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Construction Length of Total Time Statistics Methodology

Summary

This supplement presents data on the monthly progress of construction projects reported to be completed in 2000-01, 2001-02, 2002-03, 2003-04, 2004-05, 2005-06, 2006-07, 2007-08, 2008-09, 2009-10, 2010-11, and 2011-12. The tables show the average length of time from start to completion for privately-owned nonresidential construction, projects owned by state and local governments and privately-owned multi-family projects.

Tables 1 and 2 show the average number of months from start to completion by value category and type of construction for private nonresidential and state and local government projects respectively. Statistics are also shown for six value categories within type of construction. For example, for projects completed in 2011 and 2012, table 1 shows that the average number of months for all values of private commercial construction was 5.6 months. It also shows the average number of months for private commercial construction valued between $250,000 and $999,000 was 4.8 months. Table 3 shows the average number of months from start to completion by value category and number of units for private multi-family projects.

Tables 4 through 9 show the proportion of construction value put in place each month for all types of private nonresidential (tables 4 and 7), state and local (tables 5 and 8) and private multi-family (tables 6 and 9) projects. In tables 4 and 5, the projects are sorted into six value categories. Table 6 is sorted into four value categories. For each category, the total value of all projects completed was calculated. Next, the value of work done on all projects in their first month of construction, second month, and so on was expressed as a percentage of the total. For example, looking at private nonresidential projects completed in 2011 and 2012 valued between $250,000 and $999,000 on table 4, 20.6 percent of the value was put in place in the month of start and 24.3 percent of the value was put in place in the second month. These projects could have started in any month, but all were completed in 2011 or 2012. In tables 7, 8 and 9 the projects are sorted by the month that the construction started. Similar to tables 4, 5 and 6 for each month of start, the total value of all projects that were completed during 2011 and 2012 was calculated. Next, the value of work done on all projects for each month was expressed as a percentage of the total. For example, the January column consists of all projects completed in 2011 or 2012 that had a start date in January. Looking at table 7, private nonresidential projects have 6.2 percent of the value put in place for the month of start and 7.2 percent of the value was put in place in the second month.

The value of the completed project is based on the cumulative value of work done on the project each month from start to completion. Projects with selection values of less than $75,000 are not included in these statistics.

Sampling Variability

These statistics are estimated from the samples of projects used to collect monthly value of private nonresidential, state and local and private multi-family construction put in place. In the surveys, owners are asked to report the amount of work done on their projects each month from start through completion. These surveys and their methodologies are described on the Internet at: http://www.census.gov/constructionspending

The particular sample selected is one of a large number of similar probability samples that, by chance, might have been selected using the same specifications. Each of the possible samples would yield somewhat different sets of results. The standard error of an estimate is a measure of the variation among the estimates from all possible samples and, thus, is a measure of the precision with which an estimate from a particular sample approximates the average from all possible samples. Relative standard errors, or coefficients of variation, for average number of months from start of construction to completion are included in tables 1a, 2a and 3a. Standard errors of the proportion of construction value in place each month are included in tables 4a through 9a.

The sampling estimates and the standard errors of those estimates may be used to define confidence intervals; that is, ranges that would include the average estimate from all possible samples. A 90-percent confidence interval is defined to be from 1.645 standard errors below the estimate to 1.645 standard errors above the estimate. For example, table 1 shows that the average number of months from start to completion for private commercial projects completed in 2011 and 2012 valued between $250,000 and $999,000 is 4.8 months and table 1a shows the relative standard error of this estimate is 3.1 percent. Multiplying 4.8 months by 0.031, we obtain 0.149 months as the standard error. To obtain a 90-percent confidence interval, multiply 0.149 months by 1.645 and add and subtract the result from 4.8 months, yielding limits of 4.555 months and 5.045 months. One can say that the average number of months from start to completion for private commercial projects valued between $250,000 and $999,000 is between 4.6 months and 5.0 months with a specified confidence of 90%.

As calculated for this study, the standard error measures sampling errors, but does not measure all nonsampling error in the data. Nonsampling error consists of both a variance component and a bias component. Bias is the difference, averaged over all possible samples with the same size and design, between the estimates and the true value being estimated. Nonsampling errors can be attributed to definitional difficulties, inability to obtain information about all cases in the sample, differences in interpretation of the questions, and errors made in processing the data.


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Source: U.S. Census Bureau | Construction Spending Survey | (301) 763-1605  Last Revised: September 03, 2013