The NAICS codes are hierarchical, as are the rows of the data tables. It's important to add rows only at the same level of industry detail - if you add together NAICS codes at more than one level, you could get a sum greater than the true total. Theoretically, any sector (2-digit NAICS) total should equal the sum of its component subsectors (3-digit NAICS), each 3-digit total should correspond to the sum of its component 4-digits, and so forth.
There are two reasons why the NAICS detail shown in a table may not add to the total:
- Below the national level, NAICS categories are published only when they meet certain minimum criteria that vary from industry to industry. For example, county or city figures are shown only for those manufacturing industry categories that have at least 500 employees. Therefore, the detailed figures may not add to higher level totals because some categories simply aren't shown.
- Some figures are withheld (replaced with "D") to avoid disclosure of confidential information. Data suppressed with a "D" at one level are still included in higher level figures. However, the number of establishments is never suppressed, so you may be able to confirm additive relationships with establishment counts that you can't with other figures. Finally, in cases where the sales or payroll figures are suppressed, the employment data are shown in ranges (e.g., 20-49 employees).
For more information, see Data Manipulation and Reading Data Sets on the 2007 Economic Census Web page.