Engineering: The nature of problems

# 4.4.1 Selecting the best candidate

Assuming there is more than one likely looking candidate solution, we need to make a selection now so that we don't waste time taking all the candidates through the next steps, which become progressively more expensive and time-consuming. The rigour and formality of this step is very variable, but in general all schemes boil down to the same process you might use to choose some consumer item, such as a TV set. You would have a list of criteria that are important to you, and you would evaluate each candidate against those criteria. In many cases the list is short enough, or a single criterion of such importance relative to the others, for it to be possible to have it in your head. Usually it is worth writing down that list (which should look like the specification), and assigning a weighting to each criterion according to its relative importance. You then give each candidate solution a score against each criterion. When multiplied by their respective weightings, these scores add up to a figure of merit for each solution. The one with the largest number wins.

A benefit of using a system such as this is that it tells you quantitatively what kind of a 'squad' of substitute solutions you have to draw from (to use a sporting analogy). This is important because it tells you whether or not you really ought to take more than one on to the next step and beyond, until there is a clearer preference. It also tells you whether or not you have only one possible solution that's going to be worth considering. If this is the case, it may be good news if you are confident it will work, or it may prompt you to go back now and try to generate some more ideas.

Let us try this approach with the example of choosing a TV set. First, we ask what are our criteria, and what relative importance do we attach to each of them? This can be set out in a table:

##

Criterion | Relative importance (weighting 0–10)
--- | ---
1 | Large screen | 8
2 | Good picture quality | 10
3 | Good sound quality | 8
4 | Compatible with other audio/video systems | 10
5 | Attractive cabinet | 5
6 | Cost | 10

Next, we need to score each of our candidate models of TV set against each of the criteria, then multiply the scores by the appropriate 'importance' weighting.

##

| Model 1 | Model 2 | Model 3
--- | --- | --- | ---
Criterion | score (0–10) | × weight | score (0–10) | × weight | score (0–10) | × weight
1 | 10 | × 8 | 6 | × 8 | 6 | × 8
2 | 6 | × 10 | 5 | × 10 | 10 | × 10
3 | 5 | × 8 | 4 | × 8 | 6 | × 8
4 | 10 | × 10 | 0 | × 10 | 5 | × 10
5 | 3 | × 5 | 10 | × 5 | 8 | × 5
6 | 8 | × 10 | 7 | × 10 | 10 | × 10
Sum of weighted scores | 375 | 250 | 386

For each model we arrive at the sum of the weighted scores.

On the basis of this, we might eliminate Model 2, but we might need to consider some additional criteria to choose between Model 1 and Model 3.

OpenLearn - Engineering: The nature of problems
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