Median year structure built divides the distribution into two equal parts: one-half of the cases falling below the median year structure built and one-half above the median. Median year structure built is computed on the basis of a standard distribution (see the "Standard Distributions" section under "Derived Measures"). Median year structure built is rounded to the nearest whole number. Median age of housing can be obtained by subtracting median year structure built from 2000. For example, if the median year structure built is 1967, the median age of housing in that area is 33 years (2000 minus 1967). (For more information on medians, see "Derived Measures".)

Data on year structure built are more susceptible to errors of response and nonreporting than data on many other items because respondents must rely on their memory or on estimates by people who have lived in the neighborhood a long time.

Data on year structure built were collected for the first time in the 1940 census. Since then, the response categories have been modified to accommodate the 10-year period between each census. In the 1980 census, the number of units built before 1940 appeared to be underreported. In an effort to alleviate this problem, a "Don't know" category was added in 1990. Responses of "Don't know" were treated like blanks and the item was allocated from similar units by tenure and structure type. However, this led to an extremely high allocation rate for the item (28 percent). A 1996 test proved inconclusive in determining whether a "Don't know" category led to a more accurate count of older units, but the test showed the allocation rate for this item was greatly reduced by the elimination of the "Don't know" category. As a result, "Don't know" was deleted for Census 2000.