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Comparing Historical Data

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Things to Consider When Reviewing Historical Data

When working with historical data you need to keep in mind such things as the release of Economic Census data in the form of tables or publications, industry comparability, geographic changes, changes to the scope of the economic census, & definition changes.

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Considerations when Comparing Economic Censuses

Economic censuses provide comparable data collected at fixed intervals and with consistent definitions across decades. However, the reports from any one economic census typically include little historical data. The assembly of time series—such as the growth of retailing in a particular area or trends in a particular manufacturing industry—is left to you. In that process, you should consider the following comparability issues:

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Data Tables vs. Library Publications

Starting in 2002, data from the Economic Censuses was released as tables available online in American FactFinder. Pages linking to some of the most popular tables in FactFinder, by sector, are available in the Data Tables section of this site.

Before 2002, most Economic Census data was released as official Census Bureau publications. Pages highlighting each of these publications are available on the Publications page within the Library section of this site.

Additionally, scanned images in PDF and other formats are available for publications from the 1967 to 1987 Economic Censuses via the search at archive.org, using terms like "1987 census of retail trade Virginia." Collections of hard-copy printed reports are maintained at certain major libraries; and individual reports may be accessible through interlibrary loan.

The Census Bureau Library has print volumes of the economic census starting with the 1935 Census of Business through the 1997 Economic Census. The library is able to provide scans of selected pages. The library also has print copies of the County Business Patterns for 1948-1997. Reference librarians are available to consult with users by phone (301-763-2511) and e-mail (library@census.gov).

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Industry Comparability

1997 Conversion to NAICS from SIC

The implementation of the North American Industry Classification System (NAICS) in 1997 affected the availability of comparable information across time periods. Its predecessor, the Standard Industrial Classification (SIC) system, was updated in 1967, 1972, and 1987, and each time a significant number of new industries were introduced into the existing framework. The introduction of NAICS changed the entire framework.

Nearly half of the SICs in use in 1992 could be derived from 1997 NAICS industries. However, a substantial number of industries can only be approximated under NAICS. The 1997 Economic Census was particularly important because census questionnaires identified industry components finely enough that data could be categorized under either NAICS or SIC.

As a result, certain key data could be published according to the old system as well as the new. The report Comparative Statistics, 1997 and 1992, presented the number of establishments, sales, employment, and payroll for each SIC for the nation and each state, for both 1997 and 1992. Thus, basic SIC-by-state time series can be carried backward from 1997 to 1987, and farther to the extent that particular industries were not affected by SIC changes in 1987, 1972, and 1967.

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With some limitations due to revisions in 2002, 2007, and 2012 NAICS time series can go forward from 1997, but they cannot generally go backward to earlier years, because many NAICS categories require information that was not collected in 1992 and earlier censuses. For example, NAICS 45321, Office Supplies and Stationery Stores, differs from SIC 5943, Stationery Stores, primarily by the addition of certain office supply stores that were previously classified in Wholesale Trade. Economic census questionnaires prior to 1997 did not separately differentiate Office Supply Stores from other kinds of office supply wholesalers, so NAICS 45321 cannot be estimated for prior periods.

Correspondence tables between the old and new systems are available at the NAICS Web site. These tables show for each NAICS industry the SIC categories or parts thereof that comprise them, and for each SIC industry the NAICS industries or parts thereof to which their establishments were likely to be reclassified.

The 1997 Economic Census report Bridge between 1997 NAICS and 1987 SIC takes that correspondence a significant step further by showing the number of establishments, sales, employment, and payroll at the national level for each of those intersections between the old and new systems.

For example, the Bridge report shows that the number and sales of those office supply stores that were transferred into retail trade from wholesale trade increased sales of the new retail Office Supply and Stationery Stores category by nearly ten-fold over the SIC industry of the same name.

The Bridge tables can be so useful in that you might be tempted to "update" historical figures from SIC to NAICS by applying proportions derivable from the tables. You should employ such "synthetic estimation" with caution. Nationwide proportions may not reflect relationships in particular geographic areas or in prior time periods. Many new industries reflected in NAICS did not exist to a significant extent in prior periods (e.g., satellite telecommunications and video rental stores).

At broader levels of classification, the changes between SIC and NAICS were further confounded by the rearrangement of the hierarchy. The Service Industries division of the SIC was subdivided into five new sectors and parts of four others. Less noticeable were shifts affecting sectors—such as Manufacturing, Wholesale Trade, and Retail Trade—that retained their status as sector titles in NAICS but were affected by changes in scope. Retail Trade was roughly 10% smaller under NAICS than under SIC just because Eating and Drinking Places, which accounted for roughly 10% of SIC-based Retail Trade sales and a third of SIC-based Retail Trade employment, were transferred to the new Accommodation and Foodservices sector. Retail Trade losses were offset partially by transfers between Retail Trade and Wholesale Trade, such as the Office Supply Stores mentioned above. Manufacturing also shrank somewhat under NAICS because significant components were reclassified elsewhere.

