Software Test Case Design Methods - Black Box Testing Methods

Black box testing focuses on:
- testing thefunctional aspect of the software.
- input conditions are derived that will test all functional requirements.
- set of test cases that finds incorrect or missing functions.
- interface errors.
- data structure errors.
- external database excess errors.
- performance errors.
- initialization and termination errors.

Black box techniques include:

- Graph based testing is a black-box testing technique that uses objects that are modeled in software and the relationships among these objects. Steps involved in designing test cases using graph based testing are:

STEP 1: Create a graph of software objects and identify the relationship of these objects.
STEP 2: Traverse the graph to define test cases.

- Equivalence Testing
A black box testing technique which divides the input domain into partitions or into sets of data from which test cases can be derived. These test cases are used to uncover errors. It makes use of equivalence classes which are sets of valid and invalid states that an input may be in.

- Boundary Value Testing
It is a black box testing method that uses the boundaries of the input domain to derive test cases. If input condition specifies range bounded by n and m, the test cases that can be derived use values n and m, just above n and m, just below n and m.

If input condition specifies number of values, the test cases that can be derived are use the minimum, use the maximum, just above and below minimum, just above and below maximum.

Some Black box testing techniques...Graph based testing methods, Error guessing and Boundary Value Analysis.

Graph Based Testing Methods
Software testing begins by creating a graph of important objects and their relationships and then devising a series of tests that will cover the graph so that each objects and their relationship is exercised and errors are uncovered.Graph based testing begins with the definition of all nodes and node weights i.e. objects and attributes are identified.

Error Guessing
The purpose of error guessing is to focus the testing activity on areas that have not been handled by the other more formal techniques, such as equivalence partitioning and boundary value analysis. It comes with an experience of technology and the project. There are no specific tools and techniques for error guessing but you can write test cases depending on the situation. Error guessing will require tester to think out of the box and would involve intuition and experience of the tester.

Boundary Value Analysis (BVA)
It is a test data selection technique where the extreme values are chosen. It makes use of the fact that the inputs and outputs of the component under test can be partitioned into ordered sets with identifiable boundaries. A test engineer chooses values that lie along data extremes. It is expected that if the system works correctly for these special values, then it will work correctly for all values in between. The boundary value analysis test cases are obtained by holding the values of all but one obtained by holding the values of all but one variable at their nominal values, and letting that variable at their nominal values, and letting that variable assume its extreme values variable assume its extreme value.
There are two ways to generalize BVA technique :
- By the number of variables: for n variables, BVA yields 4n+1 test cases.
- By the kinds of ranges.

Advantages of Boundary Value Analysis
- Robustness Testing
- It checks values for min-1, min, nom, max-1, max, max+1
- It forces attention to exception handling.

Limitations of Boundary Value Analysis
BVA works best when the program is a function of several independent variables that represent bounded physical quantities.
Boundary value analysis works well when the program to be tested is a function of the program to be tested is a function of several severalindependent independent variables that variables that represent bounded represent bounded physical physicalquantities quantities.

Methods of black box testing - Graph based testing methods and Equivalence Partitioning

Black Box Testing is testing without knowledge of the internal workings of the item being tested. There are four black box testing methods:

Graph-based Testing Methods
Basic idea: A "cause" is an input condition, and an "effect" is a specific sequence of computations to be performed. A cause-effect graph is basically a directed graph that describes the logical combinations of causes and their relationship to the effects to be produced.
- Black-box methods based on the nature of the relationships (links) among the program objects (nodes), test cases are designed to traverse the entire graph.
- A graph is created between the objects and the relationships.
- From the graph, each object relationship is identified and test cases written accordingly to discover the errors.

Equivalence Partitioning
This method divides the input domain of a program into classes of data from which test cases can be derived. It reduces the number of test cases. The guidelines of Equivalence Partitioning are :
- If an input condition specifies a range, one valid and two invalid equivalence classes are defined.
- If an input condition requires a specific value, then one valid and two invalid equivalence classes are defined.
- If an input condition specifies a member of a set, then one valid and one invalid equivalence class are defined.
- If an input condition is boolean, then one valid and one invalid equivalence class are defined.