Identification key

In biology, an identification key is a printed or computer-aided device that aids the identification of biological entities, such as plants, animals, fossils, microorganisms, and pollen grains. Identification keys are also used in many other scientific and technical fields to identify various kinds of entities, such as diseases, soil types, minerals, or archaeological and anthropological artifacts.

Traditionally identification keys have most commonly taken the form of single-access keys. These work by offering a fixed sequence of identification steps, each with multiple alternatives, the choice of which determines the next step. If each step has only two alternatives, the key is said to be dichotomous, else it is polytomous. Modern multi-access or interactive keys allow the user to freely choose the identification steps and their order.

At each step, the user must answer a question about one or more features (characters) of the entity to be identified. For example, a step in a botanical key may ask about the color of flowers, or the disposition of the leaves along the stems. A key for insect identification may ask about the number of bristles on the rear leg.

Identification errors may have serious consequences in both pure and applied disciplines, including ecology, medical diagnosis, pest control, forensics, etc. Therefore, identification keys must be constructed with great care in order to minimize the incidence of such errors.

Whenever possible, the character used at each identification step should be diagnostic; that is, each alternative should be common to all members of a group of entities, and unique to that group. It should also be differential, meaning that the alternatives should separate the corresponding subgroups from each other. However, characters which are neither differential nor diagnostic may be included to increase comprehension (especially characters that are common to the group, but not unique).

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