SOFTWARE DEVELOPMENT MODELS

Software Engineering deals with the development of software. Hence, understanding the basic characteristics of software is essential. Software is different from other engineering products in the following ways: 

1.  Engineering products once developed cannot be changed. To modifications the product, redesigning and remanufacturing is required. In the case of software, ultimately changes are to be done in code for any changes to take effect.
2. The Other Engineering products are visible but the software as such is not visible. That‟s why, it is said that software is developed, but not manufactured. Though, like other products, it is first designed, then produced, it cannot be manufactured automatically on an assembly line like other engineering products. Nowadays, CASE (Computer Aided Software Engineering) tools are available for software development. Still it depends on the programmer‟s skill and creativity. The creative skills of the programmer is difficult to quantify and standardise. Hence, the same software developed by different programmers may take varying amount of time, resources and may have variable cost. 
3.  Software does not fail in the traditional sense. The engineering products has wear and tear in the operation. Software can be run any number of times without wear and tear. The software is considered as failed if:
   a) It does not operate correctly.
   b) Does not provide the required number of features. 
4. Engineering products can be perfectly designed, but in the case of software, however good the design, it can never be 100% error free. Even the best quality software is not completely error free. A software is called good quality software if it performs the required operation, even if it has a few errors. 
5. The testing of normal engineering products and software engineering products are on different parameters. In the former, it can be full load testing, etc., whereas in the case of software, testing means identification of test cases in which software may fail. Thus, testing of software means running of software for different inputs. By testing, the presence of errors is identified. 
6.  Unlike most of the other engineering products, software can be reused. Once a piece of code is written for some application, it can be reused.
7.  The management of software development projects is a highly demanding task, since it involves the assessment of the developers creative skills. The estimation regarding the time and cost of software needs standardisation of developers creativity, which can be a variable quantity. It means that software projects cannot be managed like engineering products. The correction of a bug in the case of software may take hours But, it may not be the case with normal engineering products. 
8. The Software is not vulnerable to external factors like environmental effects. But the same external factors may harm hardware. The hardware component may be replaced with spare parts in the case of failure, whereas the failure of a software component may indicate the errors in design. Thus, the characteristics of software are quite different from other engineering products. Hence, the software industry is quite different from other industries.