A number of reliability analysis models are available to predict the reliability of the software, in terms of the number of defects still remaining in the software. This metric is very useful to decide when to deliver the product or when to release the product into the market, particularly when the software is highly complex. The software reliability models generally follow one of the trends as shows the 'concave model' and 'S-shaped mpdeP. The assumption made in arriving at these models is that during initial stages, testing is not as efficient as at later stages (not necessarily a true assumption) and hence there is an increase in the rate of defect detection. In the two-stage model curve is shown, this model is applicable when significant functionality is added to the software during the testing process (which is normally the case!).
Software Reliability Curves
The software reliability models are still at a theoretical level and studies are being carried out at academic institutions. Very few organizations use them in practice for obtaining the reliability figures. The most important reliability figures are MTBF (Mean Time Between Failures), MTTR (Mean Time To Repair) and MTTF (Mean Time To Fail). The relation between these three values is shown in Figure.
While carrying out the testing process, the values of MTBF, MTTR and MTTF need to be recorded and if these values are within the limits only, the product can be considered as 'ready for release'.
Relation between MTBF, MTTR & MTTF
To obtain the values of MTBF, MTTR, and MTTF is difficult. You need to keep track of the total time for testing by all the test engineers). For example, let us say, the software is tested for about 1000 hours and during that time, the software failed twice. So, the MTBF value is 500 hours. After the failure, if it takes 5 hours to rectify the problem on an average, the MTTR is 5 hours. After the rectification, again if it fails after 495 hours, the MMTF is 495 hours. If the testing time is more, you get more accurate values for these parameters.
The software reliability models have to be developed using the defect analysis during the testing phase and maintenance phase. Based on these models, the reliability estimation models can be evolved in the organization. Organizations, which would like to improve their process based on quantitative measures, need to apply the reliability models as a part of their quality improvement process.
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