Equivalence Class Partitioning is a software testing technique that divides the input data of a software unit into partitions of data from which test cases can be derived. In principle, test cases are designed to cover each partition at least once. This technique tries to define test cases that uncover classes of errors, thereby reducing the total number of test cases that must be developed.
In rare cases equivalence partitioning is also applied to outputs of a software component, typically it is applied to the inputs of a tested component. The equivalence partitions are usually derived from the requirements specification for input attributes that influence the processing of the test object. An input has certain ranges which are valid and other ranges which are invalid. Invalid data here does not mean that the data is incorrect, it means that this data lies outside of specific partition. This may be best explained by the example of a function which takes a parameter "month". The valid range for the month is 1 to 12, representing January to December. This valid range is called a partition. In this example there are two further partitions of invalid ranges. The first invalid partition would be <= 0 and the second invalid partition would be >= 13.
... -2 -1 0 1 .............. 12 13 14 15 .....
-------------|-------------------|---------------------
invalid partition 1 valid partition invalid partition 2
The testing theory related to equivalence partitioning says that only one test case of each partition is needed to evaluate the behavior of the program for the related partition. In other words it is sufficient to select one test case out of each partition to check the behavior of the program. To use more or even all test cases of a partition will not find new faults in the program. The values within one partition are considered to be "equivalent". Thus the number of test cases can be reduced considerably.
Best Example:
Grocery Store Example
Consider a software module that is intended to accept the name of a grocery item and a list of the different sizes the item comes in, specified in ounces. The specifications state that the item name is to be alphabetic characters 2 to 15 characters in length. Each size may be a value in the range of 1 to 48, whole numbers only. The sizes are to be entered in ascending order (smaller sizes first). A maximum of five sizes may be entered for each item. The item name is to be entered first, followed by a comma, then followed by a list of sizes. A comma will be used to separate each size. Spaces (blanks) are to be ignored anywhere in the input.
Derived Equivalence Classes
- Item name is alphabetic (valid)
- Item name is not alphabetic (invalid)
- Item name is less than 2 characters in length (invalid)
- Item name is 2 to 15 characters in length (valid)
- Item name is greater than 15 characters in length (invalid)
- Size value is less than 1 (invalid)
- Size value is in the range 1 to 48 (valid)
- Size value is greater than 48 (invalid)
- Size value is a whole number (valid)
- Size value is a decimal (invalid)
- Size value is numeric (valid)
- Size value includes non-numeric characters (invalid)
- Size values entered in ascending order (valid)
- Size values entered in non-ascending order (invalid)
- No size values entered (invalid)
- One to five size values entered (valid)
- More than five sizes entered (invalid)
- Item name is first (valid)
- Item name is not first (invalid)
- A single comma separates each entry in list (valid)
- A comma does not separate two or more entries in the list (invalid)
- The entry contains no blanks (???)
- The entry contains blanks (????)
| # | Test Data | Expected Outcome | Classes Covered |
| 1 | xy,1 | T | 1,4,7,9,11,13,16,18,20,22 |
| 2 | AbcDefghijklmno,1,2,3 ,4,48 | T | 1,4,7,9,11,13,16,18,20,23 |
| 3 | a2x,1 | F | 2 |
| 4 | A,1 | F | 3 |
| 5 | abcdefghijklmnop | F | 5 |
| 6 | Xy,0 | F | 6 |
| 7 | XY,49 | F | 8 |
| 8 | Xy,2.5 | F | 10 |
| 9 | xy,2,1,3,4,5 | F | 14 |
| 10 | Xy | F | 15 |
| 11 | XY,1,2,3,4,5,6 | F | 17 |
| 12 | 1,Xy,2,3,4,5 | F | 19 |
| 13 | XY2,3,4,5,6 | F | 21 |
| 14 | AB,2#7 | F | 12 |
