An identification key is a printed or computer-aided device that aids the identification of biological organisms (e. g., plants, animals, or microorganisms) or other type of entity (e. g., diseases, soil types, minerals, or archaeological and anthropological artifacts). Traditionally identification keys have most commonly taken the form of single-access keys (especially dichotomous keys). These works by offering a fixed sequence of identification steps, each with multiple alternatives, the choice of which determines the next step. In contrast to single-access keys, multi-access keys enable the user to freely choose the identification steps. These keys are often called interactive keys.

Verifying identifications

An identification key is only a guide to what a plant, animal or fossil might be. Full identification requires comparison with authoritatively identified specimens or images and ideally also a full and accurate description preferably in a monograph.

Comparison with a monographic description is often difficult in practice; many monographs being expensive and hard to obtain. Monographs are often several decades old (thus not reflecting recent changes in taxonomy) and out of print, in foreign languages, or for a different geographic region which only incompletely reflects the species to be expected in the region of interest.

Comparison with authoritatively identified specimens in natural history museums or other relevant repositories is commonly practiced by scientists. Authoritatively identified images are becoming more common on the internet. Authoritative identification is indicated and made available for verification when an image is accompanied by 1) the scientist's name who made the identification of the photographed specimen, 2) a voucher specimen number, and 3) the name of the public institution where the specimen is housed. This way, interested parties can re-examine the specimen themselves.

Characteristics of good keys

• The characters or features used for identification purposes should be diagnostic and differential:
  • A diagnostic character is one that is common to all members of the group and is unique to that group.
  • A differential character is one which separates one group from another.
• Occasionally characters which are neither differential nor diagnostic may be included to increase comprehension (especially characters that are common to the group, but not unique).
• The terminology is consistent in meaning and it is uniformly used. The use of alternative terms for the same concept to achieve more "lively prose" should be avoided.
• Prefer positive over negative statements.
• Where possible, redundant information should be provided (e.g. two characters where one would already perform the splitting) to improve the reliability of identification. A character may be not observable under the conditions of observation or it may be misinterpreted by the observer.
  • In a single-access (dichotomous/polytomous) key, use two or more characters in each lead. Order them according to their reliability and convenience.
  • Create completely corresponding statements. Avoid statements like "flowers red, size 10-40 cm" versus "flowers yellow".
• Adding further error tolerance by using reticulation should be considered.

Common errors and problems

A high error rate in identification if present may render much ecological research virtually worthless. This highlights the importance of voucher specimens as described above in both pure and applied biological work. This is of great practical importance as in the case of medical, pest control or forensic work. Error rates in insect identification are discussed by Steve Marshall.http://www.biology.ualberta.ca/bsc/news_19_2/error_rates.htm

Sources of identification errors

1. Distribution characters should be used with caution. Species that are not listed for a region may still occasionally occur there. Very few regions have completely accurate lists of species occurring. Also, the organism may have been transported, particularly to locations near ports and airports, or it may extend its range (e. g., due to global warming). For Europe and, probably, North Africa a Palaearctic key is advisable.
2. Rarity is not a character. An identification may be correct even though a species is very rare.
3. Many keys contain brief descriptions to allow more certain identification. These should not be used as an alternative to full descriptions or comparison with authoritatively identified specimens or images.
4. Some keys identify only males (or, more rarely females). Keys to larvae may work only for the final instar. There are important exceptions however, some larval Diptera of forensic relevance being notable exceptions.
5. Some keys exclude difficult groups, which may be mentioned in the title or only in introductory text.

Problems

1. Lighting and magnification - very few keys give details of how the specimen was viewed (the magnification, lighting system, angle of view etc.). This can cause problems. The author may, for instance refer to tiny bristles, hairs or chaetae--but how tiny?
2. Language - almost no keys are multilingual and translations may be incorrect or misleading. Many keys contain vague words that do not translate.. For an example of a key in five languages, see Commercial Timbers.
3. Out of date keys - present a problem since they do not include more recently described species. Also, in such works the nomenclature is out of date, causing problems of synonymy.

External links

• Principles of interactive keys
• Programs for interactive identification and information retrieval
• ActKey
• Lucid - Interactive Identification and Diagnostics key software
• Discover Life - Interactive Guides and free online guide development space
• Interactive Identification Keys
• Bioimages Comments on Royal Entomological Society of London Keys

See also

• Cryptic species complex
• Scientific classification
• Single-access key
• Multi-access key

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