Using the American Community Survey Benefits and Challenges (2007) / Chapter Skim
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5 The Weighting of ACS 1-Year Period Estimates
5 The Weighting of ACS 1-Year Period Estimates
Pages 184-208
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From page 184... ...
responses are collected from a subsample of the housing units that have not yet responded. For purposes of analysis, the Census Bureau classifies a monthly sample as the sample units resolved in that month (the tabulation month)
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From page 185... ...
Given this definition of the monthly samples, all the data used for analysis for a 1-year or multiyear period are collected during the specified calendar year or years. (An attraction of using tabulation months is that data collection is completed at the end of the year; if the monthly samples were defined in terms of sample months, it would be necessary to wait until the following February before all the data were collected for a given year.)
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From page 186... ...
The person weights are adjusted to make the weighted person counts for major demographic subgroups in an estimation area conform to independent population subgroup estimates obtained by updating counts from the last census.
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From page 187... ...
In particular, substantial variation in the weighting adjustments can appreciably affect some ACS estimates, likely reducing bias, but probably also lowering precision. Information on the distribution of the weighting adjustments is provided below for some of the adjustments that were used with the 2004 ACS test survey.
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From page 188... ...
and 50 percent (1 in 2) for mailable addresses, with higher subsampling rates for census tracts expected to have lower mail and CATI response rates (see Table 2-3, Part B)
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From page 189... ...
THE WEIGHTING OF ACS -YEAR PERIOD ESTIMATES Total weight of all units sampled for the month (excluding the CAPI factor) VMS = Total weight of all units resolved in the month (including the CAPI factor)
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From page 190... ...
Over all months of the 2004 ACS test survey, the value of the VMS factor for the 5th percentile is 0.87 and that for the 95th percentile is 1.24. The effect of this additional variation in the resulting weights on sampling errors is likely to be small.
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From page 191... ...
The first stage of the noninterview adjustment (NIF1) is carried out within cells created by the cross-classification of building type and census tract, with census tracts combined if the cells contain too few responding housing units.
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From page 192... ...
The MBF adjustments are generally small, with only 5 percent being 0.96 or less and 5 percent being 1.03 or more, but the combined effects of their use, together with the noninterview adjustment factors in step 3, on the biases and sampling errors of ACS estimates are not transparent. The MBF procedure, in essence, has three steps.
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From page 193... ...
These sampling errors are likely larger than those based on the NIF1 and NIF2 (step 3) adjustments alone, because the NIFM adjustments are applied only to CAPI cases and also because CAPI cases start with higher base weights because of the subsampling.
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From page 194... ...
There are several issues concerning the use of the housing unit control factor, including the quality of the PE housing unit estimates. These issues are taken up in Section 5-C below.
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From page 195... ...
is to identify any extremely large weight adjustments to the base weights over all the subsequent steps in the weighting process. To avoid large increases to sampling errors, the weighting adjustments are revised by collapsing adjustment cells if any overall weight adjustment factor exceeds 8.
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From page 196... ...
Recommendation 5-1: The Census Bureau should conduct an in-depth review of the weighting scheme used for producing ACS 1-year period estimates and assess a range of alternative schemes that might improve the quality of the estimates. 5-C HOUSING UNIT CONTROLS After step 4 in the weighting process, the weighted total number of all units sampled from the MAF for the year is equated to the number of units in the MAF from which the sample was selected.
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From page 197... ...
is introduced to attempt to compensate for the failure of the ACS to cover units occurring since the January sample selection and for other deficiencies in the MAF as a sampling frame. Another reason given by the Census Bureau for the adjustment is so that any undercoverage or overcoverage of housing units in the ACS is addressed similarly to how undercoverage or overcoverage of population is addressed by the use of the PE population estimates in step 6.
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From page 198... ...
Nevertheless, the postcensal housing unit estimates are likely subject to appreciable error in some types of counties. Some of these errors may be random in nature, but some may be systematically biased upward or downward for certain types of counties.
