Software Quality and Metrics

Software Quality

• What is Quality ?

• Quality Characteristics

• Cost evolution : Software vs Hardware

• How to solve it ?

Software Quality

• What it is not !– Zero errors– A good GUI

• What it is !– Meeting the user ‘s needs

ISO Definition of Quality

• ‘The totality of characteristics of an entity that bear on its ability to satisfy stated and implied needs’

Software Product

Should fullfill has

characteristicsrequirementsIs related to

(contributes to the fulfillments)

ISO 9126 Characteristics

• The objective of this standard is to provide a framework for the evaluation of software quality. ISO/IEC 9126 does not provide requirements for software, but it defines a quality model which is applicable to every kind of software. It defines six product quality characteristics and in an annex provides a suggestion of quality subcharacteristics.

ISO 9126 Characteristics• Functionality is the set of attributes that bear on the existence of a set of functions and their

specified properties. The functions are those that satisfy stated or implied needs. • Reliability is the set of attributes that bear on the capability of software to maintain its level of

performance under stated conditions for a stated period of time. • Usability is the set of attributes that bear on the effort needed for use, and on the individual

assessment of such use, by a stated or implied set of users. • Efficiency is the set of attributes that bear on the relationship between the level of performance

of the software and the amount of resources used, under stated conditions. • Maintainability is the set of attributes that bear on the effort needed to make specified

modifications. • Portability is the set of attributes that bear on the ability of software to be transferred from one

environment.

Sub CharacteristicsCharacteristics Subcharacteristics Definitions

Suitability Attributes of software that bear on the presence and appropriateness of a set of functions for specified tasks.

Accurateness Attributes of software that bear on the provision of right or agreed results or effects.

Functionality Interoperability Attributes of software that bear on its ability to interact with specified systems.

Compliance Attributes of software that make the software adhere to application related standards or conventions or regulations in laws and similar prescriptions.

Security Attributes of software that bear on its ability to prevent unauthorized access, whether accidental or deliberate, to programs or data.

Maturity Attributes of software that bear on the frequency of failure by faults in the software.

Reliability Fault tolerance Attributes of software that bear on its ability to maintain a specified level of performance in case of software faults or of infringement of its specified interface.

RecoverabilityAttributes of software that bear on the capability to re-establish its level of performance and recover the data directly affected in case of a failure and on the time and effort needed for it.

Understandability Attributes of software that bear on the users’ effort for recognizing the logical concept and its applicability.

Usability Learnability Attributes of software that bear on the users’effort for learning its application.

Operability Attributes of software that bear on the users’effort for operation and operation control.

Sub CharacteristicsCharacteristics Subcharacteristics Definitions

Efficiency Time behaviour Attributes of software that bear on response and processing times and on throughput rates in performances its function.

Resource behavior Attributes of software that bear on the amount of resource used and the duration of such use in performing its function.

Analyzability Attributes of software that bear on the effort needed for diagnosis of deficiencies or causes of failures, or for identification of parts to be modified.

Maintainability Changeability Attributes of software that bear on the effort needed for modification, fault removal or for environmental change.

Stability Attributes of software that bear on the risk of unexpected effect of modifications.

Testability Attributes of software that bear on the effort needed for validating the modified software.

AdaptabilityAttributes of software that bear on the opportunity for its adaptation to different specified environments without applying other actions or means than those provided for this purpose for the software considered.

Portability Installability Attributes of software that bear on the effort needed to install the software in a specified environment.

Conformance Attributes of software that make the software adhere to standards or conventions relating to portability.

Replaceability Attributes of software that bear on opportunity and effort using it in the place of specified other software in the environment of that software.

Quality model

• The relative weight of the characteristics depends on the type of software– Prototype– Critical (ex : nuclear power plant)– Long live (ex : Telecom switch)– …

Quality model

• What should be the ranked list of Quality characteristics (most important first) for a software used in a space shuttle ?

• Give them a weight.

