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Common Aggregation Hierarchy (CAH)

The Common Aggregation Hierarchy (CAH) provides a standardised hierarchical aggregation (grouping) of subject codes and terms (developed based upon the HECoS codes and terms) suitable for the majority of uses. 

The CAH has been developed to provide standard groupings that can be applied to both HECoS and JACS subjects allowing for consistent analysis across both coding frames. 

These aggregations provide some continuity in subject analysis through the transition from JACS to the HECoS and to other schemes such as those used by QAA. This will improve consistency across the sector.  Whereas the CAH has been designed to act as a bridge between HECoS and JACS, these are, however, two distinct coding frames, with HECoS having been developed in part to allow for more robust coding to address inconsistencies in coding using the JACS framework, and to remove limitations imposed by the JACS structure. As such, analysis between HECoS and JACS using this Common Aggregation Hierarchy will only provide indicative comparisons for time series analysis.

CAH Contents

The CAH is a comprehensive aggregation of the entirety of HECoS at each of three hierarchical levels or tiers, where a “parent” group at a higher level of CAH always comprises the full set of HECoS codes represented by related “child” groups of CAH below it. 

CAH groups are numbered with a series of two-digit integers representing a group of HECoS codes at a level, with the code at each level separated by a dash. Each unique CAH group code is referred to as a node. 

CAH tier

Node coding format (where nn is a 2-digit integer)







Each node also has a label which serves to represent groups of subjects intelligibly and with rigour. Subject groupings (represented as nodes in the CAH) are not the same as subjects (represented by HECoS codes) even in cases where a CAH label is identical with a HECoS term. 

Where a higher-tier node is the parent of a single node at the next lower tier, they must share a title. 

Where a higher-tier node is the parent of more than one node at the next lower tier, they may not share the same title. 

In version 1.2 of the CAH, level 1 comprises 23 groups; level 2 comprises 35 groups, and; level 3 comprises 167 groups. 

In version 1.3.4 of the CAH, level 1 comprises 21 groups; level 2 comprises 35 groups; level 3 comprises 167 groups. 

Development of the CAH is subject to rules in the HECoS and CAH Management Guide (that can be found on the HECoS archive webpage).  In summer 2018 further research undertaken by the OfS (and DfE) identified areas where intended users of the CAH found groupings to be confusing, and as a result developed a proposal for amending the CAH. HESA worked closely with users of the CAH and Version 1.3 was created and approved by the Data Landscape Steering Group. To view changes between versions of CAH please view change log files that can be found under 'Download the CAH' section.

Version 1.2 of the CAH has been deprecated as from 31st July 2021.  

HESA continues to work with public bodies that are using CAH to support their work to support continued use of CAH for student funding and security. 

The following .csv files are published to detail version 1.3.4 of the CAH framework. 

Previous versions of the CAH framework (Versions 1.2 and 1.3) can be found under the 'Previous CAH Versions' section of the HECoS Archive webpage.

Please note that for ease of machine readability and ingestion of this data into other databases or systems and in line with HESA’s commitment to open data principles these files are provided in CSV format. 

When opening these files directly in Excel it maybe that Excel automatically convert some data into an inappropriate data type. For example, Excel may render CAH codes in the format 01-02-03 as a date.  If this issue is experienced, download the file to a local folder then, rather than opening the CSV file in Excel via File | Open, use the data import function, taking care to select the data type of ‘text’ for the relevant columns.

Options for downloads

Include prefix 'CAH' in codes (e.g. CAH01-01-01)

Include code labels

A simplified presentation of the current release of the CAH is also available, showing the relationship between the nodes 

