Wolmer’s Trust High School for Girls
INFORMATION TECHNOLOGY (GENERAL PROFICIENCY)
Grade 11 Teacher: Mrs. McCallum-Rodney
TOPIC: DECISION TREES
INTRODUCTION
Þ Decision Trees are used when complex branching occurs in a structured decision process.
Þ Trees are also useful when it is essential to keep a string of decisions in a particular sequence.
Þ Although decision trees derives its name from natural trees, decision trees are most often drawn from their side, with the root to the left-hand side of the paper; from there, the tree branches out to the right. This orientation allows the analyst to write on the branches in order to describe conditions and actions.
Þ In systems analysis trees are used mainly for identifying and organising conditions and actions in a completely structured decision process.
DRAWING DECISION TREES
· It is useful to distinguish between conditions and actions when drawings decision trees. This distinction is especially relevant when conditions and actions take place over a period of time and their sequence is important.
· The notation used in drawing decision trees are:
· Using notation makes the decision tree more readable, as does numbering the circles and squares sequentially.
· In drawing the tree:
1. Identify all conditions and actions and their order and timing (if they are critical)
2. Begin building the tree from left to right while making sure you are complete in listing all possible alternatives before moving over to the right.
· The tree does not have to be balanced and identical actions may appear more than once.
· Consider the following simple tree:
A SIMPLE DECISION TREE EXAMPLE
Figure:
Drawing a decision tree to show the non-cash purchase approval actions for a department store.
Þ The tree takes up considerable space, and consequently a minimum amount of description on conditions and actions is written on the tree.
Þ Data requirements are not specified on the tree; only the conditions and actions are specified.
ADVANTAGES OF DECISION TREES OVER DECISION TABLES
Þ It takes advantage of the sequential structure of decision trees branches, so the order of checking condition and executing actions is immediately noticeable.
Þ Condition and actions of decision trees are found on some branches but not on others; this contrasts with decision tables, where they are all part of the same table.
Þ Decision tree are more readily understood by others in the organization.
A COMPLEX DECISION TREE EXAMPLE
Question
When patrons request seats for a play, we try to fill their request, but often the seat they would like on their first performance date preference is sold out. On our order form, we ask the patron to choose three performance dates in order of preference, to indicate whether they prefer orchestra or balcony, and finally, to select the price of the seat they would like. We have three prices - $25, $20, and $15 – and each price is available in both orchestra and balcony.
Since we fill most of our ticket orders by mail, we have to assume a number of things and try to come as close as possible to the desires of the patron.
If orchestra is not available, we assign balcony and vice versa, but if the first night selected is unavailable in the price requested either in orchestra or balcony, we look at the second night. If the second night is unavailable, we look for options on the third. If the third night is unavailable, we select the next lower price and repeat the process. If the patron originally selected the lowest price, we must now issue a notice that the ticket request could not be filled.
