What are different startegies for black box testing?

Writing and executing test cases is an expensive task. Whatever test case is written, it should hint towards a different mode of failure. Testing strategies for black box testing are:
- Customer Requirements tests
Requirements are very important for black box testing. Each and every customer requirement should be tested. To do this, every requirement is traced to its test case and every test case to its customer requirement. The first test case is to write a most used success path ( a path that is error condition free) and proceed by planning more success paths. Some failure paths (a path that has errors in it) are also planned. Execution of tests is done so that risky requirements are tested first. It would give more time to fix the errors before the product delivery.

- Equivalence Partitioning
It is a strategy that reduces the number of test cases that are needed to be developed. The input domain is divided into classes. Test cases should be designed so that the inputs lie within these equivalence classes. For each equivalence class, set of data should be treated same by the module under test and should produce same result.

- Boundary Value Analysis
Mistakes generally occur at boundaries of the equivalence classes. Boundary value analysis guides you to create test cases at the boundaries of equivalence classes. Boundary value is defined as a value that relates to a minimum or maximum input.

- Decision Table Testing
To record business rules, decision tables are used. In decision tables, conditions represents input conditions, actions represent events that should trigger. Each column in table is a unique combination of input condition resulting in initiating an action associated with rule. Each rule or column becomes a test case.

- Failure Test Cases
The program or the application should be robust which means that it should respond properly in case or erroneous user input.

Black-Box Testing Method - Equivalence Partitioning

Equivalence partitioning is a method for deriving test cases.
- It divides the input domain into classes of data from which test cases are derived.
- It strives to define a test case that uncovers classes of errors, thereby reducing the total number of test cases that must be developed.
- Equivalence classes for an input condition is used for test case design.
- An equivalence class represents a set of valid or invalid states for input conditions.
- Input classes are known relatively early in the software process.
- An input condition is either a specific numeric value, range of values, set of related values or a boolean condition.

DEFINING EQUIVALENCE CLASSES FOR TESTING

If an input condition specifies a:
- SPECIFIES RANGE : one valid and two invalid equivalence classes are defined.
- REQUIRES SPECIFIC VALUE : one valid and two invalid equivalence classes are defined.
- SPECIFIES MEMBER OF SET : one valid and one invalid equivalence class are defined.
- BOOLEAN : one valid and one invalid equivalence class are defined.

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.

Methods of black box testing - Boundary Value Analysis

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 method and Equivalence Partitioning have already been discussed.

Boundary Value Analysis
Boundary Value Analysis determines the effectiveness of test cases for a given scenario. Boundary Value Analysis (BVA) is a test Functional Testing technique where the extreme boundary values are chosen. Boundary values include maximum, minimum, just inside/outside boundaries, typical values, and error values. Boundary value analysis complements and can be used in conjunction with equivalence partitioning.
For boundary value analysis, the following guidelines should be used:
- For input ranges bounded by a and b, test cases should include values a and b and just above and just below a and b respectively.
- If an input condition specifies a number of values, test cases should be developed to exercise the minimum and maximum numbers and values just above and below these limits.

Advantages of Boundary Value Analysis
- Robustness Testing – Boundary Value Analysis plus values that go beyond the limits.
- Min – 1, Min, Min +1, Nom, Max -1, Max, Max +1.
- Forces attention to exception handling.

Limitations of Boundary Value Analysis
Boundary value testing is efficient only for variables of fixed values i.e boundary.