A collection of related items, such as office supplies (stapler, ruler,
eraser, etc.), is assembled. Students are arranged in groups of two
to four, and each group is given a different item to describe; e.g.,
one group gets a stapler, another a ruler, and so on. Each group
then fills out a specified row in a table (e.g., Specimen A = stapler,
Specimen B = ruler, and so on), listing three to six qualitative characteristics of their item (see Table 1). These characteristics are then
written on a board so that all students can then fill out the table
with all the specimens and characteristics.
Step 2: Creating a flow chart
Students learn how to arrange data into groups and how to find
patterns in their observations.
To make a flow chart, students must be able to examine a collection of specimens and divide them into groups based on shared
characteristics. Students are taught that (a) not every characteristic
must be used, and (b) not all flow charts are going to be the same
( i.e., there is no one correct flow chart). This is when students learn
about hierarchical thinking (Objective 3).
Students are told that they must split the specimens into two groups
based on a characteristic that one group has and the other does not.
In so doing, they begin to understand the concept of dichotomy.
For example: based on the data in Table 1, they can divide the
specimens into two groups based on color. They then begin to fill
out the flow chart. At each juncture, they must clearly state the
defining character that is or is not present for each group. They
continue until each specimen is in its own box and every specimen
has been identified (see Figure 1). This is done with the class as a
whole, with the students participating, so that they can see how
this process works. The result is then written on the board, so all
the students see the same chart. It must therefore be emphasized
that there is no single correct chart.
Step 3: Creating a dichotomous key
Students learn how to transform a flow chart into a dichotomous
If students had seen a dichotomous key before, it would have been
in a biological or geological context. (Keys to various animals and
minerals are usually in the form of dichotomous keys.) Flow charts
are more common and can be seen in a variety of contexts, not only
scientific. This is when students learn about scientific literacy
(Objective 2). In addition, students learn that these are two different, but interconvertible, ways of organizing and showing qualitative characteristics (Objective 3).
Students take their flow chart and transform it into a series of
paired questions. The questions are based upon the features used
to distinguish between the groups and must be written in a sequential and logical manner. The dichotomy of the questions must be
emphasized. Once again, this is done on the board, with student
input, as they fill out their sheet appropriately (see Table 2).
Observe the following guidelines when constructing a dichotomous key:
1. Make sure the paired statements reflect contrasting
1a. The specimen is gray. 1b. The specimen is NOT gray.
1a. The specimen is gray. 1b. The specimen is plastic.
2. Start with a characteristic that will allow you to divide the specimens into two roughly even groups (e.g., the gray color of
Table 1. Table used to collect characteristics for
office supplies. Number of rows is determined by the
number of groups present. Students would be given
the table containing only the two column headings,
and the words “Specimen X: Name = .”
Specimen A: Name = Stapler gray, metallic, hinged,
Specimen B: Name = Ruler red, plastic, rectangular
Specimen C: Name = Eraser green, rubber, rectangle
Specimen D: Name = Paperclip gray, metallic, oblong
Specimen E: Name = Pen gray, plastic, oblong
Specimen F: Name = Tape
black, plastic, rectangular,
Figure 1. Example of a how a completed flow chart would
look. Students would get a flow chart with just the boxes and
lines. It is up to them to fill it out with the words.