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Agenda
Introduction
Technique
Examples
Applicability and Limitations
Summary
Practice
References
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Introduction
What is equivalence class testing?
What is it used
for?
Equivalence class testing is a technique used to reduce
the number of test cases to a manageable level while still maintaining reasonable test coverage.
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Introduction: Situation
We are writing a module for a
human resources system that decides how we should process
employment applications based on a person's age. Our organization's rules are:
0-16 – Don't hire
16-18 – Can hire on a part-time basis only
18-55 – Can hire as a full-time employee
55-99 – Don't hire
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Introduction: Coverage
Should we test the module for the
following ages: 0, 1, 2, 3, 4, 5, 6,
7, 8, ..., 90, 91, 92, 93, 94, 95, 96, 97, 98, 99?
If we had lots of time (and didn't mind the mind-numbing repetition and were being paid by the hour) we certainly could.
100 values
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Introduction: Solution 1
If (applicantAge == 0) hireStatus="NO";
If (applicantAge
== 1) hireStatus="NO";
…
If (applicantAge == 15) hireStatus="NO";
If (applicantAge ==
16) hireStatus="PART";
If (applicantAge == 17) hireStatus="PART";
If (applicantAge == 18) hireStatus="FULL";
If (applicantAge == 19) hireStatus="FULL";
…
If (applicantAge == 53) hireStatus="FULL";
If (applicantAge == 54) hireStatus="FULL";
If (applicantAge == 55) hireStatus="NO";
If (applicantAge == 56) hireStatus="NO";
…
If (applicantAge == 98) hireStatus="NO";
If (applicantAge == 99) hireStatus="NO";
Any set of tests passes tells us nothing about the next test we could execute. It may pass; it may fail.
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Introduction: Let’s believe
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Introduction: Solution 2
If (applicantAge >= 0 && applicantAge
hireStatus="NO";
If (applicantAge >= 16 && applicantAge <=18)
hireStatus="PART";
If (applicantAge >= 18 && applicantAge <=55)
hireStatus="FULL";
If (applicantAge >= 55 && applicantAge <=99)
hireStatus="NO";
It is clear that for the first requirement we don't have to test 0, 1, 2, ... 14, 15, and 16. Only one value needs to be tested. And which value? Any one within that range is just as good as any other one. The same is true for each of the other ranges. Ranges such as the ones described here are called equivalence classes.
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Introduction: Benefits
Using the equivalence class approach, we have
reduced the number of test cases
From 100 (testing each
age)
To 4 (testing one age in each equivalence class)
A significant savings
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Introduction: Definition
An equivalence class consists of a set
of data that is treated the same by the
module or that should produce the same result. Any data value within a class is equivalent, in terms of testing, to any other value.
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Introduction: Assumptions
Specifically, we would expect that:
If one test
case in an equivalence class detects a defect, all
other test cases in the same equivalence class are likely to detect the same defect.
If one test case in an equivalence class does not detect a defect, no other test cases in the same equivalence class is likely to detect the defect.
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Introduction: Solution 3
If (applicantAge >= 0 && applicantAge
= 16 && applicantAge
If (applicantAge >= 18 && applicantAge <=41) hireStatus="FULL";
// strange statements follow
If (applicantAge == 42 && applicantName == "Lee")
hireStatus="HIRE NOW AT HUGE SALARY";
If (applicantAge == 42 && applicantName <> "Lee")
hireStatus="FULL";
// end of strange statements
If (applicantAge >= 43 && applicantAge <=55) hireStatus="FULL";
If (applicantAge >= 55 && applicantAge <=99) hireStatus="NO";
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Introduction: Ready?
Now, are we ready to begin testing?
Probably not.
What about input values like 969, -42,
FRED, and &$#!@? Should we create test cases for invalid input?
The answer is, as any good consultant will tell you, "it depends“.
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Technique: Steps
1. Identify the equivalence classes.
2. Create
a test case for each equivalence class.
You could
create additional test cases for each equivalence class if you have time and money.
Additional test cases may make you feel warm and fuzzy, but they rarely discover defects the first doesn't find.
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Technique: Continuous
Continuous equivalence classes
For a valid input we
might choose $1,342/month. For invalids we might choose $123/month
and $90,000/month.
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Technique: Discrete
Discrete equivalence classes
For a valid input we
might choose 2 houses. Invalids could be -2 and
8.
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Technique: Array
Single selection equivalence classes
For a valid
input we must use "person." For an invalid we
could choose "corporation" or "trust" or any other random text string. How many invalid cases should we create? We must have at least one; we may choose additional tests for additional warm and fuzzy feelings.
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Technique: Array
Multiple selection equivalence class
While the rule says
choose one test case from the valid equivalence class,
a more comprehensive approach would be to create test cases for each entry in the valid class. That makes sense when the list of valid values is small.
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Technique: Contradictions
But, if this were a list of
the fifty states, and the various territories of the
United States, would you test every one of them? What if in the list were every country in the world?
The correct answer, of course, depends on the risk to the organization if, as testers, we miss something that is vital.
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Technique: Combination
Rarely we will have the time to
create individual tests for every separate equivalence class of
every input value.
Test cases of valid data values.
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Technique: All invalid
A test case of invalid data
values.
