MAKING IT USEABLE    Interpreting Estimates       Example Strata

The strata for the East North Central Division is typical of those for other Divisions. The Division is subset into four strata:

Metropolitan Statistical Areas of One Million or More Without Elevated Rail/Subway, Metropolitan Statistical Areas of One Million or More With Elevated Rail/Subway, Metropolitan Statistical Areas of Less Than One Million Without Elevated Rail/Subway, and Non-Metropolitan Statistical Areas Without Elevated Rail/Subway.

Strata	Census Division	Metropolitan Population Status	Availability of Elevated Rail or Subway	Number of Sample Households
9	East North Central	1,000,000 or more	Absent	920
10	East North Central	1,000,000 or more	Present	1,084
11	East North Central	less than 1,000,000	Absent	1,411
12	East North Central	Non-Metropolitan	Absent	810

Representative data can be run for each of the strata. For example, if vehicle trips per household are computed for the first strata, the 920 households in the sample strata should represent all households in that area, within the range of acceptable statistical variance.

Because of the overall design of the NPTS, it is possible, with care, to use the data for other geographic units. As a precursor to any discussions of this matter, it is necessary to understand more about sampling error and sampling bias.

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