step through the questionnaire, generating answers at each step along the way; by monitoring the progress, developers could find parts where the code breaks down. Moreover, automated testing could be used to assess the entire survey data collection process; the output dataset that emerges from some large number of tests could itself be analyzed to determine whether responses appear to be coded correctly. The practice of automated testing, of course, is harder than simplified visions allow, one in which active computer science research continues. Accordingly, automated testing is an area in which the survey community should monitor ongoing research to find techniques that suit their needs; the community then needs to work with their CAPI software providers to achieve the capability to implement those methods.

In his remarks, Robinson advocated one method for testing software systems that is growing in acceptance. The idea behind this method— called model-based testing—is to represent the functioning of a software system as various tours through a graph. The graph that is created for this purpose uses nodes to represent observable, user-relevant states (e.g., the performance of a computation, the opening of a file), and the arcs between a set of nodes represent the result of user-supplied actions or inputs (e.g., user-supplied answers to multiple-choice questions) that correspond to the functioning of the software in proceeding from one state of use to another. (A side benefit of developing a graphical model of the functioning of a software system is that it helps to identify ambiguities in the specifications.) The graphical representation of the software can be accomplished at varying levels of detail to focus attention on components of the system that are in need of more (or less) intensive testing. These graphical models are constructed at a very early stage in system development and are further developed in parallel with the system. Many users have found that the development of the graphical model is a useful source of documentation of the system and provides a helpful summary of its features.

Equipped with this model, automated tests can be set in motion. One could consider testing every path through the graph, but this would be prohibitively time-consuming for all but the simplest graphical models. Other strategies for which model-based testing algorithms have been applied include choosing test inputs in such a way that every arc between nodes in the graphical model is traversed at least once. Still other strategies include testing every path of less than n steps (for some integer n)or to conduct a purely random walk through the graph. Random walks are easy to implement and often useful, but they can be inefficient in terms of time because they may not cover a large graph quickly.

Markov chain usage models build on the graphical representation of a software program used in model-based testing. In this technique,



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