Once upon a time The Spear wrote specifications as part of his job. What he quickly came to realise, without really giving it much thought, was that there were essentially two very different ways of specifying requirements, each with their own strengths and weaknesses. We use both every day.
The first is an outcome-based, or ‘top-down’ specification. It focuses on what is trying to be achieved, without giving much detail as to how it is to be achieved. For instance, ‘the shirt must be cleaned,’ is a form of top down specification.
Outcome based specifications are often used when the specifier is short on time or technical expertise, where the process used to achieve the outcome is not deemed to be critical. They are a kind of ‘catch-all’ that cannot fail to capture the want.
Due to the often qualitative nature of the fulfillment of top down specifications (all projects must be accepted at some stage by a human being who gives the ‘OK’), it is typical that such a specification comes with a qualitative acceptance clause such as ‘to the satisfaction of X’. Contractors and those seeking to satisfy specs hate this ambiguity as it is hard to quantify (i.e. cost).
Their lack of detail gives the person receiving the specification leeway to satisfy the want as they see best. While this can be a way of efficiently performing the task, it may however introduce unwanted, unspecified side effects. A shirt may be cleaned, but it may be wrinkled in the process. It has positively fulfilled a specification which failed to detail undesirable negatives. As there are infinitely more things that the specifier does not want to happen to the shirt, it is impossible to capture them all in a general top-down specification.
This hunt for exactness leads to process-based, or ‘bottom-up’ specifications. After stating that only the specification as it is detailed it to be followed, a bottom-up spec aims to detail, item by item, step by step, the process of creating the final output in as much detail as possible. For example, ‘a shirt consisting of type A white, intertwined cotton fibres of 1.2 microns in diameter, sanitised for 35 minutes in 18L of grade 315 bleach at 66 degrees celcius in an air-tight chamber’ is a bottom-up spec.
These detailed specs are used when the writer has technical expertise in the area of specification and where the process requires a high degree of control, such as in an experiment or high-end manufacturing. So, whereas a top-down spec may be made by an end-user, a bottom-up spec is more likely to be written by a third-party specialist.
While infinite detail is impossible to achieve, small ‘catch-alls’ are embedded in the main specification to try and tie up any loose ends, such as ‘to manufacturer’s specifications’ or ‘to the principles of X’, in the effort to at least provide direction to those seeking more information as to how to fulfill the finest details of the spec.
While bottom-up specs are great for the person trying to fulfill the spec because their satisfaction of the spec is mostly free from ambiguity, their very rigidity may prevent the satisfaction of the desired outcome in the most efficient way, or even at all.
If a bottom-up specifier does not know what they are doing, or is not up with the latest developments in the field, a detailed specification may be nothing but a very length recipe for disaster, or the loss of a lot of money.
This is why The Spear, when making his own professional specifications, tended to cover his arse with top-down specifications in any instances where he doubts his own ability to adequately bottom-up specify on a subject. While generally specifying detail, he would use outcome-based catch-alls wherever he thought the risk of mis-specification was too great.
He wants his shirt to be machine washed with warm water with standard detergent, to his satisfaction.