NAVETTE HERMES

• QUALITY CHARACTERISTICS IN DECREASING ORDER :– RELIABILITY CHANGEABILITY– ROBUSTNESS FLEXIBILITY– AVAILABILITY PORTABILITY– CONFORMANCE REUTILISABILITY– INTEGRITY ADAPTABILITY– USABILITY– ERGONOMY– EFFICIENCY– TESTABILITY

Software Metrics• The Quality needs to be quantified

Maintainability

Testability

Cyclomatic complexity Max nber of interleaving Nbre of input/output

StabilityChangeability Analyzability

Metrics versus measurement

• A measure f is a function f:A->B where for each object a belonging to A there exists a formal object (measure’s value) f(a) belonging to B

• A function f is called a metric iff it has the three following properties:– f(x,y)=0 with x=y– f(x,y)=f(y,x) for each x and y– f(x,z)<=f(x,y)+f(y,z) for each x, y, z

Metrics versus measurement• A measure should satisfy 4 criteria– Being in relation with well understood and significant

attributes– Being supported by formal models or abstractions that

capture the attributes– Respecting the relations and the existing order between

objects (determined by the attributes of the models– Being mapped to the numeric system in order to maintain

the order relationship.• Remark: if it doesn’t respect the three first criteria, it is

a metric

Type of measurement

nominal

ordinal

interval

ratio

absolute

Type of measurement (examples)

• Nominal: number on the back of a football player

• Ordinal: Richter scale• Interval: f(x)=ax+b. Temperature is of interval

type• Ratio: f(x)=ax. A length is a ratio• Absolute: f(x)=x. Number of people working in

a company

Building of measures• Non terminal measures

– Raw value => normative valueEx: v(G) < 15 => v(G) = 2

15<v(G)<=30 => v(G) = 1v(G)> 30 => v(G) = 2

• Terminal measuresMes_1 = (mes_2 + 4*mes3)/5• Qualitative expression:[0;0.5 ] => Mes_1 = 0 NOK]0.5;1.5] => Mes_1 =1 Average]1.5;2] =>Mes_1= 2 OK

Software Metrics

• Different levels of measurement– Architecture• Call Graph

– Module• Control Graph

– Textual

Software Metrics

• Architecture– Number of paths– Hierarchical complexity– Structural complexity– Component assessibility– Path testability– System testability– System entropy

Software Metrics

• Structural complexity– Cyclomatic number– Max number of degrees– Max number of interleaving

• Textual– Halstead Software Science measures

• Programming Norms

Object-Oriented Software Metrics

• Depth of inheritance tree (DIT)– DIT is defined as the maximum number of steps

from the class node to the root of the inheritance tree

• What it measures: Since in OO design, the application domain is modelled as a hierarchy of classes represented as a tree, the DIT metric is a measure of how many ancestor classes can affect a particular class. The depth of inheritance tree metric is an indication of the potential for reuse.

Should be between 0 and 4


• Number of children (NOC)– NOC is defined as the number of immediate

descendants of a class in the hierarchy. (1..4)

• What it measures: NOC is an indicator of the potential influence a class can have on the design and on the system


• Coupling between object classes– : This is a count of the number of other classes to

which a given class is coupled.

• What it measures: Also known as message delivery channels, this metric counts the number of associations in a class and attributes whose parameters are of the class type. Messages can only be sent when an object of a class holds a reference to another object of a class

Should be between 1 and 4


• Number of the associations linked to a class– The number of associations including aggregations

is counted.

• What it measures: This metric is useful for estimating the static relationships between classes.


• Number of the elements in the transitive closure of the subclasses of a class

• What it measures: This is potentially useful for predicting the classes whose changes might affect this class.


• Number of the superclasses of a class– This counts the direct parents of a class

• What it measures: In a single inheritance implementation like Java, the value of this metric is either 0 or 1, whereas under multiple inheritance scheme it is greater than or equal to 0.


• Number of use cases in a model

• What it measures: A use case represents a coherent unit of functionality provided by a system, a subsystem, or a class.


• Number of system classes associated with a use case– This metric counts the number of classes whose objects

participate in the scenario of a use case.

• What it measures: NSCU is good for estimating the impact of a requirement change onto the system. Any changes of use cases spread to classes and the interactions of their objects, and vice versa


• Number of messages associated with a use case– This metric counts the number of messages comprising

the scenario of a use case. • What it measures: A use case is further refined through its

scenario. In UML, there are two scenario diagrams, i.e. sequence diagram and communication diagram. These two kinds of scenario diagrams are completely isomorphic meaning one kind of diagram can be automatically replaced with another kind without the loss of information contained in it. This metric is useful for tracing requirements into design-level elements.


• Number of actors in a model– This metric is the number of actors in a model,

where an actor is a special class whose stereotype is “Actor”.

• What it measures: This metric computes the number of actors in a model.


• Number of actors associated with a use case– : This metric is the number of actors that are

associated with a use case.

• What it measures: Normal system classes are not counted for this metric because this metric concerns the interactions between a system and its stakeholders.


