National Household Travel Survey
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FHWA > Policy Information > NHTS > NHTS Frequently Asked Questions |
NHTS Frequently Asked Questions
1) Why can't I use 2001 NHTS data for my city or state?In order for the 2001 NHTS to provide statistically accurate estimates for your state or your MPO, a sufficient number of households in your state or your MPO must be interviewed in this survey. Unfortunately, limited resources have prohibited this level of sampling. As such, the 2001 NHTS data can not be used to calculate trip rates and travel statistics specific to your state or MPO. The only exception to this are the add-ons. That said, it is possible for you to use the 2001 NHTS data to estimate trip rates and travel statistics for your state or MPO (albeit not with the statistical rigor of the add-ons). Please see FAQ on "Will I be able to use the 2001 NHTS data to estimate travel in my state or planning area?" for details. 2) Will I be able to use the 2001 NHTS data to estimate travel in my state or my planning area?The 2001 NHTS was designed to provide statistically valid estimates of trips rates and travel statistics at the national level, specifically at the nine Census Division level, and at the seven area-type levels. The nine Census Divisions are:
The seven area types are with respect to population size and the availability of transit services:
If you are not an add-on area, you could use the 2001 NHTS data to estimate trip rates for your state or MPO by assuming that the travel behavior in your area is similar to that of the sampled households in states within the same Census Division as yours, and in areas with a population size similar to that of your MPO. To identify sampled households in an area that is similar to yours with respect to Census Division, population size, and the availability of transit services, please use variable CDIVMSAR. There is another approach that you can use to estimate travel for your state or planning area. Please see Transferability of National Data for Local Estimates for details. 3) What is an add-on area?An add-on area is either a state or a planning organization that purchased additional sampling of households in its state/area to be surveyed. The rationale is to increase the number of sample households in the state/area so that trip rates and travel statistics can be reliably estimated at that geographic level. Four states and five planning areas purchased additional samples in the 2001 survey, adding over 40,000 sample households to the NHTS. The add-ons and the target sample sizes were:
The final sample size for the 2001 NHTS was targeted at 65,055 = (25,000 for the national sample + 40,055 for the add-on samples) households. Statistically developed sample weights (i.e., factors that expand data collected from the sample households to represent the entire nation) do not allow add-on samples to overpower the national sample and to bias the national level estimates. 4) What are the similarities and differences between the 1995 NPTS and the 2001 NHTS?There are a number of improvements and changes in the 2001 NHTS:
In addition, a number of questions were added to the 2001 NHTS to cover emerging trends pertinent to personal travel behavior: At the Household Level.
At the Person level.
At the Individual Daily Trip level.
At the Individual Long distance Trip Level.
5) Why does a household member have his/her information in the household file but not in the person file?The 2001 NHTS data collection consisted of two main phases:
It was conducted once per household member. If a household member did not complete the person-level interview and if no proxy completed this interview for him/her , then his/her information would NOT be included in the person file. Note that as long as a household member (or a proxy) completed his/her personal level interview, there will be a record for him/her in the person file regardless of whether he/she took any trips on the designated travel day. 6) What was the rule in designating whether a sample household was a "complete" household?The concept of a "complete" household is used in the context of survey responses. The NHTS used a fifty-percent rule in deciding whether a household was a complete household. The fifty-percent rule states that if fifty or more percent of the adults (18 years or older) within a household completed the Person Interview (or Extended Survey), then this household was considered a complete household. The person weights were adjusted to account for non-interviewed persons within an interviewed household. The fifty-percent rule was adopted to address a concern that excluding households where some of the household members did not respond to the survey might bias the survey results. This concern was confirmed in a Federal Highway Administration-sponsored report where low-income households and large households (with four or more members) were found more likely not to respond to surveys than other households. In the NHTS, the household file contains information on all members of the household (such as age, gender, and employment and drivers license status) regardless of whether all of the members responded to the Person Interview (or Extended Survey). |
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This page last modified on 07/29/08 |