Page 82 - Software and Systems Requirements Engineering in Practice

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C h a p t e r 3 : E l i c i t i n g R e q u i r e m e n t s 53 53
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Prioritization and Ranking of Requirements
While prioritization and ranking of requirements typically occur after
analysis (or even later), the topic is worth mentioning here, as
customer priorities are best captured during elicitation.
First, we should mention the difference between the two, as there
tends to be some confusion regarding the use of the two terms.
Prioritization is the assignment of importance to a requirement using
a tag or label. For example,
• “The base engine sold with the car shall be a 1.8 liter
turbocharged engine”—priority high.
• “18 inch wheels shall be offered as an option with the car”—
priority medium.
Priorities are usually defined at the start of a project, using either
a numerical or verbal ranking; e.g., 1 means most important and 5
means least important (a numerical ranking has the advantage of
being sortable).
When priorities are assigned to requests and requirements by
stakeholders, only one of the defined values is acceptable.
Ranking is the assignment of a unique order to each requirement
in a group, such that no two requirements have the same rank. For
example,
Under $100 street price 1 (the lower number is more
important)
Built-in camera 2
Operable with one hand 3
LCD panel can be seen in 4
daylight
When deciding which features will be in a product release, a
ranking technique is normally used, whereas prioritization is used
more for initial scoping. When questionnaires or surveys are sent out
to customers, they will typically be asked to assign a priority to a
feature (e.g., more likely to buy the product, no difference, less likely to
buy the product).
A common problem can occur when customers label their
stakeholder requests as being of “high,” “medium,” or “low” priority,
since to some customers, every request will be of “high” priority.
An effective approach when scoping a product or planning
schedules or releases is to use pairwise ranking [Karlsson 1996],
[Sobczaka et al. 2007]. Pairwise ranking, sometimes called the