Equivalence Partitioning Made Simple: A Practical Tutorial

What Is Equivalence Partitioning?

Equivalence partitioning (EP) is a black-box test design technique that divides input data into partitions of equivalent data. The principle is straightforward: if one value in a partition works correctly, all other values in that partition should also work correctly. This lets you reduce a potentially infinite number of test inputs to a manageable set without sacrificing coverage.

This equivalence partitioning tutorial walks you through the technique step by step with practical examples you can apply immediately.

The Core Idea

Every input to a system can be divided into groups where the system is expected to behave the same way. These groups are called equivalence partitions or equivalence classes. You need to test at least one value from each partition. Testing more values from the same partition adds cost but rarely finds new defects.

There are two types of partitions: valid partitions (inputs the system should accept) and invalid partitions (inputs the system should reject). Both must be tested.

Equivalence Partitioning Example 1: Exam Score Grading

A grading system assigns letter grades based on exam scores (0-100): - 0-39: Grade F - 40-59: Grade D - 60-69: Grade C - 70-79: Grade B - 80-100: Grade A - Below 0 or above 100: Invalid

This gives us six equivalence partitions. A minimum test set picks one value from each: -5 (invalid), 20 (F), 50 (D), 65 (C), 75 (B), 90 (A), and 105 (invalid). That is seven test cases to cover all partitions.

Equivalence Partitioning Example 2: User Registration

A registration form has a username field that accepts 3-15 alphanumeric characters. The equivalence partitions are:

Valid partitions: - Alphanumeric string, 3-15 characters long (e.g., "user123")

Invalid partitions: - Empty string - 1-2 characters (too short) - 16+ characters (too long) - Contains special characters (e.g., "user@name") - Contains spaces (e.g., "user name")

Each invalid partition represents a different reason for rejection and should be tested separately. A common mistake in this equivalence partitioning tutorial scenario is combining all invalid cases into one partition.

Equivalence Partitioning Example 3: Payment Processing

A payment system accepts amounts in USD. Business rules state: - Minimum transaction: $1.00 - Maximum transaction: $10,000.00 - Only two decimal places allowed

Partitions: - Below $1.00 (invalid — too small) - $1.00 to $10,000.00 (valid) - Above $10,000.00 (invalid — too large) - Negative amounts (invalid) - More than two decimal places (invalid — e.g., $5.123)

How EP and BVA Work Together

Equivalence partitioning and boundary value analysis are complementary techniques. EP identifies the partitions, and BVA focuses testing on the boundaries between them. In practice, you almost always use both together. First define your partitions with EP, then identify boundary values with BVA.

EP on the ISTQB Exam

On the ISTQB exam, equivalence partitioning questions typically ask you to identify the minimum number of test cases needed to cover all partitions. Remember: you need at least one test case per partition. The minimum number of test cases equals the number of partitions.

A common trap is forgetting invalid partitions. If a question describes a valid range, there are always at least two invalid partitions (below the range and above the range), plus the valid partition itself.

Practice Makes Perfect

The best way to master equivalence partitioning is to practice identifying partitions in different scenarios. Try our free ISTQB practice questions to test your ability to apply EP under exam conditions. Each question includes a detailed explanation showing exactly how to identify and count partitions.