The denominators (population estimates) used to calculate rates in four WISH
modules (Mortality, Injury Mortality, Injury Hospitalizations, and Injury
Emergency Department Visits) are revised each year to incorporate new data from
the National Center for Health Statistics (NCHS). An additional methodological
revision was made to the WISH denominator series in May, 2007. This note briefly
describes these revisions.
The denominator revisions may affect rates by race/ethnicity
generated in WISH. Rates by age group and sex are not affected, except for minor
differences due to rounding. Users are advised to document the WISH access
date to assist in reconciling differences in rates prepared at different times.
To provide rates by race and Hispanic ethnicity, WISH uses bridged
race population estimates (exit DHS) from NCHS. These are published in annual
"vintages" that correspond to the year of release (e.g., vintage 2006
estimates are released in 2006). When NCHS releases a new vintage, estimates for
previous years extending back to the 2000 Census are revised to incorporate the
newly available data. These race/ethnicity revisions are incorporated into the
WISH denominators, meaning that rates by race/ethnicity for specific years may
change as new vintages are incorporated.
However, we also adjust the NCHS estimates so that when summed
by county, age group and sex they equal the estimates produced by the
of Health Informatics (OHI), which are not revised. This means that
rates by county, age group and sex for specific years in WISH will remain
constant, despite the addition of a new NCHS vintage each year.
WISH users are advised to investigate the potential impact of
revised denominators on conclusions drawn from rates by race/ethnicity generated
in the affected WISH modules. In practical terms, compare rates generated at
different times and see how big a difference the revised denominators make.
Adjusting the NCHS estimates so they equal the OHI estimates means that the
distributions of populations by race/ethnicity in the two data series differ
from one another. However, a methodological change was made in May, 2007 that
reduced the extent to which the OHI estimates differ from the NCHS estimates.
This change was applied to all estimates extending back to 1990. The change may
affect rates by race/ethnicity generated prior to May, 2007, so WISH users are
again advised to investigate the potential impact on conclusions derived from
The mean absolute percent differences between the OHI and the
NCHS estimates for both methods ("New" versus "Old") are
shown in the table below. These percentages are for 1990-2005 and are shown by
race and Hispanic ethnicity.
Mean absolute percent difference between
OHI and NCHS estimates: 1990-2005
|Non-Hispanic American Indian
|Non-Hispanic Black/African American
The table shows that the largest improvements are among the
non-White populations: percent differences between the OHI and NCHS estimates
were cut at least in half among three of four non-White groups and the percent
difference among Hispanics was reduced by 2.2 percentage points, or 36 percent.
Briefly, the methodological change involved inserting an additional step
into the adjustment process. Under the old method the estimates were produced
and adjusted so they equaled the OHI estimates in a single step, while the new
method first produces "raw" estimates, then adjusts them to equal the
Under the new method, the first step in creating the
"raw" estimates is to divide the NCHS estimates in individual cells by
the NCHS total county population. This produces a set of proportions that
correspond to the distributions found in the NCHS bridged race file. These
proportions are then applied to the OHI total county population, which results
in the set of raw estimates. The adjustment step then consists of dividing the
OHI estimates by the sum of the raw estimates for each corresponding county,
sex and age group, which produces a set of adjustment factors. The raw estimates
are then multiplied by the corresponding adjustment factor. This produces the
WISH denominators by county, age group, sex, race, and Hispanic ethnicity.
The two-step procedure in the new method results in bridged race
denominators that a) are adjusted so they equal the OHI population estimates;
and b) generally are closer to the NCHS bridged race estimates compared with
those produced under the old method.
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Last Revised: March 01, 2012