What is Reliability Testing? (Example)

What is Reliability Testing?

Reliability Testing is a software testing process that checks whether the software can perform a failure-free operation in a particular environment for a specified time period. The purpose of Reliability testing is to assure that the software product is bug-free and reliable enough for its expected purpose.

Reliability means “yielding the same,” in other terms, the word “reliable” means something is dependable and that it will give the same outcome every time. The same is true for Reliability testing.

Reliability Testing Example

The probability that a PC in a store is up and running for eight hours without crashing is 99%; this is referred to as reliability.

Reliability Testing can be categorized into three segments,

  • Modeling
  • Measurement
  • Improvement

The following formula is for calculating the probability of failure.

Probability = Number of failing cases/ Total number of cases under consideration

Reliability Testing Example

Factors Influencing Software Reliability

  1. The number of faults present in the software
  2. The way users operate the system

Reliability Testing is one of the keys to better software quality. This testing helps discover many problems in the software design and functionality.

The main purpose of reliability testing is to check whether the software meets the requirement of customer reliability.

Reliability testing will be performed at several levels. Complex systems will be tested at the unit, assembly, subsystem, and system levels.

Why do Reliability Testing?

Reliability testing is done to test the software performance under the given conditions.

The objective behind performing reliability testing are,

  1. To find the structure of repeating failures.
  2. To find the number of failures occurring is the specified amount of time.
  3. To discover the main cause of failure.
  4. To conduct Performance Testing of various modules of software applications after fixing a defect.

After the release of the product too, we can minimize the possibility of the occurrence of defects and thereby improve the software reliability. Some of the tools useful for this are- Trend Analysis, Orthogonal Defect Classification, and formal methods, etc.

Types of reliability Testing

Software reliability testing includes Feature Testing, Load Testing, and Regression Testing

Feature Testing:-

Featured Testing checks the feature provided by the software and is conducted in the following steps:-

  • Each operation in the software is executed at least once.
  • Interaction between the two operations is reduced.
  • Each operation has to be checked for its proper execution.

Load Testing:-

Usually, the software will perform better at the beginning of the process, and after that, it will start degrading. Load Testing is conducted to check the performance of the software under the maximum workload.

Regression Test:-

Regression testing is mainly used to check whether any new bugs have been introduced because of fixing previous bugs. Regression Testing is conducted after every change or update of the software features and their functionalities.

How to do Reliability Testing

Reliability Testing is costly compared to other types of testing. So Proper planning and management is required while doing reliability testing. This includes the testing process to be implemented, data for the test environment, test schedule, test points, etc.

To begin with reliability testing, the tester has to keep following things,

  • Establish reliability goals
  • Develop operational profile
  • Plan and execute tests
  • Use test results to drive decisions

As we discussed earlier, there are three categories in which we can perform Reliability Testing,- Modeling, Measurement, and Improvement.

The key parameters involved in Reliability Testing are:-

  • Probability of failure-free operation
  • Length of time of failure-free operation
  • The environment in which it is executed

Step 1) Modeling

Software Modeling Technique can be divided into two subcategories:

1. Prediction Modeling

2. Estimation Modeling

  • Meaningful results can be obtained by applying suitable models.
  • Assumptions and abstractions can be made to simplify the problems, and no single model will be suitable for all situations. The major differences between the two models are:-
Issues Prediction Models Estimation Models
Data Reference It uses historical data It uses current data from software development.
When used in Development Cycle It will usually be created before the development or testing phases. It will usually be used later in the Software Development Life Cycle.
Time Frame It will predict reliability in the future. It will predict the reliability either for the present time or in the future time.