Industry Revisions within NAICS: 2002, 2007, and 2012

Industry revisions are a regular feature of NAICS, occuring every 5 years. They are typically minor in scope (relative to the massive change that was undertaken in 1997).

2002 NAICS

For 2002, industries were redefined and renumbered within Construction and Wholesale Trade, and there was some rearrangement of industries within the Information sector. Several new industries were defined within Information and Retail Trade without disrupting the availability of data comparable to that published for 1997. These modifications did not affect the comparability of sector totals.

2007 NAICS

Changes between 2002 NAICS and 2007 NAICS were relatively minor, but did affect four sector totals. NAICS 2007 introduced two new industries:

  • Biotech Research and Development
  • Executive Search Services

At the same time, several industries in the information sector were consolidated:

  • Paging into Wireless Telecommunication
  • Cable Program Distribution and most Internal Service Providers (ISPs) into Wired Telecommunication
  • Web Search Portals into Internet Publishing and Broadcasting

Real Estate Investment Trusts (REITs) were dispersed and mostly moved from the Finance and Insurance sector to Real Estate.

2012 NAICS

The 2012 Economic Census introduces new NAICS categories for solar, wind, geothermal, and biomass electric power generation, but the most significant change is substantial consolidation of industries within the manufacturing sector, reducing 473 industries to 364.

Resources for comparing different NAICS years.

For each period of changes there are three resources:

  • Correspondence tables on the NAICS Web site, generally available before the reference year.
  • Bridge Between [year] and [prior year] NAICS, showing essentially the same changes as the correspondence tables, but with statistics that allow the signifcance of each change to be evaluated, generally available three to four years after the reference year.
  • Comparative Statistics, [year] and [prior year], a presentation of the latest economic census data reclassified according to the previous system for direct comparison with data from the previous census, generally available three to four years after the reference year.

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Scope of Economic Census Programs

Prior to 1992, the economic census program covered less of the American economy. In 1987 and earlier years, the economic census did not include the Finance, Insurance, and Real Estate division of the SIC; and it included only selected transportation industries within the Transportation, Communication, and Utilities division of the SIC. The addition of those components boosted the coverage of the economic, agriculture, and governments censuses collectively from roughly 76% of the gross domestic product in 1987 to about 98% in 1992. The coverage of service industries had also expanded in 1967, 1977, and 1987.

In 2002, the economic census expanded to include four relatively small industries previously out of scope: Landscape Architecture, Landscaping Services, Veterinary Services, and Pet Care.

In 2007, the economic census expanded to include all scheduled passenger air transportation. Even with these additions, total coverage still rounds to 98% of the United States Economy.

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Geographic Comparability

Trends over time are most readily measured for the nation, states, and counties. County boundary changes are few and far between, while many metropolitan areas, places, and ZIP Codes change boundaries over time.

Metropolitan areas are redefined after each population census and new criteria are generally introduced at that point. Most metropolitan areas tabulated for 2007 are defined the same as they were for 2002, while most metro areas were redefined between the 1997 and 2002 Economic Censuses, and in a similar manner many metro areas were be redefined in between the 2007 and 2012 Economic Census.

The boundaries of cities and other places may have been modified due to annexations or other changes during the intervening period. In 2007, the scope of places published changed, including Census Designated Places for the first time, but excluding small incorporated places with 2,500 to 4,999 inhabitants that had been included in earlier censuses. For 2012, the criteria changed again to include any place with 2,500 people as of the 2012 Population Estimates or 2,500 jobs.

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Definition Changes

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Industry Classification Changes

The North American Industry Classification System (NAICS) is updated periodically. Changes are implemented during economic census years.

To confirm that data are comparable, data users need to review:

  • the NAICS and Standard Industry Classification (SIC) correspondence tables and
  • the Bridge Reports, which show the effect of industry classification changes between economic censuses.

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Geographic Boundary Changes

Geography boundaries may change over time. The geographic areas covered by County Business Patterns and Nonemployer Statistics are benchmarked to the year of the last economic census.

To confirm that data are comparable, data users need to review the Geographic Change Notes.

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Data Field Changes

Data field names and definitions may change over time.

To confirm that data are comparable, data users need to review:

  • the data field definitions. Click on the data field name to get the definition.

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