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4.5 Requirements elicitation and analysis 107

Register Export
Patient Statistics

View Manager
Medical Receptionist Personal Info. Generate
Report

View
Record

Doctor
Edit
Nurse Record

Setup
Figure 4.15 Use cases Consultation
for the MHC-PMS

the actors involved in an interaction and names the type of interaction. This is then
supplemented by additional information describing the interaction with the system.
The additional information may be a textual description or one or more graphical
models such as UML sequence or state charts.
Use cases are documented using a high-level use case diagram. The set of use
cases represents all of the possible interactions that will be described in the system
requirements. Actors in the process, who may be human or other systems, are repre-
sented as stick figures. Each class of interaction is represented as a named ellipse.
Lines link the actors with the interaction. Optionally, arrowheads may be added to
lines to show how the interaction is initiated. This is illustrated in Figure 4.15, which
shows some of the use cases for the patient information system.
There is no hard and fast distinction between scenarios and use cases. Some peo-
ple consider that each use case is a single scenario; others, as suggested by Stevens
and Pooley (2006), encapsulate a set of scenarios in a single use case. Each scenario
is a single thread through the use case. Therefore, there would be a scenario for the
normal interaction plus scenarios for each possible exception. You can, in practice,
use them in either way.
Use cases identify the individual interactions between the system and its users or
other systems. Each use case should be documented with a textual description. These
can then be linked to other models in the UML that will develop the scenario in more
detail. For example, a brief description of the Setup Consultation use case from
Figure 4.15 might be:

Setup consultation allows two or more doctors, working in different offices, to
view the same record at the same time. One doctor initiates the consultation by
choosing the people involved from a drop-down menu of doctors who are on-
line. The patient record is then displayed on their screens but only the initiating
doctor can edit the record. In addition, a text chat window is created to help