# Software Engineering | Schick-Wolverton software reliability model

Prerequisite – Jelinski Moranda software reliability model

The Schick-Wolverton (S-W) model is a modification to the J-M model. It is similar to the J-M model except that it further assumes that the failure rate at the ith time interval increases with time ti since the last debugging. In the model, the program failure rate function between the (i-1)th and the ith failure can be expressed as

where and N are the same as that defined in the J-M model and ti is the test time since the (i-1)th failure.

The pdf(probability distribution function) of can be obtained as follows:

Hence, the software reliability function is

We now wish to estimate N assuming that is given. Using the MLE method, the log likelihood function is given by,

Taking the first derivative with respect to N, we have,

Therefore, the MLE of N can be obtained by solving the following equation:

Next, we assume that both N and ? are unknown. Hence we obtain,

and

Therefore, the MLEs of N and can be found by solving the two equations simultaneously as follows:

where

## Recommended Posts:

- Software Engineering | Project size estimation techniques
- Types of Software Testing
- Software Testing | Basics
- Software Engineering | Architectural Design
- Software Engineering | Halstead’s Software Metrics
- Beta Testing | Software Testing
- Software Engineering | Debugging Approaches
- Software Engineering | COCOMO Model
- Software Engineering | Classification of Software Requirements
- Software Engineering | Classical Waterfall Model
- Software Engineering | Iterative Waterfall Model
- Software Engineering | Spiral Model
- Software Engineering | Requirements Engineering Process
- Software Engineering | Requirements Elicitation
- Software Engineering | System configuration management

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.