CSE2305 - Object-Oriented Software Engineering
Week 7

Topic 14: Software Characteristics and Metrics

Synopsis

The issue is...how to measure these things
Why measure at all?
Human subjective perception is notoriously inaccurate (how many shark attacks in the last 200 years?)
Numbers give us a way of comparing, controlling and predicting
Measurements give us a way of tracking progress (and rescheduling if necessary)
Also provide an assessment of product quality
Measurement is the difference between "craft" and "engineering"

"A quantitative measure of the degree to which a system component or process possesses a given attribute (IEEE)
Hence, for each metric, we require...
...a measurable property
...a relationship between that property and what we wish to know
...a consistent, formal, validated expression of that relationship
For example: who is the greatest actor of all time?

The number of independent paths through the code
The number of separate test cases required to reach every line of code at least once.
Usually computed by representing program as a flow control graph and counting the distinct regions

[Diagram showing a graph represnting the flow control in a piece of software]

A language-independent measure of testing difficulty, and probable number of errors.
Some evidence that the values produced correspond to the intuitive notion of "software complexity"

An empirical formula based on...
...the number of inputs to a (sub-)system
...the number of outputs it produces
...the number of files (data repositories) used
...the number of interfaces^¤ to other systems
...the number of algorithms encapsulated
Numerical measure the "functionality" or "utility" of a system.
But is empirically derived and does not correspond to any directly perceivable quantity (i.e. just a number)

Class^¤-oriented metrics (Proc. OOPSLA, 1991)...
...Weighted methods per class^¤ (number of methods weighted by static complexity)
...Depth of inheritance tree (number of ancestral classes^¤)
...Number of children (number of immediate subclasses^¤)
...Degree of coupling (how many other classes^¤ rely on the class^¤, and vice versa)
...Response (number of public methods)
...Method cohesion (degree to which data members^¤^¤ shared by two or more methods)

(OO SW Metrics: A Practical Guide, 1994)
Class^¤- and operation- oriented
Class^¤ size (CS) (number of methods, number of attributes)
Number of operations overridden (NOO) by a subclass^¤ (number of virtuals subsequently redefined)
Specialization^¤ Index (NOO x level / CS)
Average size of operation (number of messages sent by operation)
Operation complexity (number of parameters to operation)

Pressman: Chapter 2, Chapter 17.4, Chapter 18.3.2, Chapter 19 (Optional), Chapter 24
An overview of object-oriented design metrics
Harrison, R.; Counsell, S.; Nithi, R.
Software Technology and Engineering Practice, 1997. Proceedings., Eighth IEEE International Workshop on [incorporating Computer Aided Software Engineering] , 1997, Pages: 230 -235 [Available on-line via the Monash library].