the Journal of the
National Cancer Institute (volume 94, pages
1537-1545). This study focused on areas where quicker
accumulation of regional cancer statistics could give
NCI more reliable numbers for the most recent rates.
The best estimate of rectification of reporting delays
and corrections to the most recent year of incidence
data would be a change in existing rates by about 3
percent to 6 percent.
8. Does the 1975-2000 SEER Cancer Statistics Review
reflect corrections to statistics that have been wrong
for many years?
No. NCI is making small adjustments (about 1 percent
to 3 percent) to some cancer statistics, and larger
adjustments to statistics in certain areas of the
country and to selected cancers. NCI statistical
reports from previous years have been the best rates
that could be calculated given the data that were
available at those times. Adjustments currently taking
place primarily relate to rates in minorities based on
the new categories, to improved completeness and
coverage of cancer statistics in certain geographic
areas, or to population updates based on the recent
2000 Census. Overall, however, the trends and rates
reported annually by NCI and its partners will not
change substantially.
9. Where can I learn more about bridging and get other
information about NCI cancer statistics?
For more information on bridged population estimates,
please go to
http://www.cdc.gov/nchs/about/major/dvs/popbridge/popbridge.htm
.
For complete information on cancer statistics, please
go to NCI's "Finding Cancer Statistics" Web site at
http://surveillance.cancer.gov/statistics/.
For additional information on bridging and other
modifications to NCI population estimates, please go
to
http://seer.cancer.gov/popdata/.
=====
-- John
John Jacobus, MS
Certified Health Physicist
e-mail: crispy_bird@yahoo.com
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