MAT 7310                                                                                          Summer 2002


Constructing a Dichotomous Key

freely adapted from Perry, Morton, Perry 2002


To classify organisms, you must first identify them.  A taxomonic key is a device for identifying an object unknown to you but that someone else has described.  The user chooses between alternative characteristic of the unknown object, and by making the correct choices, arrives at the name of the object. 


Keys that are based on successive choices between two alternatives are known dichotomous keys (dichotomous means to fork into two equal parts).  The set of choices may be regarded as a flow chart with each step as a branch.  In graph theory, diagrams of these choices are called binary trees.  Searching algorithms can be built this way.  The identification procedure is deterministic, and terminates after a finite number of steps.  So this process goes by many different names in various science disciplines.


Suppose the geometric objects below have unfamiliar names.  Look at the dichotomous key following the figures.  Notice there is a 1a and a 1b.  Start with 1a.  If the description in 1a fits the figure you are observing better than the description in 1b, then proceed to the choices listed under 2, as shown at the end of line 1a.  If 1a does not describe the figure in question, 1b does.  Looking at the end of line 1b, you see the figure’s name is Eric.

Using the key provided, determine the name for each object.  Write the name beneath the object and then check with your instructor to see if you have used the key correctly.


        __________          ___________                _________            _________              _________





Figure with distinct corners



Figure without corners



Figure with 3 sides



Figure with more than 3 sides



All sides equal length



Sides not all equal length



Figure with only right angles



Figure with angle other than right




This is not the only such key; there are lots of others.  Can you make one that is longer (requires more than four decisions)?  Can you make one that is shorter (requires less than four decisions)?





Now we will construct a dichotomous key for the participants in this workshop.  We will work in two groups.


To plan a dichotomous key, we need to set up binary (two choice) categories that will differentiate members in your group.  The most obvious split it male/female, but this need not be used, even as a first cut.  Look at the sample branch diagram below.




Select characteristics for your group (they must be distinguishable by visual inspection), and construct a branch diagram; condense it into a dichotomous key.  (Every member of your group will carry the same key.)  Once both groups have complete keys, exchange your key with an individual in the other group.  Key out the individuals in the other group without speaking until you believe you know the name of the individual you are examining.  Ask that individual if you are correct.  If not, go back and find out where you made a mistake, or possibly where the key was misleading.