Writing a Test Strategy

Torbjørn Skramstad

Comment: Slides 1-8 were lectured in week 7 (14.02.2013)The rest of the slides were lectured in week 8 (21.02.2013)

Why a testing strategyWe need a testing strategy to help• System and software testers plus Test-and-

Evaluation staff to determine the overall test strategy when developing or modifying a software intensive system

• Project stakeholders – customers and senior management – to approve the test strategy

• Testers and system and software analysis to determine– Test objectives– Qualification requirements– Verification and validation criteria

Testing strategy conceptsWe will discuss the following concepts:• Purpose of a test strategy• Testing focus• Contents of a test strategy• Software integrity levels• Test objectives and priorities

Purpose of a test strategy – 1

The test strategy is important in order to• Obtain consensus on test goals and objectives

from stakeholders – e.g. management, developers, testers, customers and users

• Manage expectations right from the start• Be sure that we are heading in the right

direction• Identify the type of tests to be conducted at

all test levels

Purpose of a test strategy – 2

When we write a test strategy is important to remember that:

• Whatever we do, some kind of test strategy will emerge. Thus, we might as well specify the one we think is the best one

• A documented strategy is the most effective way to get an early agreement on goals and objectives

• We need to address:– Human factors – usability– Interoperability, except for stand-alone systems

Testing focus Our focus will depend on which stakeholder we

are considering at the moment:• Users – acceptance test and operational tests• Analysts – systems test, qualification tests• Designer – integration tests• Programmer – unit testsThe main point is that we need to define the

stakeholder first – then the tests to be run.

Contents of a test strategy – 1 The following is a list of what can be specified in

a test strategy. Not all of it is needed in all cases – only use what is necessary.

• Project plan, risks and activities• Relevant regulations – depending ofn

application area• Required processes and standards• Supporting guide lines

Contents of a test strategy – 2• Stakeholders – e.g. users, testers, maintainers

– and their objectives• Necessary resources – people, computers• Test levels and phases• Test environment – e.g. lab equipment• Completion criteria for each phase• Required documentation and review method

for each document

Test Strategy continued

• Started from here 21.02.2013

Software integrity levelsThere are several ways to define software

integrity levels. When we choose an integrity level this will strongly influence the way we do testing.

We will look at three definitions of integrity levels:

• IEEE 1012 – general software• ISO 26262 – automotive software• IEC 61508 – general safety critical software

Warning • The next slides are taken from IEEE 1012 and the two

standards for safety critical software ISO 26262 (automotive) and IEC 61508 (industrial automation).

• They show the recommended techniques for some testing activities – integration testing, unit testing and module testing.

• Note that these lists– Show what is recommended depending on SIL or ASIL level

HR, R, NR– Do not include any techniques related to TDD or other

agile techniques.

IEEE 1012 – general software

• 4, High – some functions affect critical system performance.

• 3, Major – some functions affects important system performance

• 2, Moderate – some functions effect system performance but workarounds can be implemented to compensate.

• 1, Low – some functions have noticeable effect on system performance but creates only inconveniences

V&V Activities

V&V activityDevelopment requirements

level

Design level

Implementation level

Test level

SW Integrity Level 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1

Acceptance test execution X X X

Acceptance test plan X X X

Interface analysis X X X X X X X X X

Management and review support

X X X X X X X X

Management review of V&V

X X X X X X X X X X X

ISO 26262 – automotive software

The ASIL level – A, B, C or D – is the outcome of the combination of three factors:

• S – Severity. How dangerous is a event• E – Probability. How likely is the event• C – Controllability. How easy is the event to

control if it occurs

Methods for software integration testing

Methods and Measures Accordingto req.

ASIL

A B C D

1 Requirements based test 9.4.4 ++ ++ ++ ++

2 External interface test 9.4.4 + ++ ++ ++

3 Fault injection test 9.4.4 + + ++ ++

4 Error guessing test 9.4.4 + + ++ ++

Methods for software unit testing

Methods and Measures Accordingto req.

ASIL

A B C D

1 Functional tests 8.4.2 See table 8.2

2 Structural coverage 8.4.2 See table 8.3

3 Resource usage measurement 8.4.2 + + + ++

4 Back-to-back test between simulation model and code, if applicable

8.4.2 + + ++ ++

Safety integrity

level

High demand or continuous mode of operation

(Probability of a dangerous failure per hour)

4 10 -9 to 10 -8

3 10 -8 to 10 -7

2 10 -7 to 10 -6

1 10 -6 to 10 -5

IEC 61508 – safety critical software

Module testing and integration Technique/Measure Ref SIL1 SIL2 SIL3 SIL4

1 Probabilistic testing C.5.1 --- R R HR

2 Dynamic analysis and testing B.6.5Table B.2

R HR HR HR

3 Data recording and analysis C.5.2 HR HR HR HR

4 Functional and black box testing B.5.1B.5.2

Table B.3

HR HR HR HR

5 Performance testing C.5.20Table B.6

R R HR HR

6 Interface testing C.5.3 R R HR HR

a) Software module and integration testing are verification activities (see table A.9).b) A numbered technique/measure shall be selected according to the safety integrity level.c) Appropriate techniques/measures shall be selected according to the safety integrity level.

