Single-access key

In Phylogenetics, a single-access key (also called dichotomous key, sequential key, analytical key, or pathway key) is an identification key where the sequence and structure of identification steps is fixed by the author of the key. At each point in the decision process, multiple alternatives are offered, each leading to a result or a further choice. The alternatives are commonly called "leads", and the set of leads at a given point a "couplet".

Single access keys are closely related to decision trees or self-balancing binary search trees. However, to improve the usability and reliability of keys, many single-access keys incorporate reticulation, changing the tree structure into a directed acyclic graph. Single-access keys have been in use for several hundred years. They may be printed in various styles (e. g., linked, nested, indented, graphically branching) or used as interactive, computer-aided keys. In the latter case, either a longer part of the key may be displayed (optionally hyperlinked), or only a single question may be displayed at a time.

If the key has several choices it is described as polychotomous or polytomous. If the entire key consists of exactly two choices at each branching point, the key is called dichotomous. The majority of single-access keys are dichotomous.

Any single-access key organizes a large set of items into a structure that breaks them down into smaller, more accessible subsets, with many keys leading to the smallest available classification unit (a species or infraspecific taxon typically in the form of binomial nomenclature). However, a trade-off exists between keys that concentrate on making identification most convenient and reliable (diagnostic keys), and keys which aim to reflect the scientific classification of organisms (synoptic keys). The first type of keys limits the choice of characteristics to those most reliable, convenient, and available under certain conditions. Multiple diagnostic keys may be offered for the same group of organisms: Diagnostic keys may be designed for field (field guides) or laboratory use, for summer or winter use, and they may use geographic distribution or habitat preference of organisms as accessory characteristics. They do so at the expense of creating artificial groups in the key.

...
Wikipedia