.. Continuing the Beginners Guide to Software Testing series

Concepts:   Equivalence partitioning is a method for deriving test cases.  In this method, classes of input conditions called equivalence classes are identified such that each member of the class causes the same kind of processing and output to occur.
In this method, the tester identifies various equivalence classes for partitioning.  A class is a set of input conditions that are is likely to be handled the same way by the system.  If the system were to handle one case in the class erroneously, it would handle all cases erroneously.
Equivalence partitioning significantly reduces the number of test cases required to test a system reasonably.  It is an attempt to get a good ‘hit rate’, to find the most errors with the smallest number of test cases.
To use equivalence partitioning, you will need to perform two steps
1. Identify the equivalence classes
2. Design test cases
Take each input condition described in the specification and derive at least two equivalence classes for it.  One class represents the set of cases which satisfy the condition (the valid class) and one represents cases which do not (the invalid class).
Following are some general guidelines for identifying equivalence classes:
a) If the requirements state that a numeric value is input to the system and must be within a range of values, identify one valid class inputs which are within the valid range and two invalid equivalence classes inputs which are too low and inputs which are too high. 
For example, if an item in inventory (numeric field) can have a quantity of +1 to +999, identify the following classes:  
1. One valid class:   (QTY is greater than or equal to +1 and is less than or equal to +999).   This is written as (+1 < = QTY < = 999)
2. The invalid class (QTY is less than 1), also written as (QTY < 1) i.e. 0, -1, 2, … so on
3. The invalid class (QTY is greater than 999), also written as (QTY >999) i.e. 1000, 1001, 1002, 1003… so on.

Invalid class Valid Class Invalid class
0 1 1000
-1 2 1001
-2 3 1002
-3 4 1003
-4… So on 5… up to 999 1004.. So on

b) If the requirements state that the number of items input by the system at some point must lie within a certain range, specify one valid class where the number of inputs is within the valid range, one invalid class where there are too few inputs and one invalid class where there are too many inputs.
For example, specifications state that a maximum of 4 purchase orders can be registered against anyone product.  The equivalence classes are: the valid equivalence class: (number of purchase an order is greater than or equal to 1 and less than or equal to 4, also written as (1   < = no. of purchase orders < =   4) the invalid class  (no.  of purchase orders> 4) the invalid class  (no.  of purchase orders <  1)
c) If the requirements state that a particular input item match one of a set of values and each case will be dealt with the same way, identify a valid class for values in the set and one invalid class representing values outside of the set. 
Says that the code accepts between 4 and 24 inputs; each is a 3-digit integer:
– One partition: number of inputs
– Classes “x<4”, “4<=x<=24”, “24<x”
– Chosen values: 3,4,5,14,23,24,25
Go back to Test Design Techniques
Related Posts: Methods of Black Box Testing

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