ingly, persons who would have been eligible for TES but were not in a sampled block cluster were assigned a zero weight.

The TES had the desired effect of reducing the variance of the DSE estimates for poststrata. The reduction in the average and median coefficient of variation (the standard error of an estimate as a percentage of the estimate) for poststrata was 22 percent, similar to an average reduction of 20 percent for the nationwide extended search operation in 1990 (Navarro and Olson, 2001:7).

The TES operation in the A.C.E. was methodologically more complex than the corresponding operation in the 1990 PES. At the time of the decision not to use the original DSE estimates of the population to adjust census data for redistricting in March 2001, the Census Bureau cited concerns that the TES may have been un-balanced, thereby introducing bias into the DSE. Suggestive of an imbalance, which could occur if the P-sample and E-sample search areas were not defined consistently for the TES, was the larger increase in the P-sample match rate (3.8 percentage points) compared with the E-sample correct enumeration rate (2.9 percentage points) (Navarro and Olson, 2001:Table 1). Such an imbalance may also have occurred in 1990, when the extended search increased the P-sample match rate by 4.1 percentage points and the E-sample correct enumeration rate by 2.3 percent. A follow-up study to the 1990 census was not able to determine whether balancing error had occurred (Bateman, 1991).

A subsequent evaluation, which used data from two TES follow-up studies that rechecked certain kinds of housing units (Adams and Liu, 2001:i), determined that the larger increase in the TES in the P-sample match rate compared with the E-sample correct enumeration rate was due to P-sample geocoding errors and E-sample classification errors that did not affect the DSE. P-sample geocoding errors were the primary explanation; they occurred when P-sample address listers mistakenly assigned addresses from surrounding blocks to A.C.E. block clusters. When the original A.C.E. clerk did not find matches for these cases in the A.C.E. block cluster because there were no corresponding census addresses, then a search for matches in the surrounding ring was likely to be successful. If the TES had not been conducted, these matches would have been missed, resulting in an underestimate of the P-sample match rate and an overestimate of the DSE population estimate and the net undercount.

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