A prototyping example

Details of the study can be found in Mayhew, P. J., Worseley, C. J. & Dearnley, P. A. 'Control of Software Prototyping Process: Change Classification Approach'. Information and Systems Technology 13(2) 59-66 published in 1989.

The use of prototyping on the COMET Commercial Estimating System project at the Royal Dockyards was the subject of a study, information about which has been published. The Royal Dockyards had been transferred to a system of 'commercial management7 where they had to produce estimates of the cost of doing tasks that could then be compared against the cost of contracting the work out. Staff at the dockyards were inexperienced with this method of working.

It was decided to implement a computer system to support the estimating process and it was realized that the human-computer interface would be important. For this reason, the software house implementing the system suggested a prototyping approach

Among the questions that had to be decided was who should be present at evaluation sessions. If managers were there, would they dominate the proceedings at the expense of those who would actually use the system? Furthermore, if the designers were present they might well feel defensive about the system presented and try and argue against changes suggested.

The results of the prototype

The impact of prototyping may be judged by the fact that although the preliminary design had been done using SSADM, as a result of the evaluation of the prototype the number of screens in the system was doubled.

A major problem was that of controlling changes to the prototype. In order to record and control the changes suggested by users, the changes were categorized into three types.

Cosmetic (about 35% of changes). These were simply changes to the layout of the screen. They were:

Local (about 60% of changes). These involved changes to the way that the screen was processed but did not affect other parts of the system. They were:

• backed-up so that they could removed at a later stage if necessary;

Inspections are discussed in Chapter 12. • inspected retrospectively.

4.14 INCREMENTAL DELIVERY

Global (about 5% of changes), These were changes that affected more than one part of the processing. All changes here had to be the subject of a design review before they could be implemented.

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