If no failures occur during testing, a conservative estimate of reliability may be obtained through simple hypothesis testing based on the binomial distribution. The following table shows the number of test cases that must be run without failure in order to claim a specified level of reliability at a specified level of confidence. The reliability of the software, which may be calculated at any point in testing, is the probability of taking a random walk through the testing chain without encountering a failure state. In other words, reliability is the probability that every event in a complete usage scenario will be processed successfully.
Reliability is calculated from the testing chain with confidence intervals.
Poore, J. H., H. D. Mills and D.Mutchler. Planning and Certifying Software System Reliability. IEEE Software , January 1993, pp 88-99. Whittaker, J. A. and M. G. Thomason. A Markov Chain Model for Statistical Software Testing.