Survey of Market Absorption of Apartments (SOMA)

Source and Accuracy of the Estimates - Fourth Quarter 2006

The Survey of Market Absorption (SOMA) is a sample survey and 
consequently all statistics in the report are subject to sampling variability. 
Estimates derived from different samples would likely differ from these.
90-percent confidence intervals for statistical comparisons are shown with each 

The SOMA adopted new ratio estimation procedures in 1990 to derive
more accurate estimates of completions.  Please use caution when 
comparing the number of completions in 1990 and following years with  
those in earlier years.

The U.S. Census Bureau designed the survey to provide data concerning the rate at which
privately financed, nonsubsidized, unfurnished units in buildings with five or more units
are rented or sold (absorbed).  In addition, the survey collects data on characteristics 
such as number of bedrooms, asking rent, and asking price.

Buildings for the survey came from those included in the Census 
Bureau's Survey of Construction (SOC).  (Refer to Section V of for further details on the SOC 
sample design.) For the SOC, the United States is first divided into primary sampling units 
(PSUs), which are stratified based on population and building permits.  The PSUs to be used 
for the survey are then randomly selected from each stratum.  Next, a sample of geographic 
locations that issue permits is chosen within each of the selected PSUs.  All newly 
constructed buildings with five units or more within the sampled places and a 
subsample of buildings with one to four units are included in the SOC.

For the SOMA, the Census Bureau selects, each quarter, a sample of buildings with 
five or more units that have been reported in the SOC as having been completed 
during that quarter.  The SOMA does not include buildings in areas that do not issue 
permits.  In each of the subsequent four quarters, the proportion of units in the 
quarterly sample that are sold or rented ("absorbed") are recorded, providing data  
for absorption rates 3, 6, 9, and 12 months after completion. 


The Census Bureau publishes preliminary estimates for a given quarter and 
may revise these estimates in ensuing quarters.  Each quarter, some of the 
absorption data for some buildings arrive after the deadline for that quarter's 
report; these late data appear in a revised table in the next quarterly report.   
Finalized data appears in the H-130, Market Absorption of Apartments
annual report.

Beginning with data on completions in the fourth quarter of 1990
(which formed the basis for absorptions in the first quarter of 1991),
the Census Bureau modified the estimation procedure and applied the 
new procedure to the data for the other three quarters of 1990
so that annual estimates using the same methodology for four quarters
could be derived.  The Census Bureau did not perform any  additional 
re-estimation of past data.

Using the original estimation procedure, the Census Bureau created unbiased 
estimates by multiplying the counts for each building by its base weight 
(the inverse of its probability of selection) and then summing over all 
buildings.  Multiplying the unbiased estimate by the following ratio estimate
factor for the country as a whole provided the final estimate:

     total units in buildings with five or more units in permit-issuing
     areas as estimated by the SOC for that quarter* divided by 
     total units in buildings with five or more units as estimated by
     the SOMA for that quarter

*(Beginning with January 2001 completions, the SOC revised its methodology 
for estimating the number of units completed for 5+ multi-unit structures.  See
for these changes.  Thus, use caution when comparing data from 2001 and 
forward to any estimates prior to 2001.) 

In the modified estimation procedure, instead of applying a single
ratio-estimate factor for the entire Nation, the Census Bureau computes 
separate ratio-estimate factors for each of the four census regions.  
Multiplying the unbiased regional estimates by the corresponding 
ratio-estimate factors provides the final estimates for regions.  The 
Census Bureau obtains the final  estimate for the country by summing 
the final regional estimates.

This procedure produces estimates of the units completed in a given
quarter which are consistent with the published figures from the SOC 
and also reduces, to some extent, the sampling variability of the
estimates of totals.

Absorption rates and other characteristics of units not included in
the interviewed group or not accounted for are assumed to be identical
to rates for units about which data were obtained.  The noninterviewed and
not-accounted-for cases constitute less than 2 percent of the sample
housing units in this survey.


The SOMA is a sample survey and consequently all statistics in this report
are subject to sampling variability.  Estimates derived from different samples 
would likely differ from these. 

Two types of possible errors are associated with data from
sample surveys: nonsampling and sampling errors. 

In general, nonsampling errors can be attributed to many sources:
inability to obtain information about all cases in the sample,
difficulties with definitions, differences in interpretating questions,
inability or unwillingness of the respondents to provide correct
information, and data processing errors. Although no direct measurements 
of any bias that might result from nonsampling errors has been obtained, 
the Census Bureau thinks that most of the important response and operational 
errors were detected during review of the data for reasonableness and consistency. 

Sampling Errors
The particular sample used for this survey is one of many
possible samples of the same size that could have been
selected using the same sample design. Even if the same
questionnaires, instructions, and interviewers were used, estimates
from different samples would likely differ from each other. The
deviation of a sample estimate from the average of estimates from all 
possible samples is defined as the sampling error. The standard error of 
a survey estimate provides a measure of this variation and, 
thus, is a measure of the precision with which an estimate from 
a sample approximates the average result from all possible samples.

If all possible samples were selected, if each was surveyed under 
the same general conditions, and if an estimate and its estimated 
standard error were calculated from each sample, then:

     Approximately 90 percent of the intervals from 1.645 standard
     errors below the estimate to 1.645 standard errors above the
     estimate (i.e., the 90-percent confidence interval) would include
     the average result from all possible samples.

This report uses a 90-percent confidence level as its standard for statistical 

Beginning with data for completions in the second quarter of 1999, the
Census Bureau implemented a new procedure for computing standard errors. 
The new procedure may result in differences in standard errors derived using
the prior methodology, so standard errors were revised back to the third  
quarter of 1998.

The estimates in this report show the totals, percents, and medians with the 
90-percent confidence interval.
For very small estimates, the lower limit of the confidence interval
may be negative. In this case, a better approximation to the true
interval estimate can be achieved by restricting the interval estimate
to positive values; that is, by changing the lower limit of the
interval estimate to zero.

The average result from all possible samples either is or is not
contained in any particular computed interval. However, for a
particular sample, one can say with specified confidence that the
average result from all possible samples is included in the constructed

Contact George Boyd at 301-763-3199 or mail to for further information on the Survey of Market Absorption of Apartments Data.

Source: U.S. Census Bureau, Social, Economic, and Housing Statistics Division