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From page 199... ...
In the case of undercoverage, the factors increase the weights of MAF housing units to represent those not included in the MAF frame under an assumption that the missed housing units are missing at random. This assumption would be false if, for instance, the proportion of vacant units in the postcensal estimates is higher than in the MAF, as it might well be.
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From page 200... ...
The differences between the increase in the MAF and the increase in the housing unit estimates varied among counties as a function of county size, with larger differences occurring for counties with larger numbers of housing units. The Census Bureau plans to conduct more research to gain an understanding of large discrepancies between the MAF counts and the postcensal housing unit estimates.
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From page 201... ...
As with the PE housing unit estimates, the population estimates start from the 2000 census counts and adjust for estimated changes between April 1, 2000, and July 1 of the year in question. At the outset, the 2000 census population is divided into the household population and the group quarters population.
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From page 202... ...
The Census Bureau has conducted an evaluation of the 2000 county total population estimates by comparing them with the 2000 census counts, in a similar way to the evaluation of the housing unit estimates described in the previous section (Blumerman and Simon, 2006)
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From page 203... ...
For this purpose we dropped the race classification because of the problems with the differences in that classification between the population estimates and the 2000 census, retaining only Hispanic versus non-Hispanic (equivalent to collapsing the first five of the six race/ethnicity categories listed above)
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From page 204... ...
TABLE 5-4 Mean Absolute Percentage Error (MAPE) of July 1, 1999, Estimation Area Population Estimates Compared with April 1, 2000, Census Counts, by Cells Based on Combinations of Sex, Ethnicity, and Age (Excludes Cells with Fewer Than 500 People in the 2000 Census)
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From page 205... ...
In particular, the population estimates classified by ethnicity are subject to appreciable error. The large number of excluded cells with the cross-classifications involving ethnicity and age groups should be noted: the MAPE values given in Table 5-4 represent only a part of the total population, and the values for the excluded part may be very different.
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From page 206... ...
0 TABLE 5-5 Percentage Ratio of July 1, 1999, Estimation Area Population Estimates to April 1, 2000, Census Counts, by Cells Based on Combinations of Sex, Ethnicity, and Age (Excludes Cells with Fewer Than 500 People in the 2000 Census) Number of Population Classification Cells 80%– 90%– 95%– 100%– 105%– 110%– 120%+ Total <80% No subclassification 1,950 0.1 2.4 14.6 61.4 20.6 0.7 0.2 0.0 100.0 Sex 3,900 0.1 2.9 15.0 58.9 21.8 0.9 0.2 0.0 100.0 Ethnicity 3,401 19.7 7.5 9.1 36.8 18.3 3.1 3.0 2.4 100.0 Age group 23,397 2.0 13.9 20.0 26.4 20.6 10.2 5.8 1.1 100.0 Sex by ethnicity 6,130 16.1 7.3 9.3 39.1 19.8 3.3 2.7 2.4 100.0 Sex by age group 46,430 2.5 14.8 19.2 24.8 19.7 11.0 6.6 1.3 100.0 Ethnicity by age group 28,553 8.5 13.1 16.9 22.4 19.2 10.3 7.0 2.5 100.0 Sex by ethnicity by age group 53,020 6.6 13.9 17.1 22.3 19.1 11.0 7.7 2.3 100.0 NOTE: Estimation areas are large counties and groups of smaller counties with at least 16,000 people; the median size is 55,000 people; the average size is 145,000 people; the District of Columbia is included as an estimation area (information from the U.S.
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From page 207... ...
. In contrast, the population controls used in the ACS are midyear population estimates based on different residence rules and different sources than the yearly accumulation of ACS monthly samples.
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From page 208... ...
Concomitantly, the panel views research on methods to improve the postcensal population estimates as a priority area for the Census Bureau. An important component of that research should be to investigate using ACS data more fully than currently in producing national estimates of international migration and particularly for estimating domestic migration.
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