Test Cases and Scenarios - Equivalence Class

Equivalence Class

* Equivalence class testing is a technique used to reduce the number of test cases to a manageable level while still maintaining reasonable test 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.
* Many testers have logically deduced its usefulness, while others have discovered it simply because of lack of time to test more thoroughly.

Equivalence Class

* Consider this 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:
Equivalence Class of Hire
* 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[*]

Equivalence Class of Hire

* 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 ?
Develop coding
* If (applicantAge == 0) hireStatus="NO";
* If (applicantAge == 1) hireStatus="NO";
* ...
* If (applicantAge == 14) hireStatus="NO";
* If (applicantAge == 15) hireStatus="NO";
* If (applicantAge == 16) hireStatus="PART";
* If (applicantAge == 17) hireStatus="PART";
Develop coding
* 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";
Develop coding
* Luckily, programmers don't write code like this (at least not very often). A better programmer might write:
Develop coding
If (applicantAge >= 0 && applicantAge <=16)
hireStatus="NO";
If (applicantAge >= 16 && applicantAge <=18)
hireStatus="PART";
If (applicantAge >= 18 && applicantAge <=55)
hireStatus="FULL";
If (applicantAge >= 55 && applicantAge <=99)
hireStatus="NO";

Equivalence Class

* Given this typical implementation, 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.

Equivalence Class

We will expect:
* 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.

Equivalence Class

A group of tests forms an equivalence class if you believe that:
* They all test the same thing.
* If one test catches a bug, the others probably will too.
* If one test doesn't catch a bug, the others probably won't either

Continuous equivalence classes

Discrete equivalence classes

Single selection equivalence classes
* GMC will make mortgages only for a person. They will not make mortgages for corporations, trusts, partnerships, or any other type of legal entity.
Multiple selection equivalence class
* GMC will make mortgages on Condominiums, Townhouses, and Single Family dwellings. They will not make mortgages on Duplexes, Mobile Homes, Treehouses, or any other type of dwelling

Identifying Equivalence Class

* If an input condition specifies a continuous range of values, select one valid class and two invalid classes
* If an input condition specifies a discrete range of permissible values, select one valid class and two invalid classes
Identifying Equivalence Class
* If an input condition enumerates a set of values, identify one valid equivalence class and one invalid equivalence class
* If a 搈ust be?condition is required, identify one valid equivalence class and one invalid equivalence class

Reference

* A Practitioner's Guide to Software Test Design
by Lee Copeland ISBN:158053791X
Artech House ?2004
* Beizer, Boris (1990). Software Testing Techniques (Second Edition). Van Nostrand Reinhold.
* Binder, Robert V. (2000). Testing Object-Oriented Systems: Models, Patterns, and Tools. Addison-Wesley.
　

Choosing Test Data

In system testing, test data should cover the possible values of each parameter based on the the requirements. Since testing every value is impractical, a few values should be chosen from each equivalence class. An equivalence class is a set of values that should all be treated the same.

Ideally, test cases that check error conditions are written separately from the functional test cases and should have steps to verify the error messages and logs. Realistically, if error test cases are not yet written, it is OK for testers to check for error conditions when performing normal functional test cases. It should be clear which test data, if any, is expected to trigger errors.

Example equivalence classes:

Strings

empty string
String consisting solely of white space
String with leading or trailing white space
syntactically legal: short and long values
syntactically legal: semantically legal and illegal values
syntactically illegal value: illegal characters or combinations
Make sure to test special characters such as #, ", ', &, and <
Make sure to test "Foreign" characters typed on international keyboards

Numbers

empty string, if possible
0
in range positive, small and large
in range negative, small and large
out of range positive
out of range negative
with leading zeros
syntactically invalid (e.g., includes letters)

Identifiers

empty string
syntactically legal value
syntactically legal: reference to existing ID, invalid reference
syntactically illegal value

Radio buttons

one item checked
nothing checked, if possible

Checkboxes

checked
unchecked

Drop down menus

select each item in turn

Scrolling Lists

select no item, if possible
select each item in turn
select combinations of items, if possible
select all items, if possible

File upload

blank
0 byte file
long file
short file name
long file name
syntactically illegal file name, if possible (e.g., "File With Spaces.tar.gz")