The OSCA structure

The OSCA structure has five levels – major group, sub-major group, minor group, unit group and occupation.

Major groups are the broadest groups of jobs in OSCA. They are formed using a combination of skill level and broad skill specialisation. Major groups are designed for producing summary data. Each major group is denoted by a unique one-digit identifier.

Sub-major groups are subdivisions of each major group. They are formed using a combination of skill level and finer skill specialisation. Each sub-major group is denoted by a unique two-digit identifier. Within each major group, sub-major groups are ordered by skill level, and then alphabetically. Residual ‘other’ sub-major groups are always shown last.

Minor groups are subdivisions of each sub-major group. They are formed using a combination of skill level and finer skill specialisation. Each minor group is denoted by a unique three-digit identifier. Within each sub-major group, minor groups are alphabetically ordered. ‘General’ minor groups are listed first. Residual ‘miscellaneous’ minor groups are listed last.

Unit groups are subdivisions of each minor group. They are formed using a finer application of skill specialisation and, where necessary, skill level. Most unit groups have a single skill level. Each unit group is denoted by a unique four-digit identifier. Within each minor group, unit groups are alphabetically ordered. ‘General’ unit groups are listed first. Residual ‘other' unit groups are listed last.

Occupations are subdivisions of each unit group. They are the finest application of skill specialisation. Each occupation has a single skill level. Each occupation is denoted by a unique six-digit identifier. Within each unit group, occupations are alphabetically ordered. ‘General’ occupations are listed first. Residual ‘not elsewhere classified’ (nec) occupations are listed last.

The following diagram summarises the OSCA structure. It includes an illustrative example.

Each major group comprises a different number of sub-major groups, minor groups, unit groups and occupations. The following table illustrates the distribution between the major groups.

The elements of each major, sub-major, minor and unit group are:

• identifier – the numerical representation of the group
• title – describes the group
• lead statement – describes the skill specialisations relevant to the group
• inclusion and exclusion statement(s) (optional) – clarifies the scope of the group. They list specific occupations that are included or excluded from the group.
• skill level statement – describes the skill level(s) relevant to the group
• subcategories – lists the groups in the level immediately below the group

The elements of each occupation:

identifier – the numerical representation of the occupation

principal title – the title which best describes the occupation. The most common title is usually used. There are exceptions in cases where the most common title is too broad or too narrow in meaning.

lead statement – describes the skill specialisation relevant to the occupation. Any registration or licensing requirements are also shown.

inclusion and exclusion statement(s) (optional) – clarifies the scope of the occupation. They list specific occupations that are included or excluded from the occupation.

alternative title(s) – any commonly used alternative title (or titles) for the occupation. They have the same meaning as the principal title but are less commonly used.

specialisation(s) – any common subset of jobs within the occupation. They involve the performance of a subset of specialised tasks within the occupation.

skill level – one of OSCA’s five skill levels which is relevant to the occupation

main tasks – the set of tasks associated with the occupation. The set represents the main tasks only. The set is not intended to be comprehensive of all tasks associated with the occupation.

A one, two, three, four and six-digit identifier is assigned to each major, sub-major, minor group, unit group, and occupation respectively. Each identifier is unique and shows hierarchical relationships within the classification. For example, Occupation 371132 Stonemason is part of:

• Major Group 3 – Technicians and Trades Workers
• Sub-major Group 37 – Building Structural Trades Workers
• Minor Group 371 – Bricklayers, Stonemasons and Concreters
• Unit Group 3711 – Bricklayers and Stonemasons

An identifier may not refer to the same group of jobs between versions of OSCA. The ABS will aggregate and disaggregate occupations over time to reflect changes in the labour market. An example is the separate identification of an emerging occupation from a not elsewhere classified occupation. The emerging occupation is assigned the next available six-digit identifier in the sequence. The not elsewhere classified occupation retains its six-digit identifier (ending in ‘99’) however now refers to a smaller group of jobs.

Identifiers ending with a ‘9’ denote a residual category. For example, Sub-major Group 89 Other Labourers refers to all jobs in Major Group 8 that are not classified within the following Sub-major Groups:

• 81 Cleaner and Laundry Workers
• 82 Construction and Mining Labourers
• 83 Factory Process Workers
• 84 Farm, Garden and Forestry Workers, or
• 85 Fast Food Cooks and Food Preparation Assistants.

Similarly, Occupation 462699 Sportspersons nec refers to all jobs within Unit Group 4626 Sportspersons which are not elsewhere included in the following Occupations:

• 462631 Footballer
• 462632 Golfer, or
• 462633 Jockey.

