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The current version of the standard occupation classification (SOC) was updated in 2000. The next revision is underway and will be published in 2010. Currently, the last revision was in 1980.

Here is the link to the OMB notice about the final decisions for the 2010 SOC:

http://www.bls.gov/soc/home.htm#Revision

There are two differences between the 1980 and 2000 standard. On a technical note, there is more detail in 2000. The 2000 code is a six-digit code, whereas in 1980 it was a four-digit code. The dash between the third and fourth digits is for presentation purposes only. The coding scheme is as follows:

Description                  Location                     Number of categories
Major group                 First two digits                  23
Minor group                 Third digit                       96
Broad occupation            Fourth and fifth digits          449
Detailed occupatin          Sixth digit                      821

For example, "Life, Physical and Social Science Occupations" (19-0000) is divided into four minor groups, "Life Scientists" (19-1000), "Physical Scientists" (19-2000), "Social Scientists and Related Workers" (19-3000), and "Life, Physical and Social Science Technicians" (19-4000). Life Scientists contains broad occupations such as "Agriculture and Food Scientists" (19-1010), and "Biological Scientists" (19-1020). The broad occupation Biological Scientists includes detailed occupations such as "Biochemists and Biophysicists" (19-1021), and "Microbiologists" (19-1022).

The second change is the coding scheme is substantive. In 2000, people who work together in the same job family are grouped together in their major occupational group. There are 23 major groups. For instance, the major group for doctors, nurses, technicians, etc. is:

29-1000 Health Diagnosing and Treating Practitioners.

An exception to the rule about grouping workers in a job family together is supervisors. First-line managers and supervisors of production, service, and sales workers who spend more than 80 percent of their time performing supervisory activities are classified separately in the appropriate supervisor category, since their work activities are distinct from those of the workers they supervise.

Thus, all farm workers are not in the same major group. The farmer or rancher is coded as 11-9012, where 11-0000 is [Management occupations], while farm workers are in the following major group: 45-0000 [Farming, Fishing, and Forestry occupations].

The following are some additional Standard Occupational Classification resources. They include crosswalks between the census and SOC codes, downloadable versions of the codes, coding instructions for surveys, frequently asked questions, etc.

Census Bureau site on SOC

Bureau of Labor Statistics site on SOC

Related to occupational changes over time (based on census occupational codes):

BLS has produced a document that proposes a harmonized system for 1960-2000 census occupation codes:

BLS Working Paper: Proposed Category System for 1960-2000 Census Occupations

Another resource is out of the Integrated Public Use Microdata Samples (IPUMS). They have a standard coding scheme for census occupation codes based on a 1950 starting point. Here is a link to their coding:

http://usa.ipums.org/usa-action/codes.do?mnemonic=OCC1950