DesignWIKI

Fil Salustri's Design Site

Site Tools


design:decision_matrix

Decision Matrix

A decision matrix is a tool used to compare alternatives with respect to multiple criteria. It can be used to rank all the alternatives relative to a “fixed” reference and thus create a partial order of the alternatives.

MEC723 students must use weighted decision matrix.

Introduction

Making decisions is both important and difficult. You must make decisions that are justified and in which all stakeholders have confidence. It is also important to document decisions in clear, structured ways to ensure that others will understand your reasons for having made a decision, long after the decision is made.

There are often many different criteria that need to be considered in making a decision. It is essential to identify the criteria, and to make the decision with respect to those criteria as precisely as possible. You cannot just maximize each criterion - that's just not possible - and maximizing one criterion very likely restricts you to fail to meet other important criteria. That is, the “best” design almost always involves a design trade-off.

The matter is further aggravated when there are many alternatives from which to choose; in these cases, not only does each alternative need to be examined, but all the alternatives must be treated consistently to ensure that a final comparison of all the alternatives is justifiable.

A decision matrix (DM) is a simple tool that can be very useful in making complex decisions, especially in cases where there are many alternatives and many criteria to be considered.

DMs are often used in design engineering as a qualitative tool to evaluate alternatives. DMs are also used in many settings outside of engineering because as its name implies, it is a tool to aid in general decision making. Financial institutions use this kind of tool to determine who should or should not get a loan; health organizations use them to determine which implementations of policies is best suited to specific situations; even the process of medical triage assumes a certain decision making process that can be modelled with decision matrices. They can even be used to decide what kind of vacation you should take.

This page explains how they work in general; other topics will show how they are used in specific design tasks.

DMs are often used in concept evaluation.

A template Google Sheet for DMs is available here.

Structure of a DM

This section just points out general features of DMs.

To find out how to apply DMs in this course, you must read about concept evaluation.

A sample DM is given below.

Consider Concept 2. It was scored as -2 on functionality; this means is was much worse than the reference design with respect to the FRs and constraints that were listed in the PRS under functionality.

When you are filling in a DM, you must have your PRS at hand.

  • Go through one concept at a time.
  • Consider each PC in turn. For each PC:
    • With respect to all the FRs and constraints for that PC, consider how the concept rates against the reference design.
    • Note that rating in the DM
    • Go on to the next PC.
  • Repeat the above steps for each concept.

Make sure to keep notes on why you gave a particular rating. Remember: designing is about justifying your decisions. The more challenging you found a particular decision to make, the more likely you'll need to justify it properly.

The TOTALS and RANK columns of the DM will update themselves as you add more ratings. Please be careful to leave alone the “math” in those cells.

The RANK column is particularly important because this gives the rank order of concepts from best to worst. Notice that the reference design will also be ranked; this can help you to distinguish between concepts that beat the reference design and those that did not.

However

It should be clear that a DM is not a static document; it can change and evolve in parallel to both your understanding of the problem and the development of a solution. So it's important to keep old versions of your DMs, to track the history of your work.

design/decision_matrix.txt · Last modified: 2020.07.25 21:43 by Fil Salustri