Equivalence Class Testing

Among the various software testing techniques performed by the team of testers, there is one important technique- Equivalence Class Testing -that assists the team in getting accurate and expected results, within the limited period of time and while covering large input scenarios.Since, it plays such a significant role in software testing life cycle (STLC), following is a comprehensive discussion on Equivalence Class Testing and its various important components.

What is meant by the Equivalence Class Testing?

Equivalence Class Testing, which is also known as Equivalence Class Partitioning (ECP) and Equivalence Partitioning, is an important software testing technique used by the team of testers for grouping and partitioning of the test input data, which is then used for the purpose of testing the software product into a number of different classes.

These different classes resemble the specified requirements and common behaviour or attribute(s) of the aggregated inputs. Thereafter, the test cases are designed and created based on each class attribute(s) and one element or input is used from each class for the test execution to validate the software functioning, and simultaneously validates the similar working of the software product for all the other inputs present in their respective classes.


Let us further clear the concept of equivalence class testing with the assistance of the following example:

Consider a software application, which takes not less than two digit number and not more than 3 digit number, for its execution. Given below is the huge amount of input to test and validate the functioning of the software application.


  • Now, as per the requirement specifications, these inputs are grouped together to form some classes. Now, instead of testing 1500 inputs, we have formed 4 classes and are accordingly dividing the inputs into a category of valid and invalid inputs, which reduces the work of the test case preparation.
  • A single element, chosen from each class, as a test input, represents the whole class. For example, number 121 is used from the class "three digit numbers" as the test input. On using 121, it was found that software application functions properly and passes the test. Therefore, it is assumed that all the other numbers of the class "three digit number" will work well for the software application. And if the software fails the test, then it is assumed, that all the three digit numbers will generate error in the software application.
  • Similarly, a number 7 is used from the class "single digit numbers", i.e. invalid input. It is expected by the software application, to generate error, on using number 7, and if does then the software is functioning appropriately, and it is also assumed that the remaining single digit number will also produces error(s), in the software application.

Features of Equivalence Class Testing:

Equivalence class testing can be termed as a logical step in the model of functional testing. It improves the quality of test cases, which further enhances the quality of testing, by removing the vast amount of redundancy and gaps that appear in the boundary value testing. Other features of this testing technique are:

  • It is a black box testing technique which restricts the testers to examine the software product, externally.
  • Also known by the name of equivalence class partitioning, it is used to form groups of test inputs of similar behaviour or nature.
  • Based on the approach, if one member works well in the family then the whole family is considered to function well and if one members fails, whole family is rejected.
  • Test cases are based on classes, not on every input, thereby reduces the time and efforts required to build large number of test cases.
  • It may be used at any level of testing i.e. unit, integration, system & acceptance.
  • It is good to go for the ECT, when the input data is available in terms of intervals and sets of discrete values.
  • However, there is no such specific rule to use only input from each class. Based on the experience and need, a tester may opt for more than one input.
  • It may results into good amount of decrease in the redundant test cases, if implemented properly.
  • It may not work well with the boolean or logical types variables.
  • A mixed combination of Equivalence class testing and boundary value testing produces effective results.
  • The fundamental concept of equivalence class testing/partition comes from the equivalence class, which further comes from equivalence relations.

Equivalence Class Testing Types:

The equivalence class testing can be categorized into four different types, which are integral part of testing and cater to different data set. These types of equivalence class testing are:

  1. Weak Normal Equivalence Class Testing: In this first type of equivalence class testing, one variable from each equivalence class is tested by the team. Moreover, the values are identified in a systematic manner. Weak normal equivalence class testing is also known as single fault assumption.
  2. Strong Normal Equivalence Class Testing: Termed as multiple fault assumption, in strong normal equivalence class testing the team selects test cases from each element of the Cartesian product of the equivalence. This ensures the notion of completeness in testing, as it covers all equivalence classes and offers the team one of each possible combinations of inputs.
  3. Weak Robust Equivalence Class Testing: Like weak normal equivalence, weak robust testing too tests one variable from each equivalence class. However, unlike the former method, it is also focused on testing test cases for invalid values.
  4. Strong Robust Equivalence Class Testing: Another type of equivalence class testing, strong robust testing produces test cases for all valid and invalid elements of the product of the equivalence class. However, it is incapable of reducing the redundancy in testing.

Advantages & Disadvantages of Equivalence Class Testing:

Equivalence class testing or equivalence partitioning plays a potent role in reducing redundancy in testing and making the process agile and powerful. It is among those testing techniques that offer numerous benefits to the team and ensures compliance of the product with customer requirements. However, there are few drawbacks associated to this type of testing, which are listed below along with its various advantages.


  • Equivalence class testing helps reduce the number of test cases, without compromising the test coverage.
  • Reduces the overall test execution time as it minimizes the set of test data.
  • It can be applied to all levels of testing, such as unit testing, integration testing, system testing, etc.
  • Enables the testers to focus on smaller data sets, which increases the probability to uncovering more defects in the software product.
  • It is used in cases where performing exhaustive testing is difficult but at the same time maintaining good coverage is required.


  • It does not consider the conditions for boundary value.
  • The identification of equivalence classes relies heavily on the expertise of testers.
  • Testers might assume that the output for all input data set are correct, which can become a great hurdle in testing.

Guidelines for Equivalence Class Testing:

By following a set of guidelines while implementing the process of testing, the team of testers can ensure better outputs from the tests and make sure all scenarios are being tested accurately. Therefore, listed below are some tips/guidelines for equivalence class testing:

  • Use robust forms if the error conditions in the software product are of high priority.
  • It can be used by the team in projects where the program function is complex.
  • To ensure the accuracy and precision of equivalence class testing, define the input data in terms of intervals and sets of discrete values.
  • Use of robust from is redundant of the implemented language is strongly types and when invalid values cause runtime errors in the system.
  • The team needs to select one valid and one invalid input value each, if the input conditions are broken or not stated accurately.
  • Establishing proper equivalence relation might require several tries and extra efforts of the team.

Difference Between Equivalence Class Testing & Boundary Value Analysis:

Boundary value analysis and equivalence class testing are two strategies used for test case designing in black box testing, which makes it crucial for us to differentiate them from one another and define their specific relevance in software testing. The differences between these two are:

Equivalence Class Testing Boundary Value Analysis
1. Equivalence Class Testing is a type of black box technique. 1. Next part of Equivalence Class Partitioning/Testing.
2. It can be applied to any level of testing, like unit, integration, system, and more. 2. Boundary value analysis is usually a part of stress & negative testing.
3. A test case design technique used to divide input data into different equivalence classes. 3. This test case design technique used to test boundary value between partitions.
4. Reduces the time of testing, while using less and effective test cases. 4. Reduces the overall time of test execution, while making defect detection faster & easy.
5. Tests only one from each partition of the equivalence classes. 5. Selects test cases from the edges of the equivalence classes.


A type of specification based testing or black box testing technique, Equivalence Partitioning or Equivalence Class Testing is a widely used method that decreases the number of possible test cases that are required to a software product. Moreover, its ability to generate greater testing coverage, without compromising time and efforts, makes Equivalence Class Testing a popular testing technique worldwide.