• Number of classes in a model

• What it measures: A class in a model is an instance of the metaclass “class”. This metric is comparable to the traditional LOC (lines of code) for estimating the size of a system. This metric can be used to compare sizes of systems.


• Number of inheritance relations in a model

• What it measures: This metric counts the number of generalisation relationships between classes existing in a model.


• Number of packages in a model

• What it measures: Package is a way of managing closely related modelling elements together. Also by using packages, naming conflicts can be avoided.


• Number of objects in a model

• What it measures: In a similar manner that a class is an instance of the metaclass “Class”, an object is an instance of a class.


• Number of associations in a model– This metric measures the number of connections

between classes

• What it measures: An association is a connection, or a link, between classes. This metric is useful for estimating the scale of relationships between classes.


• Number of aggregations in a model

• What it measures: An aggregation is a special form of association that specifies a whole-part relationship between the aggregate (whole) and a component part


• Number of messages in a model

• What it measures: A message is an instance of the metaclass “Message”. Messages are exchanged between objects manifesting various interactions


• Number of methods (NOM)– NOM is defined as the total number of methods in a

class, including all public, private and protected methods.

• What it measures: It has been suggested that this metric is a useful indication of the classes that may be trying to do too much work themselves; i.e., they provide too much functionality. NOM cannot be viewed as an indicator of the effort to develop a class, since a class containing a single large method may take as long to develop as a class containing a large number of small methods.

• Number of methods added (NMA)

• What it measures : The number of operations added plays a role in the specialization of the class and must be maintained in a proportion which continues to justify inheritance, otherwise known as generalization.

Too many added operations signify too big a difference with the parent class. The inheritance would then make less sense.

The more added operations there are, the more the class must be redeveloped, and the less the inheritance is justified.


• Number of Methods Inherited (NMI)

• What it measures: For a class, this metric gives the percentage of the number of non-redefined operations with regard to the number of operations inherited.

The percentage of operations inherited should be high. This is the opposite of the Number of Methods Overridden (NMO) threshold. A low percentage of inherited operations indicates poor sub-classing.

The maximum of 100 % is ideal, but is never attained, given the fact

that we often need to adapt inherited services.


http://support.objecteering.com/objecteering6.1/help/us/metrics/metrics_in_detail/number_of_methods_overridden.htm

• Number of Methods Overridden (NMO)

• What it measures: The number of redefined operations plays a role in the specialization of the class and must be maintained in a proportion that continues to justify inheritance.

Too many redefined operations implies too big a difference with the parent

class and inheritance then makes less sense.For a class, this is the count of the number of inherited operations that are

redefined by the class.A class which inherits services must use them with a minimum of

modifications. If this is not the case, the inheritance loses all meaning and becomes a source of confusion.


http://support.objecteering.com/objecteering6.1/help/us/metrics/metrics_in_detail/number_of_methods_inherited.htm


• Response for a class– This is the size of the set of methods that can

potentially be executed in response to a message received by an object.

Class AA::m1() calls B::m1(), B::m2()A::m2() calls A::m1(),C::m1()A::m3()RFC = 7


• Lack of cohesion in methods– This is a measure of the number of disjoint sets

formed by the intersection of the sets of instances variables used by methods.

• Lack of cohesion in methods (another one)

The LCOM value for a class C is defined as LCOM(C) = 1 - |E(C)| / (|V(C)| x |M(C)|), where V(C) is the set of instance variables, M(C) is the set of instance methods, and E(C)

is the set of pairs (v,m) for each instance variable v in V(C) that is used by method m in M(C).



• Number of attributes (NOA)– NOA is defined as the total number of attributes in a

class.

• What it measures: The ratio of private and protected attributes to total number of attributes indicates the effort required by that class in providing information to other classes. Private and protected attributes are therefore viewed merely as data to service the methods in the class


• Number of public attributes• Number of protected attributes• Number of private attributes• Number of class attributes• Number of public methods• Number of protected methods• Number of private methods• Number of class methods

Software Analysis Process and Report

• Architectural Level

• Structural Level

• Report

Architectural Level

• General view of the call graph– Depth, width, hierarchy, levels jump

• Singularities– Isolated components, roots, strongly connected

components, recursivity

Call Graph examinationSymptom diagnosis decision

Large number of calls No hierarchy in tasks Splitting of the component

Large number of recursivities

Very high risk: problem of ending recursivity

Careful reading of codeIntensive testing of components

Components largely used Essential resource of architecture => high risk

Careful reading of codeIntensive testing of components

Isolated component Doesn’t belong to this version

Deleting the component

Root for a very limited number of paths in the graph

Auxiliary Entry point (ex: debugging)