Step 2) Measurement

Software reliability cannot be measured directly; hence, other related factors are considered to estimate software reliability. The current practices of Software Reliability Measurement are divided into four categories:-

Mesurement 1: Product Metrics

Product metrics are the combination of 4 types of metrics:

  • Software size: – Line of Code (LOC) is an intuitive initial approach for measuring the size of the software. Only the source code is counted in this metric, and the comments and other non-executable statements will not be counted.
  • Function point Metric:- Function Pont Metric is the method for measuring the functionality of Software Development. It will consider the count of inputs, outputs, master files, etc. It measures the functionality delivered to the user and is independent of the programming language.
  • Complexity is directly related to software reliability, so representing complexity is important. The complexity-oriented metric determines the complexity of a program’s control structure by simplifying the code into a graphical representation.
  • Test Coverage Metrics:- It is a way of estimating fault and reliability by completing software product tests. Software reliability means it is the function of determining that the system has been completely verified and tested.

Measurement 2: Project Management Metrics

  • Researchers have realized that good management can result in better products.
  • Good management can achieve higher reliability by using better development, risk management,and configuration management processes.

Measurement 3: Process Metrics

The quality of the product is directly related to the process. Process metrics can be used to estimate, monitor, and improve the reliability and quality of software.

Measurement 4: Fault and Failure Metrics

Fault and Failure Metrics are mainly used to check whether the system is completely failure-free. Both the types of faults found during the testing process (i.e. before delivery) as well as the failure reported by users after delivery are collected, summarized, and analyzed to achieve this goal.

Software reliability is measured in terms of the mean time between failures (MTBF). MTBF consists of

  • Mean to failure (MTTF): It is the time difference between two consecutive failures.
  • Mean time to repair (MTTR): It is the time required to fix the failure.

Reliability for good software is a number between 0 and 1.

Reliability increases when errors or bugs from the program are removed.

Step 3) Improvement

Improvement completely depends upon the problems occurred in the application or system, or else the characteristics of the software. According to the complexity of the software module, the way of improvement will also differ. Two main constraints, time and budget will limit the efforts put into software reliability improvement.

Example Methods for Reliability Testing

Testing for reliability is about exercising an application to discover and remove failures before the system is deployed.

There are mainly three approaches used for Reliability Testing

  • Test-Retest Reliability
  • Parallel Forms Reliability
  • Decision Consistency

Below we tried to explain all these with an example.

Test-Retest Reliability

Test-Retest Reliability

To estimate test-retest reliability, a single group of examinees will perform the testing process only a few days or weeks apart. The time should be short enough so that the examinee’s skills in the area can be assessed. The relationship between the examinee’s scores from two different administrations is estimated through statistical correlation. This type of reliability demonstrates the extent to which a test is able to produce stable, consistent scores across time.

Parallel Forms Reliability

Parallel Forms Reliability

Many exams have multiple formats of question papers, these parallel forms of exam provide Security. Parallel forms reliability is estimated by administrating both forms of the exam to the same group of examinees. The examinee’s scores on the two test forms are correlated in order to determine how similar the two test forms function. This reliability estimate is a measure of how consistent examinees’ scores can be expected across test forms.

Decision Consistency

After doing Test-Retest Reliability and Parallel Form Reliability, we will get a result of examinees either passing or failing. The reliability of this classification decision is estimated in decision consistency reliability.

Importance of Reliability Testing

A thorough assessment of reliability is required to improve the performance of software products and processes. Testing software reliability will help software managers and practitioners to a great extent.

To check the reliability of the software via testing:-

  1. A large number of test cases should be executed for an extended period to determine how long the software will execute without failure.
  2. The test case distribution should match the software’s actual or planned operational profile. The more often a function of the software is executed, the greater the percentage of test cases that should be allocated to that function or subset.

Reliability Testing Tools

Some of the Reliability testing tools used for Software Reliability are:

1. WEIBULL++:- Reliability Life Data Analysis

2. RGA:- Reliability Growth Analysis

3. RCM:-Reliability Centered Maintenance


Reliability Testing is an important part of a reliability engineering program. More correctly, it is the soul of a reliability engineering program. Furthermore, reliability tests are mainly designed to uncover particular failure modes and other problems during software testing.

In Software Engineering, Reliability Testing can be categorized into three segments,

  • Modeling
  • Measurement
  • Improvement

Factors Influencing Software Reliability

  • The number of faults present in the software
  • The way users operate the system