Techniques: Parameters and perimeters
by David Blakey
A method of refining assignment briefs.
[Monday 21 July 2003]
For some assignments, it may be useful to make a first version of the assignment brief in terms of parameters, and then refine this brief in terms of perimeters before translating it back in terms of parameters.
Parameters
The parameters of an assignment define what the assignment should produce and how it should be produced. They describe, often in simple words, what the consultant needs to do.
Imagine an orchard of apple and pear trees. Your assignment is to pick all the ripe apples in the orchard.
Your assignment has a number of parameters.
- You must pick only in the orchard.
- You must pick only apples.
- You must pick only ripe apples.
Perimeters
These can be expressed also as perimeters. Instead of describing the items that are enclosed within the assignment, we can describe the enclosures that surround those items.
- You must pick only in the orchard. The perimeter is the geographic border of the orchard. You cannot pick apples from the next orchard.
- You must pick only apples. The perimeter includes all those fruit that have the characteristics of apples.
- You must pick only ripe apples. The perimeter is a degree of ripeness of the apples.
These perimeters raise some questions.
- What about fruit that overhangs the client's orchard from a tree that is actually in a neighbour's orchard?
- What about fruit that overhands the neighbour's orchard from a tree in the client's orchard?
- Is the criterion the position of the tree or the position of the apple or both?
- Can apples resemble pears in appearance?
- If so, what other tests can be applied to identify apples?
- How do we know when fruit is ripe?
- Does it matter if some apples are not quite ripe?
Some other perimeters are presented.
- Can you include the stalk as part of an apple?
- Is there a maximum length of stalk?
- Can you include leaves?
- What about apples that are ripe but that are worm-eaten or frost-bitten?
Focus
You must be careful about the focus when you switch from defining an assignment in terms of parameters to defining it in terms of perimeters. There are parameters beyond the perimeters. The parameters inside the perimeters are the things that must be done. The parameters beyond the perimeters are the things that must not be done.
You should avoid just switching from a view of ‘positive’ parameters to one of ‘negative’ parameters. The perimeters are actually the borders. In our example, we switch from looking at the orchard to looking at the fences that surround it.
Presentation
It may often help to switch from parameters to perimeters so that you can refine the definition of an assignment. Once you have produced a set of sound perimeters, you can switch back to a set of parameters. This can often help with presentation.
For our apple-picking assignment, our revised parameters could be:
- You must pick only in the orchard, enclosed as follows ...
- You must pick only apples, which have a shape as follows ...
- You must pick only healthy apples, which must meet the following criteria ...
- You must pick only ripe apples, whose characteristics are ...
- You may include stalks and leaves, to the following limit ...
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