Deleting of this root (doesn’t feature a normal case)

Call Graph

Call Graph (analyzed components)

Call Graph (root = ana_appel)

Call Graph (root = display)

Control Graph examination

• General view of the control graph– Sequences, nesting, pending nodes

• Singularities– Isolated code, graphical similarities, intern calls

Control graph examinationSymptom Diagnosis decisionDestructuring back call Bad maintainability, infinite

loop risk, bad understandability

Back to the algorithm to rewrite the component

Non reachable code Many changes done, no test, bad readability

Delete the dead code

Illogical structure, many auxiliary exits

Poor programming. Bad readability

Delete the exits but one => homogenisation

Several complex structures in sequence

Very height intern complexity. Hard to test

Split the component in several ones

Several nested complex sequences

Lack of modularity Organize these structures in components and create a new level in the call graph

Graphical similarities Non factorized code Factorization of code – Creation of a new component

Auxiliary entry point Lack of modularity. Poor maintainability

New component. Delete the entry point

Control Graph

Quality Report• Quality model– Quality characteristics (from the most to the least important)

for the project– Sub characteristics– Measures related to these characteristics and sub

characteristics (upper and lower bounds)• Global quality results– Kiviat, histograms, …

• Detailed analysis– Architecture– Components (accepted, rejected, …)

Example

Maintainability

Testability

Cyclomatic complexity Max nber of interleaving Nbre of input/output

StabilityChangeability Analyzability

Testability

• Testability– Cyclomatic complexity v(G)• V(G) <= 15 => OK 2• V(G) > 15 => NOK 0

– Max_nb_interleaving• Max_level <=4 => OK 2• Max_level > 4 => NOK 0

– Nb_input/output• Nb_IO =2 => OK 2• Nb_IO >2 => KO 0

Testability=(4*V(G)+4*Max_level+2*Max_IO)/10

V(G) Max_Level Max_IO Diagnosis

OK OK OK 2 = Accepted

OK OK 1.6 = restructure

OK OK 1.2 = to split

OK 0.8 = to split

OK OK 1.2 = to split

OK 0.8 = to split

OK 0.4 = to rewrite

0 = to rewrite

TestabilityUpper bound

Lower bound

V(G)

Max_level

Max_IO

1

15

2

2

1 4

V(G) = 12Max_Level =3Max_IO =2

TestabilityV(G)

Max_level

Max_IO

1

15

2

2

1 4

V(G) = 17Max_Level =5Max_IO =2

Maintainability

Testability Changeability

AnalysabilityStability

V(g)

Max_level

Max_IO

fcom

Nb_Ins

V(g)

Nb_Ins

Max_IOfcomMax_IO

Nb_Ins

Max_level

V(g)

Example in OO Programming

• Quality Model

• Measures and bounds

Quality Model

• Reliability– WMPC– CBO– FD– RFC

• Efficiency– CBO– AC– LCOM

• Maintainability– ASC - AR– DIT - ComR– AC– RFC

• Portability– NMCI - DIT– AR - EH– ER - PR

Class Level Measures

• ComR (Comment Rate)• ER (Encapsulation Rate =nber of private

variables/total number of variables)• NCI (Nber of Classes)• NMCI (Average Nber of Methods per class)• WMPC (Weighted Methods per Class)• LCOM (Lack of COhesion in Methods)• DIT (Depth of inheritance Tree)

Inheritance Measures

• EH (Entropy of inheritance Tree)• AR (Abstraction Rate = Nber of abstract

classes/Total nber of classes)• PR (Polymorphism Rate = (Nber of virtual

methods /total number of methods)

Object interactions Complexity measures

• AC (Association Complexity = R-NCI +2P with R nber of interactions between objects and P nber of disconnect parts in the graph)

• FD (Flow Density = sum of FI *p(start node exists) with FI, number of invoked methods between 2 objects and start node, object invoking methods)

Object interactions Complexity measures

• CBO (Coupling between Objects)

• RFC (Response for a class)

MeasuresLCOM Lack of COhesion in Methods <3

CBO Coupling between objects <10

EH Entropy of inheritance tree <2

AR Abstraction Rate >0.2

PR Polymorphism Rate >0.2

AC Association Complexity <15

WMPC Weighted Methods Per Class <15

RFC Response for a Class <10

FD Flow Density <100

ASC Average Size per Class <24

DIT Depth of inheritance tree <5

ComR Comment Rate >0.2

NCIER

Number of classesEncapsulation Rate

<20>0.9

Software Quality and Metrics

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