A limited list of preferred terms from which the user can draw one or more to express a concept such as patient data, a disease or drug name, etc. An alphanumeric code corresponding to the term is then stored by the computer. This approach makes it easier for a computer to analyse data than the use of free text words or phrases. Examples of clinical coding systems include SNOMED-CT (divergent codes used to capture patient data), MeSH (terms used to index biomedical literature) and ICD-10 (convergent codes for international comparisons). Clinical coding systems play a key role in epidemiological studies and health service research, from the use of MeSH terms to conduct literature searches for systematic reviews to numerous studies which use ICD codes to classify and compare diseases. To prevent information loss, it is vital that the terms and codes are never changed or dropped, only added to. Obsolete terms can be marked as such to deter inappropriate use. Continuing maintenance is needed to incorporate new terms and codes for new concepts and new synonyms as they arise.