Scale specification

From © Tom Gilb, 2011, Chapter 5: How to Quantify: Scales of Measure.

Principles: Scale Specification

1. The Principle of ‘Defining a Scale of Measure’
   • If you can’t define a scale of measure, then the goal is out of control.
   • Specifying any critical variable starts with defining its units of measure.
2. The Principle of ‘Quantification being Mandatory for Control’
   • If you can’t quantify it, you can’t control it.
   • If you cannot put numbers on your critical system variables, then you cannot expect to communicate about them, or to control them.
3. The Principle of ‘Scales should control the Stakeholder Requirements’
   • Don’t choose the easy scale, choose the powerful scale.
   • Select scales of measure that give you the most direct control over the critical stakeholder requirements. Chose the Scales that lead to useful results.
4. The Principle of ‘Copycats Cumulate Wisdom’
   • Don’t reinvent scales anew each time – store the wisdom of other scales for reuse.
   • Most scales of measure you will need, will be found somewhere in the literature, or can be adapted from existing literature.
5. The Cartesian Principle
   • Divide and conquer said René – put complexity at bay.
   • Most high-level performance attributes need decomposition into the list of sub-attributes that we are actually referring to. This makes it much easier to define complex concepts, like ‘Usability’, or ‘Adaptability,’ measurably.
6. The Principle of ‘Quantification is not Measurement’
   • You don’t have to measure in order to quantify!
   • There is an essential distinction between quantification and measurement.
   • “I want to take a trip to the moon in nine picoseconds” is a clear requirement specification without measurement.”
   • The well-known problems of measuring systems accurately are no excuse for avoiding quantification. Quantification allows us to communicate about how good scalar attributes are or can be – before we have any need to measure them in the new systems.
7. The Principle of 'Meters Matter'
   • Measurement methods give real world feedback about our ideas.
   • A ‘Meter’ definition determines the quality and cost of measurement on a scale; it needs to be sufficient for control and for our purse.
8. The Principle of 'Horses for Courses'
   • Different measuring processes will be necessary for different points in time, different events, and different places.
9. The Principle of ‘The Answer always being ‘42’’
   • Exact numbers are ambiguous unless the units of measure are well-defined and agreed.
   • Formally defined scales of measure avoid ambiguity. If you don’t define scales of measure well, the requirement level might just as well be an arbitrary number.
10. The Principle of ‘Being Sure About Results’
    • If you want to be sure of delivering the critical result – then quantify the requirement.
    • Critical requirements can hurt you if they go wrong – and you can always find a useful way to quantify the notion of ‘going right.’

Source

• http://www.idi.ntnu.no/emner/tdt4240/Syllabus/GilbChapter5.pdf