1. For the 2005 NYCHVS, the same sample was used as in 2002, drawn from Census 2000 but updated by including newly created housing units, while the 1991 NYCHVS sample was selected from the 1990 census and updated for the 1993, 1996, and 1999 NYCHVSs.

2. The 2005 and 2002 NYCHVSs were weighted based on the results from Census 2000 , while the 1991, 1993, 1996, and 1999 NYCHVSs were all weighted based on results from the 1990 decennial census. Since surveys based on different censuses produce different results (2000 versus 1990), it may be difficult to compare various data from the 2002 and 2005 NYCHVS to the 1999 NYCHVS and earlier years.

1. After reviewing all major issues concerning reweighting earlier data, we have decided not to reweight any prior NYCHVS surveys for the following reasons:
   
   • The NYCHVS surveys from the 1990s and from 2002/2005 are from different samples and are weighted based on different decennial censuses (1990 and 2000). We know that Census 2000 had a better coverage of population and housing in the United States and in New York City than did the 1990 census. Comparing estimates from the 1999 and previous NYCHVSs with the 2002 NYCHVS reflects this improved coverage. Simply reweighting earlier data would likely not solve inconsistencies in housing unit and population estimates between the two survey years.
   • In Census 2000, there was a major change in the way race data was collected in the decennial census. Respondents were allowed to choose more than one race, whereas prior to Census 2000, respondents could only select one race. This was an important difference in the Census 2000 from earlier censuses. All current surveys had to make this change in data collection. So, for the 2002/2005 NYCHVS, respondents were allowed to select more than one race, whereas in the 1999 NYCHVS and earlier surveys, only one race could be selected. Since race is an important part of the weighting process and because of this change in data collection, we don’t believe there is any way to make data for the 2002/2005 NYCHVSs consistent with earlier NYCHVSs.
   • If the 1999 NYCHVS data were reweighted, the 2002 NYCHVS data would also have to be reweighted. Reweighting the 2002 NYCHVS data would necessitate revising estimates of the components of inventory change from 1999 to 2002 and from 2002 to 2005. Therefore, reweighting the 1999 NYCHVS data would cause a chain reaction for three or more NYCHVSs: 1999, 2002, 2005, and probably beyond. In the year following each NYCHVS survey, we release the best data that we have at that time. Once these estimates are officially released, we believe they should be considered final. The NYCHVS survey results are ratio estimated to match independent housing unit estimates by borough AND independent population estimates by borough, age, race, and sex. As time goes on, these independent housing and population estimates may change as more updated data become available. We don’t believe these changes warrant reissuing the NYCHVS data every time this occurs.
2. In comparing data from 1999 and previous NYCHVSs with data from 2002 and 2005 NYCHVSs data users should compare percentages, means, and medians, rather than absolute numbers of housing units, households, and/or persons from survey to survey.

Some rental housing units in the survey occur in more than one rent regulation category. In order to assign such units to one mutually exclusive category, units matched to the sample file that were both HUD-regulated [because they receive subsidies from the U.S. Department of Housing and Urban Development (HUD)] and also rent stabilized, were classified in 2002 and previous HVSs in the final Control Status Recode as 'HUD Regulated'.

However, under this classification system fluctuations in the number of rent-stabilized units which were also HUD Regulated units (matched to the sample file) have created fluctuations in estimates of the number of rent-stabilized units. In order to have a consistent estimate of rent-stabilized units, the Census Bureau has revised the rent regulation classification system in 2005 to classify all rent-stabilized units as rent-stabilized, even if they are also HUD regulated. The revised classification provides the number of rent stabilized units in one survey year that is comparable with the number in another year.

The micro-data files for 2005 and 2002 (re-issued) incorporate two rent regulation classification codes, one under the old system and one under the revised system. For 2002, the new Control Status Recode Variable is located at position: 35-36 (occupied records) and 98-99 (vacant records). For 2005, the new Control Status Recode Variable is located at position: 260-261 (occupied records) and 100-101 (vacant records).

The New York City Housing and Vacancy Survey (NYCHVS) is a sample survey. Data from the NYCHVS are subject to sampling and non-sampling errors. Data users should have an understanding of standard errors of the data, as a measure of statistical reliability, particularly where sample sizes are small.

The NYCHVS is principally designed to provide statistically reliable data for New York City as a whole and for each of the five boroughs. Data for sub-borough areas are generally much less reliable than city-wide and borough data. Comparisons of sub-borough area data between survey years should be done with great caution, since the sample size for housing and household characteristics for sub-borough areas is small, relative to the entire city. The reliability of estimates of changes in such characteristics between surveys is likely to be extremely low, as seen below.

The sample estimate and its standard error enable one to construct a confidence interval. A confidence interval is a measure of an estimate's reliability. The larger the confidence interval in relation to the size of the estimate, the less reliable the estimate. For example, the standard error on the estimated rental vacancy rate of 4.36 in Borough Park in Brooklyn is 2.14 percentage points. The 90-percent confidence interval is calculated as 4.36 percent +/- (1.645 x 2.14), or 4.36 percent +/- 3.52, or from 0.84 percent to 7.88 percent. This means that if a census of every housing unit in the sub-borough had been taken, using exactly the same procedures as in the NYCHVS, there is a 90 percent chance that the actual rental vacancy rate would fall within the confidence interval.

Another example: In Borough Park, Brooklyn the estimated standard error on the number of renter housing units (33,784) in Borough Park, Brooklyn is 4,696. The 90-percent confidence interval is calculated as 33,784 +/- (1.645 x 4,696), or 33,784 +/- 7,725, or from 26,059 to 41,509.

As a final example - the rental vacancy rate in Stuyvesant Town/Turtle Bay in Manhattan was 2.89 percent in 2005, compared to 4.33 percent in 2002. Thus, the apparent difference between the two rates is 1.44 percent. The standard errors were 0.91 percent and 1.15 percent respectively. This means that the 90-percent confidence interval for the 2005 estimate of 2.89 covered the range 1.39 through 4.39. The 90-percent confidence interval for the 2002 estimate of 4.33 includes the range 2.44 through 6.22. The two confidence intervals overlap from 2.44 to 4.39. Therefore, we can conclude that the rental vacancy rates in Stuyvesant Town/Turtle Bay in 2002 and 2005 are not statistically different from each other at the 90-percent confidence interval.

For more details, the document on sample design, estimation procedures, accuracy of the estimates, and Topcoding is presented in Source and Accuracy of the Estimates. Estimated standard errors for many HVS data items are provided at Standard Errors of the Estimates.