Mind, Body, World- Foundations of Cognitive Science, 2013a

technique, the ID3 algorithm (Quinlan, 1986) to induce a decision tree for the
mushroom problem. A decision tree is a set of tests that are performed in sequence
to classify patterns. After performing a test, one either reaches a terminal branch of
the tree, at which point the pattern being tested can be classified, or a node of the
decision tree, which is to say another test that must be performed. The decision tree
is complete for a pattern set if every pattern eventually leads the user to a terminal
branch. Dawson et al. (2000) discovered that a decision tree consisting of only five
different tests could solve the Schlimmer mushroom classification task. Their decision
tree is provided in Table 4-4.
Step
Tests and Decision Points
1
What is the mushroom’s odour?
If it is almond or anise then it is edible. (Rule 1 Edible)
If it is creosote or fishy or foul or musty or pungent or spicy then it is poisonous.
(Rule 1 Poisonous)
If it has no odour then proceed to Step 2.
2
Obtain the spore print of the mushroom.
If the spore print is black or brown or buff or chocolate or orange or yellow then
it is edible. (Rule 2 Edible)
If the spore print is green or purple then it is poisonous. (Rule 2 Poisonous)
If the spore print is white then proceed to Step 3.
3
Examine the gill size of the mushroom.
If the gill size is broad, then it is edible. (Rule 3 Edible)
If the gill size is narrow, then proceed to Step 4.
4
Examine the stalk surface above the mushroom’s ring.
If the surface is fibrous then it is edible. (Rule 4 Edible)
If the surface is silky or scaly then it is poisonous. (Rule 4 Poisonous)
If the surface is smooth then proceed to Step 5.
5
Examine the mushroom for bruises.
If it has no bruises then it is edible. (Rule 5 Edible)
If it has bruises then it is poisonous. (Rule 5 Poisonous)
Table 4-4. Dawson et al.’s (2000) step decision tree for classifying mushrooms.
Decision points in this tree where mushrooms are classified (e.g., Rule 1 Edible)
are given in bold.
Elements of Connectionist Cognitive Science 179