Page 30 - Biomedical Engineering and Design Handbook Volume 2, Applications

MEDICAL PRODUCT DESIGN 9

An additional concern that should be recognized here is that many aspects of medical practice
vary, often geographically and sometimes in more complex patterns. A procedure may be done
frequently in one facility and rarely in another, which would seem to have a similar patient base. At
4
the international scale, the variations can be striking. Rutter and Donaldson provide an interesting
examination of some of these issues. In assessing the needs for a device, patterns of treatment that
are relative to the problem should be examined carefully.
The material collected from this exercise should be sorted, characterized, and summarized into a
document that is the first real step toward the design. Individual statements from stakeholders should
be preserved verbatim. Condensation should only remove redundancy. If we have seven people
telling us that the design must be such that it can be operated with one hand, at least one of them
should be quoted in the needs document. At a later point in the process, it will be important to examine
whether the request was met, and it should be tested against the user’s words, not the developer’s
interpretation of the user’s need.
When a list of the needs has been established, the team should assign weights to each item. This
can be done on a scale of 1 to 5 or 1 to 10, or even a three-level scale of “highly,” “moderately,” and
“minimally” important.
This is the first time that the team doing numerical evaluations has been mentioned, and it brings
up several potential problems. In this instance, there may be some tendency to bias the scores
according to what is perceived to be obtainable. If we know that keeping the weight of the product
down is going to be difficult, we know that we will feel better if somehow it is not so important.
Unfortunately, how we feel has little to do with how important it might be, so the team must gather
itself up and evaluate things from the point of view of the user, regardless of the difficult position
into which it may put itself.
The second risk in team scoring is team members who strategize. Typically, this takes the form
of stating a position that is different and more extreme than that which is held, in order to offset the
position of another team member. If I think the item is a 6, and I think you want to score it an 8, I
would give it a 4 to offset your opinion. Some individuals have difficulty resisting the desire to do
this, and some may even do it unconsciously. It is one of the responsibilities of the team leader to
recognize that this is happening and to deal with it. One measure of an effective team is the absence
of this effect. If it were to happen frequently, the team could be considered dysfunctional.
The most highly developed and detailed processes for obtaining user input have been described
under the title quality function deployment (QFD). For a more detailed description of this process,
see Hauser and Clausing. 5

1.9 PRODUCT SPECIFICATIONS

The document that clarifies the needs is expressed in the language of the user. The next step in our
process is to transform these needs into specifications that can be used to guide the design decisions.
These product specifications become the targets for the product development and as such they need
to be measurable. In general there should be a specification for each identified need, but there are
exceptions to this that run in both directions.
The important characteristic of specifications is that they must be expressed in measurable terms,
for example, “The device must weigh less than 2.3 lb,” or “The power consumption must be less than
350 W.” While the user needs were often qualitative, the purpose now is to write requirements that
can be used to evaluate the ideas and designs produced. Before proceeding to define the specifica-
tions, the metric or unit of measure for each of the needs must be identified. These are often obvi-
ous, but sometimes require some thought. Most requirements will state maximum or minimum
values, like weight and power consumption. Sometimes there are actual target values that are desired,
such as “Must fit in size B front end.”
The metrics selected for each requirement should be considered carefully. They should be the
common measures for simple characteristics. The difficulty of making the measurements must be
considered. These specifications could easily turn into quality assurance requirements in the product,