CAH Level 1 CAH Level 2 CAH Level 3
medicine and dentistry
medicine and dentistry
medical sciences (non-specific)
medicine (non-specific)
medicine by specialism
subjects allied to medicine
pharmacology, toxicology and pharmacy
nursing and midwifery
nursing (non-specific)
adult nursing
community nursing
children's nursing
dental nursing
mental health nursing
learning disabilities nursing
others in nursing
medical sciences
medical technology
healthcare science (non-specific)
biomedical sciences (non-specific)
anatomy, physiology and pathology
allied health
health sciences (non-specific)
nutrition and dietetics
environmental and public health
complementary and alternative medicine
counselling, psychotherapy and occupational therapy
biological and sport sciences
biosciences (non-specific)
biology (non-specific)
ecology and environmental biology
microbiology and cell science
plant sciences
molecular biology, biophysics and biochemistry
hair and beauty sciences
others in biosciences
sport and exercise sciences
sport and exercise sciences
psychology (non-specific)
applied psychology
developmental psychology
psychology and health
others in psychology
veterinary sciences
veterinary sciences
veterinary medicine and dentistry
others in veterinary sciences
agriculture, food and related studies
agriculture, food and related studies
animal science
agricultural sciences
rural estate management
forestry and arboriculture
food and beverage studies (non-specific)
food sciences
food and beverage production
physical sciences
physics and astronomy
general, applied and forensic sciences
physical sciences (non-specific)
forensic and archaeological sciences
sciences (non-specific)
natural sciences (non-specific)
mathematical sciences
mathematical sciences
operational research
engineering and technology
engineering (non-specific)
mechanical engineering
production and manufacturing engineering
aeronautical and aerospace engineering
naval architecture
bioengineering, medical and biomedical engineering
civil engineering
electrical and electronic engineering
chemical, process and energy engineering
others in engineering
materials and technology
minerals technology
materials technology
polymers and textiles
maritime technology
others in technology
materials science
computer science
information technology
information systems
software engineering
artificial intelligence
computer games and animation
business computing
others in computing
architecture, building and planning
architecture, building and planning
landscape design
planning (urban, rural and regional)
social sciences
sociology, social policy and anthropology
social sciences (non-specific)
social policy
development studies
cultural studies
health and social care
social work
childhood and youth studies
health studies
business and management
business and management
business and management (non-specific)
business studies
management studies
human resource management
tourism, transport and travel
others in business and management
language and area studies
English studies
English studies (non-specific)
English language
literature in English
studies of specific authors
creative writing
others in English studies
Celtic studies
Celtic studies (non-specific)
Gaelic studies
Welsh studies
others in Celtic studies
languages and area studies
French studies
German and Scandinavian studies
Italian studies
Iberian studies
Slavic studies
Asian studies
African and modern Middle Eastern studies
American and Australasian studies
others in language and area studies
historical, philosophical and religious studies
history and archaeology
history of art, architecture and design
heritage studies
philosophy and religious studies
theology and religious studies
education and teaching
education and teaching
teacher training
combined and general studies
combined and general studies
combined, general or negotiated studies
personal development
humanities (non-specific)
liberal arts (non-specific)
media, journalism and communications
media, journalism and communications
information services
publicity studies
media studies
design, and creative and performing arts
creative arts and design
creative arts and design (non-specific)
design studies
cinematics and photography
others in creative arts and design
performing arts
performing arts (non-specific)
geography, earth and environmental studies
geography, earth and environmental studies
geography (non-specific)
physical geographical sciences
human geography
environmental sciences
others in geographical studies
earth sciences

Changes between versions of the CAH

To assist users of the CAH to understand the differences between versions a change log has been produced.

Options for downloads

Include prefix 'CAH' in codes (e.g. CAH01-01-01)

In preparation for the release of the HESA 2019/20 Student Statistical Bulletin, HESA investigated the quality characteristics of the CAH. 2019/20 data was compared with data from 2018/19. HESA derived the CAH groups for each dataset, to look at changes in how students were distributed in each year. Preliminary analysis revealed that a small overall average change in the distribution of students masks wider underlying variations. Many individual CAH level 1 and CAH level 3 groups show more substantial changes in the numbers of students allocated to them. These differences appear to reflect underlying changes to courses, as well as movement from the many ‘others in…’ categories within JACS to the precisely defined subjects in HECoS. HESA  determined that these inconsistencies meant it should not use CAH v1.2 for presentation of its initial outputs containing time series.

CAH to HECoS Mapping

Use this search function to view HECoS Subject Codes mapped a CAH Code.

A .csv file containing all CAH to HECoS mappings is availible under the 'Download the CAH' section above.

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