Black box testing

Black box testing attempts to derive sets of inputs that will fully exercise all the functional requirements of a system. It is not an alternative to white box testing. This type of testing attempts to find errors in the following categories:

1. incorrect or missing functions,
2. interface errors,
3. errors in data structures or external database access,
4. performance errors, and 5. initialization and termination errors.

Tests are designed to answer the following questions:

1. How is the function's validity tested?
2. What classes of input will make good test cases?
3. Is the system particularly sensitive to certain input values?
4. How are the boundaries of a data class isolated?
5. What data rates and data volume can the system tolerate?
6. What effect will specific combinations of data have on system operation?

White box testing should be performed early in the testing process, while black box testing tends to be applied during later stages. Test cases should be derived which

1. reduce the number of additional test cases that must be designed to achieve reasonable testing, and
2. tell us something about the presence or absence of classes of errors, rather than an error associated only with the specific test at hand.

Equivalence Partitioning

This method divides the input domain of a program into classes of data from which test cases can be derived. Equivalence partitioning strives to define a test case that uncovers classes of errors and thereby reduces the number of test cases needed. It is based on an evaluation of equivalence classes for an input condition. An equivalence class represents a set of valid or invalid states for input conditions.

Equivalence classes may be defined according to the following guidelines:

1. If an input condition specifies a range, one valid and two invalid equivalence classes are defined.
2. If an input condition requires a specific value, then one valid and two invalid equivalence classes are defined.
3. If an input condition specifies a member of a set, then one valid and one invalid equivalence class are defined.
4. If an input condition is boolean, then one valid and one invalid equivalence class are defined.

Boundary Value Analysis

This method leads to a selection of test cases that exercise boundary values. It complements equivalence partitioning since it selects test cases at the edges of a class. Rather than focusing on input conditions solely, BVA derives test cases from the output domain also. BVA guidelines include:

1. 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.
2. 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.
3. Apply guidelines 1 and 2 to the output.
4. If internal data structures have prescribed boundaries, a test case should be designed to exercise the data structure at its boundary.

Cause-Effect Graphing Techniques

Cause-effect graphing is a technique that provides a concise representation of logical conditions and corresponding actions. There are four steps:

1. Causes (input conditions) and effects (actions) are listed for a module and an identifier is assigned to each.
2. A cause-effect graph is developed.
3. The graph is converted to a decision table.
4. Decision table rules are converted to test cases.
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