If the system accepts this input as valid, clearly
the system is not validating the four input fields properly. If the system rejects this input as invalid, it may do so in such a way that the tester cannot determine which field it rejected. For example: ERROR: 653X-2.7 INVALID INPUT
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Technique: One invalid
In many cases, errors in one
input field may cancel out or mask errors in
another field so the system accepts the data as valid. A better approach is to test one invalid value at a time to verify the system detects it correctly.
A set of test cases varying invalid values one by one.
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Technique: Varying values
For additional warm and fuzzy feelings,
the inputs (both valid and invalid) could be varied.
A
set of test cases varying invalid values one by one but also varying the valid values.
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Technique: Tips
Another approach to using equivalence classes is
to examine the outputs rather than the inputs.
Divide
the outputs into equivalence classes, then determine what input values would cause those outputs. This has the advantage of guiding the tester to examine, and thus test, every different kind of output. But this approach can be deceiving.
In the previous example, for the human resources system, one of the system outputs was NO, that is, Don't Hire. A cursory view of the inputs that should cause this output would yield {0, 1, ..., 14, 15}. Note that this is not the complete set. In addition {55, 56, ..., 98, 99} should also cause the NO output.
It's important to make sure that all potential outputs can be generated, but don't be fooled into choosing equivalence class data that omits important inputs.
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Examples: 1
No invalid choices.
It reduces the number of
test cases the tester must create.
Only the valid inputs
{Buy, Sell} need to be exercised.
Valid inputs: {Buy, Sell}.
Invalids: {Trade, Punt, ...}.
What about "buy", "bUy", "BUY"? Are these valid or invalid entries? The tester would have to refer back to the requirements to determine their status.
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Examples: 2
Input to this field can be between
one and four numeric characters (0, 1, ..., 8,
9) with a valid value greater or equal to 1 and less than or equal to 9999.
Valid inputs are {1, 23, 456, 7890}.
Invalid inputs are {-42, 0, 1.2, 12345, SQE, $#@%}.
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Examples: 3
The valid symbols are {A, AA, AABC,
AAC, ..., ZOLT, ZOMX, ZONA, ZRAN}. The invalid symbols
are any combination of characters not included in the valid list.
Valid inputs are {A, AL, ABE, ACES, AKZOY}.
Invalid inputs are {C, AF, BOB, CLUBS, AKZAM, 42, @#$%}.
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Examples: 4
Rarely will we create separate sets of
test cases for each input. Generally it is more
efficient to test multiple inputs simultaneously within tests. For example, the following tests combine Buy/Sell, Symbol, and Quantity.
A set of test cases varying invalid values one by one.
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Applicability and Limitations
Equivalence class testing can significantly reduce
the number of test cases that must be created
and executed. It is most suited to systems in which much of the input data takes on values within ranges or within sets. It makes the assumption that data in the same equivalence class is, in fact, processed in the same way by the system. The simplest way to validate this assumption is to ask the programmer about their implementation.
Let your designers and programmers know when they have helped you. They'll appreciate the thought and may do it again.
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Applicability and Limitations
Very often your designers and programmers
use GUI design tools that can enforce restrictions on
the length and content of input fields. Encourage their use. Then your testing can focus on making sure the requirement has been implemented properly with the tool.
Equivalence class testing is equally applicable at the unit, integration, system, and acceptance test levels. All it requires are inputs or outputs that can be partitioned based on the system's requirements.
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Summary
Equivalence class testing is a technique used to
reduce the number of test cases to a manageable
size while still maintaining reasonable coverage.
This simple technique is used intuitively by almost all testers, even though they may not be aware of it as a formal test design method.
An equivalence class consists of a set of data that is treated the same by the module or that should produce the same result. Any data value within a class is equivalent, in terms of testing, to any other value.
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Practice
ZIP Code – five numeric digits.
Last Name –
one through fifteen characters (including alphabetic characters, periods, hyphens,
apostrophes, spaces, and numbers).
User ID – eight characters at least two of which are not alphabetic (numeric, special).
Student ID – eight characters. The first two represent the student's home campus while the last six are a unique six-digit number. Valid home campus abbreviations are: AN, Annandale; LC, Las Cruces; RW, Riverside West; SM, San Mateo; TA, Talbot; WE, Weber; and WN, Wenatchee.
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Practice: Answers 1
ZIP Code – five numeric digits.
Length
Valid:
5
Invalid: 3; 20
Characters
Valid: numeric digits
Invalid: special; alphabetical
Is this Zip
Code really valid? Is it real?
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Practice: Answers 2
Last Name – one through fifteen
characters (including alphabetic characters, periods, hyphens, apostrophes, spaces, and
numbers).
Length
Valid: 7
Invalid: 0; 19
Characters
Valid: alphabetic; numeric; .; -; ; “
Invalid: all other special
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Practice: Answers 3
User ID – eight characters at
least two of which are not alphabetic (numeric, special).
Length
Valid:
8
Invalid: 2; 11
Number of numeric and special characters
Valid: 2
Invalid: 1; 10
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Practice: Answers 4
Student ID – eight characters. The
first two represent the student's home campus while the
last six are a unique six-digit number. Valid home campus abbreviations are: AN, Annandale; LC, Las Cruces; RW, Riverside West; SM, San Mateo; TA, Talbot; WE, Weber; and WN, Wenatchee.
Length
Valid: 8
Invalid: 5; 10
Characters position
Valid: first 2
Invalid: 3d and 4th
Campus
Valid: in the list
Invalid: other
Unique
Valid: unique
Invalid: not unique