OSCA 2024 v1.0 contains 86 ‘not elsewhere classified’ (nec) occupations.

Each residual group is part of the OSCA structure used to classify jobs that do not meet design constraints for example, Harbour Master. These groups of jobs may be listed within each nec occupation.

Identifiers ending in zero are used to process responses which cannot be coded to an occupation, but can be coded to a major, sub-major, minor or unit group. These are called ‘not further defined’ (nfd) identifiers. For example, a response of ‘Internal Medicine Specialist’ is not detailed enough to be classified to any occupation. It can be coded to Unit Group 2643 Physicians which comprises all internal medicine specialists. The ‘Internal Medicine Specialist’ response is coded to supplementary identifier 264300 Physicians, nfd.

Identifiers commencing with a ‘0’ (for example 099888 ‘Inadequately Described’) are used to process and store responses which do not contain sufficient information to be coded to any part of OSCA. These supplementary identifiers are also used for responses which are outside of the scope of OSCA such as ‘housewife’, ‘pensioner’ and ‘student’.

Supplementary identifiers are not part of the OSCA structure. They exist for operational reasons only. No responses would be coded to these identifiers if sufficient information or information within the scope of the classification was available.

The design of OSCA is informed by three constraints – statistical balance, statistical feasibility, and mutual exclusivity.

Statistical balance ensures groups at the same level in OSCA have similar numbers of jobs. Statistical balance exists to minimise large variations in relative standard errors as large relative standard errors result in suppressing data produced by sample surveys.

Meaningful groups of jobs may not result if statistical balance is the only consideration. For example, groups of jobs which are growing at a faster rate than all other groups of jobs in the classification, or which represent large numbers of jobs but which cannot be meaningfully split may not be separately identified.

Statistical balance is achieved by applying ideal size ranges for each level of the classification. The ideal size ranges specify the ideal minimum and maximum number of employed persons that should be classified to each of OSCA’s five levels. The ideal size range for each of OSCA’s five levels and a specialisation is shown in the below table.

Data from the 2021 Census of Population and Housing was the primary source for determining if statistical balance is achieved. Other data sources were given less emphasis as they can be less reliable. For example, the number of members of an association is of limited value because:

• employed persons can belong to multiple associations
• membership does not guarantee the member is working in a job associated with the occupation, and
• many retirees maintain their membership of an association after they cease employment.

The ideal size range was used for most groups of jobs in OSCA. Some groups of jobs are below the ideal minimum size. This is because these groups cannot be meaningfully combined (for example, Unit Group 1213 Legislators). Other groups exceed the ideal maximum size. In many cases, these groups cannot be meaningfully disaggregated (for example, Unit Group 2654 Registered Nurses, Occupation 273333 Software Engineer).

Statistical feasibility refers to whether meaningful data can be compiled. It considers whether the information typically provided in source data, such as the Census of Population and Housing or Labour Force Survey, is sufficiently detailed to enable accurate classification. Statistical feasibility ensures OSCA only includes an occupation for which data can be produced. For example, there may be more than 300 employed persons in Australia undertaking the tasks of a fertility nurse. A separate occupation cannot be created if most of these employed persons state their main occupation is ‘nurse’, their tasks are ‘nursing’, the goods or services they produce or provide are ‘health’ and their industry of employment is ‘health’.

Mutual exclusivity ensures that each job is classified to one, and only one, group of jobs at each level of the classification with no overlaps. It means that OSCA can be applied without ambiguity to every job in the Australian labour market.

OSCA establishes mutual exclusivity through the set of tasks associated with the occupation. Individual tasks may be undertaken by multiple occupations. However, each occupation’s set of tasks is unique to the occupation and not performed by any other. The set of main tasks listed for each occupation in OSCA has been defined to be unique. This ensures the mutual exclusivity of all occupations.

For example, the set of tasks for the occupations Carpenter and Bricklayer involve reading and interpreting plans and erecting structures. However, the main tasks for Carpenter also include using nails to erect structures made of wood and similar materials, while the main tasks for Bricklayer involve erecting structures made of bricks and mortar. These occupations are mutually exclusive because each has a unique set of tasks.

The design constraint of mutual exclusivity means that separate occupations cannot be identified in OSCA if the set of tasks are essentially the same. For example, registered nurse jobs in acute care are classified to Occupation 265432 Registered Nurse (Acute Care). Whether the job is in a private or public hospital makes no difference as the jobs perform the same main tasks. Similarly, beef cattle farmer jobs are classified to Occupation 152131 Beef Cattle Farmer. It is irrelevant whether they raise Angus or Hereford cattle as the main tasks are identical.