Recent research characterizes two types of program errors. The repeatable class, called Bohrbugs, always cause the same error with the same inputs. Prior thinking and reliability growth models were based on Bohrbugs, (Bbugs). Another class of errors called Mandelbugs, (Mbugs), are difficult to reproduce even with the same inputs, [1], [2]. Mbugs may be an interaction of inputs with computer storage, where computer storage represents the past history of inputs to the computer system [3]. A small fraction of Mbugs which increase with time, aging-related Mbugs, are ignored in the model. This paper formulates a software reliability growth model for predicting operational reliability of the software as development progresses. Based on prior experience, constant and exponentially decreasing error removal models are assumed. The inputs to the model are error removal data for both Bbugs and Mbugs, recorded during development testing and mean time to failure data from simulated operational tests. Simplified values for the model parameters are developed based on moment estimates. More sophisticated estimates using least squares and maximum likelihood techniques are discussed. The potential for improving the accuracy of reliability estimates by using models that incorporate both Bbugs and Mbugs is discussed with respect to two examples.

This paper discusses a feasibility study which developed estimates of the occurrence rate for significant avionics software problems. Two FAA databases, airworthiness directives (ADs) and service difficulty reports (SDRs) were used. A study of the AD database for large aircraft (1984-1994) revealed 33 avionics ADs, 13 of which were software related. Estimates were made of the operational hours for the fleet of commercial aircraft with computer avionics and the number of problem occurrences. Minimum, maximum and average occurrence rates were established. The average occurrence rate for the 6 resulting data sets was 0.15 per million operating hours. The nonoccurrence of ADs for the remaining avionics was "bounded on the average"; yielding less than 0.02 occurrences/per million hrs. The significant problem occurrence rate for the TCAS system (collision avoidance) software has motivated others to apply proof of correctness techniques to the specification and design of this software (Craigen et al., 1994).

A discussion is given on a new micro model which allows reliability estimation to begin at the module test phase, continue during integration testing and carry over to field deployment. The model first decomposes the structure of the software into a set of execution paths. The failure rate of the software system is related to the frequency and time of path traversal, and the probability of encountering an error during traversal. A second stage of decomposition is necessary to relate the path reliability to the module reliabilities. In the second decomposition the failure probabilities are expressed by combinatorial expressions involving the probabilities of failure of the individual modules. Since the basic model decomposes the structure into execution paths the model can be used to apportion reliabilities and test efforts among the various execution paths. The optimum allocation is computed for a particular effort model and applied to a numerical example.>

It has become abundantly clear to all that during the last two decades of the twentieth c e n t u r y and long into the twenty first, software will be both the h e a r t and the binding force of all our large technological developments.Two decades ago large software systems began to be born.Within the last decade, leaders in i n d u s t r y , government, and the universities have realized that software can r e p r e s e n t up to 90% of the cost of large computer projects.During this time period, the term Software Engineering has emerged, which can be defined as: Software Engineering:The collection of analysis, design, test, documentation, and management techniques needed to produce timely software within b u d g e t e d cost.One of the major challenges facing computer science departments is how to teach software engineering to the large number of B .S .and M.S. students who are now s t u d y i n g Computer Science. DIFFICULTIES IN TEACHING SOFTWARE ENGINEERINGThere are many difficulties associated with the teaching of software engineering at a u n i v e rsity.Some of the factors are associated with the fact that software engineering has a strong design component.Other difficulties relate to the r equired computer facilities.