Test objectives and priorities

• Only in rather special cases (binary input / output and few parameters) can we test all input. Thus, we need to know – The overall objective of testing– The objective of every test case– The test case design techniques needed to

achieve our goals in a systematic way.• The test objectives are our requirements

specification for testing.

Test data selection

• One of the important decisions in selecting a test strategy is how to select test data. We will look at five popular methods– Random testing– Domain partition testing– Risk based testing– User profile testing– Bach’s heuristic risk-based testing

Random testing• The idea of random testing is simple:

1. Define all input parameters – e.g. integer, real, string2. Use a random test / number generator to produce inputs

to the SUT

• The main problem with this method is the lack of an oracle to check the results against. Thus, manual checking is necessary.

• The method is mostly used for crash testing (robustness testing) – will the system survive this input?

Domain partition testing – 1

Definitions:• A domain is a set of input values for which the

program performs the same computation for every number of the set. We want to define the domains so that the program performs different computations on adjacent domains

• A program is said to have a domain error if the program incorrectly performs input classification – selects the wrong domain

Domain partition testing – 2• The main challenge with domain partition is to find

the domains. For small components we can identify the domains from the code.

• For large components or when we do not have access to the code, we may use the algorithm that is used for the implementation, e.g.

X interval 1 => F1X interval 2 => F2X interval 3 => F3

– will give us three input domains defined by the three intervals

Domain partition testing – 3• Textual use case can also be used to identify

input domains. Important sources of information are– Preconditions and triggers. Each set may identify a

new input domain– Alternate flows. Each alternate flow may identify a

new input domain

Domain partition testing – 4

• When we discuss testing, it is reasonable to also include erroneous inputs.

• If we have several input parameters, incorrect values for each may identify a new domain since the error handling, triggered by a test or an exception will activate a new path through the code.

Domain testing – simple example(Second degree equation)

02 cbXaX

If a ≠ 0, this equation has the following solution

a

c

a

b

a

bX

2

2

42

Otherwise we have that

b

cX

Testing domains – 1

A = 0a = 0

A = 0a ≠ 0

A = 0

A = 0 04 2

2

a

c

a

b

04 2

2

a

c

a

b

D3

D2

D1

Testing domains – 2

We have four input domains:•D0: a = b = 0, which is a line in the three-dimensional space •D1: a = 0 and b ≠ 0, which a plane in the three-dimensional space •D3: b*b >= 4*ac and a ≠ 0. •D2: b*b < 4*ac and a ≠ 0.

Testing domains – 3

a

b

D0

D1

D1

Domains for a fixed c - value

D3

D2

D3

D2

2* sqrt(ac )

a

b

D

Domains for a fixed c - value

D3

D3

2* sqrt(ac )

Risk based testing

The idea of risk based testing is to 1.Identify the risk or cost of not delivering a

certain functionality.2.Use this info to prioritize tests. We will cover

this in more details later under the topic “Test prioritation”

User profile testing• The main idea with this type of testing is to

generate tests that mirror the user’s way of using the system.

• Consider a situation where we know that the users in 80% of all case – Fetch a table from the database– Update one or more info items– Save the table back to the database

• Then 80% of all tests should test these three actions.

Bach’s risk-based testing• Bach’s heuristics is based on his experience as

a tester. Based on this experience he has identified– A generic risk list – things that are important to

test– A risk catalogue – things that often go wrong

• Following slides give a short summary of the first of Bach’s lists.

Bach’s generic risk list – 1 Look out for anything that is:• Complex – large, intricate or convoluted• New – no past history in this product• Changed – anything that has been tampered with or

“improved”• Upstream dependency – a failure here will cascade

through the system• Downstream dependency – sensitive to failure in the

rest of the system• Critical – a failure here will cause serious damage

Bach’s generic risk list – 2• Precise – must meet the requirements exactly• Popular – will be used a lot• Strategic – of special importance to the users or

customers• Third-party – developed outside the project• Distributed – spread out over time or space but still

required to work together• Buggy – known to have a lot of problems• Recent failure – has a recent history of failures.

Test and system level – 1

Test and system level – 2• From the diagram on the previous slide we

see that we can test on the1. Electronics level – e.g. DoorActuator sends the

right signal2. State / signal level – e.g. door is closed iff

DoorStateClosed3. Logical level – e.g. the door remain closed as

long as the speed is non-zero4. Safety level – e.g. the door remain closed as long

as the train is moving

Test and system level – 1

1

2

3

4

Acknowledgement

The first part of this presentation is mainly taken from Gregory T. Daich’s presentation “Defining a Software Testing Strategy”, 30 April 2002.