Household Identification Number: HOUSEID (sometimes called ID9)

The Household ID number was constructed for the January 2004 Release of the National Household Travel Survey Data Set.  It is a unique household identification number for each household in the sample.  Data users should interpret the ID only as a unique identifier and not attempt to ascribe further meaning to the number.  If needed, other variables associated with each household, person, vehicle and trip (i.e., SMPLFIRM, SMPLSRCE, SMPLAREA) should be used to determine the data collection contractor, the sample source location, and the geographic sample component (e.g., National sample NY Add-on, etc.).

The ID9 was constructed in the following manner:

1.  The data were collected by two firms: Westat and Morpace.

2.  Westat collected the National Sample households (a population-weighted random sample representing the entire US) and households from two enhanced Add-on samples (New York and Wisconsin).  Morpace collected data from seven enhanced Add-on samples (Baltimore, Des Moines, Hawaii, Kentucky, Lancaster, Oahu and Texas).

3.  As independent data collection contractors, Westat and Morpace differed in the way each constructed case IDs at the time that the samples were drawn.  Westat constructed a unique 8-digit all-numerical ID for each household both within and across the National, NY and WI samples - there were no duplicated numbers either within or across the combined National, NY and WI samples.  Morpace provided a unique 5 character numerical ID for each household within each of its add-on samples.  To facilitate the determination of weights for the combined Westat and Morpace samples, Morpace also provided a 2-character location identifier (e.g., TX for Texas) with each case.  These were combined into a 7-character ID.  It's format was AAnnnnn, with the first two alpha characters (AA) representing the Add-on sample location and the next five numeric digits (nnnnn) an ID number within the location.  Morpace did not prevent duplication of the numerical components of its IDs across samples - consequently there were duplicated numerical components of its IDs across the seven Morpace samples.

4.  For the January 2004 Release of the NHTS data, DOT and Westat prepared a combined Westat and Morpace data set, thus a unique ID was needed for each household. 

5.  Westat converted the various case ID formats from each data collection firm's samples to a single ID format, producing a unique ID for each household.   This format both preserved the numerical component of each data collection contractor's original case IDs, identified the data collection firm and identified the sampling "site" of each sample household (where site is the sample location in which the household's telephone number is associated). 

"HOUSEID is the variable representing the new ID in the January Release of the NHTS."  It is a character variable that is 9 characters in length (consequently called "ID9").  It was constructed as follows:

• 1st Character:  Sample "site".  A numeric identifier representing the household's location in terms of it's area code/exchange/hundred block.  It was an intermediate descriptor that was useful during the weighting process but was not retained among the delivered variables.  It does not represent either the contractor that interviewed the household nor the actual geographic location of the household.  It may have the following values:
  • 1=Lancaster, PA
  • 2=Baltimore, MD
  • 3=Kentucky
  • 4=Des Moines, IA
  • 5=Texas
  • 6=Hawaii
  • 7=Oahu
  • 8=New York
  • 9=Wisconsin
  • 0=Remainder of US
    
• 2nd Character:  Firm identifier.  A Numeric or Alphabetic character that indicates whether the household was interviewed by Westat or Morpace.  It's values were "M" if the case was surveyed by Morpace, or a numerical value (the first character of Westat's original 8-digit ID) if the case was surveyed by Westat.
• 3rd and 4th Characters:  A filler set to "00" for Morpace sample, or the 2nd and 3rd characters of Westat's original 8-digit ID.
• 5th - 9th Characters: All five numeric characters from the original Morpace sample ID, or the remaining 5 numeric characters from the original Westat ID.