Single-access Key

In biology, a 'key' is a modeling method used for categorizing species using logical choices. A single-access key (dichotomous key, also called a sequential key, analytical key, or pathway key) is a 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.