Multi-Criteria Decision-Making

It’s a bit of a long post, but I think weighted-sums as a decision making tool is awesome! I would whole-heartedly recommend it to anyone who’s has to make a complicated decision.

With so many options at hand which more or less matched the design brief, and so many factors to consider, I was going to need a structured approach to have any chance of arriving at a conclusive decision. My dad lent me a book “Contemporary Management Science with Spreadsheets“, and recommended that I read the chapter on multi-criteria decision-making. Being in a bit of a state of despair, but determined to resolve the issue, I flipped open the book and started reading. The chapter featured two techniques, weighted sums and the more complex analytic hierarchy process, which relies on a very large number of pairwise decisions. I chose weighted sums because it’s the simpler method of the two.

Method – Weighted Sums

  1. Pick out clear candidates (decision alternatives) for the process
  2. Decide which criteria are relevant for your decision
  3. Give each criterion a relative weight, for how important it is to you
  4. For each candidate, rate how well it satisfies each of the criteria
  5. Calculate rating x weight for each entry, and then add those up for a total score for each candidate

While some of my candidates were pretty hypothetical, I did have a rough list of candidates, so I took sheet of paper and started brainstorming which criteria are important to me. Once I had my list, I cut the sheet of paper into several cards, one for each criterion, and kept arranging them until I had them in an order, from most important to least important, to make it a bit easier to assign the weights. I think that is by far the easiest, most intuitive, and fastest way of prioritizing things.

The list of criteria which are important to me, ranked from most important (top) to least important (bottom).

I also formulated each criterion as a question, so that later on it would be as clear as possible in my own mind, what exactly I was trying to assess. Initially I had written down ‘cost’ and ‘construction time’ as criteria, but while deciding the ratings, I kept subconsciously making the mistake of thinking “it’s expensive, so I have to give it a high rating”. Oops! I quite quickly changed those criteria to ‘inexpensive’ and ‘construction-time saved’, so that ‘more‘ of the criterion, is also entails a higher rating.

I decided to go for decimal weights, from 0.0 to 1.0, instead of 1 to 5, but it doesn’t really matter. The ratings are straight from the book “Contemporary Management Science with Spreadsheets”.

Values

With 9 candidates and 12 criteria, this took quite a while. I think for the typical problem, you would have fewer criteria and also fewer candidates, so the process would be a lot faster, but that’s just how it is. 😉 It wasn’t perfect on the first go either, it’s important that the table is more or less consistent, so that boats with similar performance for example, actually have similar ratings for performance. I tweaked it until I felt like the values were fair. This is obviously subjective, some one else, with different priorities and different skills would have filled the table out differently, and probably even have different criteria, but making your own decision is the whole point, weighted sums just gives it a bit of structure. Here are the values I assigned:

Results

The final step was to use the spreadsheet to calculate the scores. I also applied a bit of conditional formatting to get color gradients to better visualize the data, so that it’s easier to see where the different candidates are getting their points. That actually allows you to sum up their respective strengths and weaknesses quite well:

Conclusion

  1. The proas are the clear losers of this comparison, mainly because I’m not confident in my ability to resolve the design issues in the available time.
  2. The clear overall winners are the 7 meter tacking outrigger and the open sailing/rowing dory.
  3. Interestingly, the 7m Tacker is not the best solution in any one of the main criteria, but it earns consistently good scores across the board.
  4. The open dory on the hand scores very poorly in performance and protection from the elements, but also racks up a really high score because it’s a small and simple boat, which is can be built rapidly from readily available plans. There is a high adventure aspect in sailing in an open boat, though it would obviously have to be sailed in day trips, camping at night. It departs a bit from the design brief in that regard, but it’s a good Plan B.

Overall, from the moment I picked up the book and started reading to when I had my conclusion, took me about one and a half days. It sounds like a lot, but given how many candidates and relevant aspects there are, thinking about it in an unstructured way could easily have taken weeks, most likely without arriving at a definite conclusion at all. Instead, not two days later, I knew exactly which option is the most deserving of further investigation: the tacking outrigger. More on that soon! 🙂

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