Problem completion - constraints & criteria

The problem has been treated broadly up till this stage in order to derive as many candidate solutions as possible. It is now time to complete the problem, to focus on exactly what is required of solutions by way of constraints and criteria.

• Constraints
  A constraint is a bound, a limit with which every candidate must comply if it is to be a valid solution. Problems are usually characterised by a number of constraints.
  Satisfaction of a constraint by a candidate is binary - the candidate either does or does not satisfy the constraint, there are   no ifs, no buts, no maybes and no 85% about it.
  Typical constraints are :

  1. "To solve my transport problem I need a vehicle which must cost less than $10000"
  2. "Our device for the D&B competition has to fit inside a 400mm cube"
  3. "Any solution to the greenhouse gas problem must not put any of our people out of work"
  There are constraints - often implied and not spelled out - which are obvious and particularly important to D&B competition groups. Each group designs, constructs, operates and repairs its own solution device for the competition. The group therefore must be able to build the device, the device must operate as intended, and so on.

• Criteria
  A criterion is a yardstick by which the suitability of candidates may be judged - some candidates may satisfy the criterion well, some poorly. Problems are usually characterised by a number of criteria.
  The degree to which the candidate i satisfies the criterion j may be expressed by the utility, u_ij - often a real number between 0 and 100%, though other scales are used :
  • A high utility indicates that the candidate satisfies the criterion to a high degree.
  • A low utility signifies that the candidate satisfies the criterion poorly.
  • A utility of 100% means that the candidate satisfies the criterion perfectly.
  • Zero utility means that the candidate is completely useless as far as the criterion is concerned.
  NB : A zero utility does NOT imply that the candidate is useless as a whole, so a zero utility for one candidate with respect to one criterion must not be cause for trashing the candidate.
  Typical criteria are :
  4. "To solve my transport problem a vehicle will have to be fast, cheap to run, manoeuverable, safe and commodious"
  5. "Simple manufacture without powertools will be a requirement for any device we adopt to solve the D&B problem"
  6. "Any solution to the greenhouse gas problem must minimise the number of our people put out of work"

A constraint is more limiting than an equivalent criterion - the $10000 limit of constraint 'i' above leaves less scope for a solution than a criterion along the lines of 'cheap as possible' or 'minimum cost'. You should therefore try to increase the solution space wherever possible, by converting constraints into equivalent criteria. For example if Australia had adopted the criterion 'vi' above rather than the constraint 'iii' during the 1997 Kyoto talks then its stance would likely have attracted less criticism - ie. more folk might have been kept happy. Unfortunately many political decisions appear to be argued on the basis of constraints rather than criteria, thus antagonising whole sections of the community.

In this completion activity we are not interested in whether a candidate complies with a constraint or not, neither are we interested in how well a candidate satisfies a criterion - all we are doing is identifying and recording the constraints and criteria, so that they might be applied to candidates during later activities in the design process.

One of the most crucial tasks in the whole design process is to :

One of the reasons for mounting D&B competitions is that members of student design groups are involved with all life stages of their solution, and so must themselves establish the completion wishlist. Although other folks' agenda are not addressed, students still have to foresee the solution's future and to set constraints and criteria accordingly. These should not have to be inferred during later activities, but must be written down in black and white in the present completion activity.

One common criterion which is often overlooked is the need for simplicity. Here are a couple of cases where simplicity features :

• Two devices for the PEP design and build competition are contrasted - the first demonstrates a total disregard for simplicity, the second is the epitome of simplicity.
• I love my Citroën motor car! but it has some infuriating features - the accelerator cable gland is one of them.