Multi-Criteria Decision Analysis
Multi-Criteria Decision Analysis, or MCDA, is a valuable tool that we can apply to many complex decisions. It is most applicable to solving problems that are characterized as a choice among alternatives. It has all the characteristics of a useful decision support tool: It helps us focus on what is important, is logical and consistent, and is easy to use. At its core MCDA is useful for:
- Dividing the decision into smaller, more understandable parts
- Analyzing each part
- Integrating the parts to produce a meaningful solution
When used for group decision making, MCDA helps groups talk about their decision opportunity (the problem to be solved) in a way that allows them to consider the values that each views as important. It also provides a unique ability for people to consider and talk about complex trade-offs among alternatives. In effect, It helps people think, re-think, query, adjust, decide, rethink some more, test, adjust, and finally decide.
MCDA problems are comprised of five components:
2. Decision maker or group of decision makers with opinions (preferences)
3. Decision alternatives
4. Evaluation criteria (interests)
5. Outcomes or consequences associated with alternative/interest combination
Example: Choosing a Location for a Municipal Aquatics Facility
Step 1: Define the Decision Opportunity
Goal: Choose a site for a 40,000 sq. ft. municipal aquatics facility in a city of 120,000 people.
Decision Makers: 10-member Parks and Recreation Advisory Board.
Decision Alternatives: Decide among four alternative sites:
- Hawes Tract – a vacant site on the rapidly growing outskirts of town
- Chatham Street – an infill site near the center of town
- Bartley Park – a new park on an old agricultural site in the south
- North Cary Park – a site adjacent to a community park and a well-traveled riparian greenway
Step 2: Identify Stakeholder Interests
Board member interests serve as the criteria by which to evaluate each site. Interests are identified through a facilitated discsussion of the board members.. The Parks and Recreation Advisory Board members identified 10 key interests, many of which were further broken down into sub-interests.
Interests and Sub-Interests Identified by Board Members:
Proximity to other facilities
a. Maintain distance from year-round aquatics
b. Maintain distance from seasonal aquatic centers
c. Maintain distance from community centers
d. Be accessible to public parks
e. Be accessible to high schools
Proximity to users
a. Be easily accessible to existing neighborhoods
b. Be easily accessible to planned neighborhoods to be built in the near-term
Ease of access
a. From surrounding neighborhoods
b. By greenway
d. By roadway
a. Minimize disruption to mature trees and vegetation
b. Avoid topographic challenges
c. Minimize impact to watershed
a. New traffic can be accommodated by existing street network
Compatibility with Surrounding Area
a. Facility footprint and use must be compatible with surrounding neighborhood
Visibility and Profile of Facility
a. Facility footprint must be compatible with, and not detract from surrounding neighborhood
Ability to Meet Future Demand
a. Tract must be able to accommodate expansion of facility and uses
a. Tract must be of an appropriate dimension to accommodate parking
a. Reduce on-site development costs
b. Reduce off-site development costs
Step 3: Build a Decision Framework
Now that the problem has been specified as goals, interests, and alternatives, it can be depicted as a set of individual components, and their linkages made explicit. Illustrating the framework in this way allows Parks Board members to understand the relationships between the overall goal, the interests they view as important to the decision, other board members' interests, and the alternatives available to them.
Step 4: Rate the Alternatives
Once they identified the key interests that would be considered in selecting a site, the Board took on the task of rating the alternatives relative to how well they satisfied each interest. Because this was a technical exercise, they sought the input of experts. They asked planners, transportation engineers, and environmental experts for their input.
-- How close is each site to other aquatic centers? (the farther away the better)
-- How challenging is the topography at each site?
-- How severe would the traffic impacts be at each site?
Two common rating scales that that are used in MCDA are (1) a relative scale, and (2) an ordinal scale. Rate each alternative relative to the others.
Each alternative is rated relative to the others in satisfying a particular interest. For example, among the 4 alternatives, assign each a 1, 2, 3, or 4 depending on which satisfies the interest: the best = 4; second best = 3; third best = 2; and the worst at satisfying the interest = 1.
Using a scale of your choosing (e.g. a 5-point scale, or a 10-point scale) assign each alternative a rating for how well it satisfies a particular interest: For example, a five point scale might be: 5 = excellent; 4 = good; 3 = satisfactory; 2 = below average; 1 = poor.
Step 5: Weight Stakeholder Interests
This is where personal preferences matter. Each board member assigns his/her own weights to each interest or sub-interest. An individual's weighting preferences are kept intact, they are not averaged or blended with other board members' weights. In fact, it is the differences in how people assign weights that engenders discussion among the group.
- Start with a simple ranking, from most important to least important
- Use common language to begin the ranking process, e.g.,
- Assign weights by allocating 100 points among the interests. Like grading a test.
- Add more detail – include sub-interests
- Assign weights to sub-interests
- Weights for sub-interests can take on any value between zero and the value of the weight you gave the interest
Step 6: Score the Alternatives
Each board member identifies his/her preferred site by multiplying the sub-interest weight by its corresponding rating value to calculate a score for that sub-interest. Summing the score yields his/her most preferred site. In the example shown below, this particular board member scored the Chatham site (C) highest, followed by the Hawes Tract (H) and Bartley Park (B). The North Cary site (NC) received the lowest score.
Step 7: Discuss Results, Re-Score, Discuss Again, Decide
At this point, the board members are able to compare their scores. If one site is preferred by everyone, the decision is made! However, if no single site rises to the top, the board has several possibilities for next steps:
- If one site is rated lowest or second lowest by all board members, then it is considered a "dominated alternative" and can be dropped from the list. This makes the choice somewhat simpler.
- Even if no site can be designated a dominated alternative, the board can review the scores and discuss the weights and scores. Alternatives with close scores are good candidates for discussion. Board members can compare and discuss their interest weights, and experiment by assigning higher or lower weights to different sub-interests. This experimentation may yield some insights and lead to agreement.
- The alternative ratings -- how well each alternative does in satisfying a particular interest -- can be discussed with planners and other experts. Some ratings might be better expressed as a range of numbers rather than a single number (e.g. 3-5 instead of a 4). Board members can conduct sensitivity analyses on these ratings to see how that affects their scores.
- Discussion, experimentation and more discussion may result in a particular site rising to the top.