Using Interaction Design to Identify Actors and Use Cases

The techniques described in this chapter can be used to identify concrete examples of the problem domain (represented as personas and interaction scenarios). These can then drive the identification of the more abstract concepts of use cases and actors.

The process is as follows:

1. Identify the personas. Write a short description of each persona.

2. Identify the interaction scenarios.

3. Combine the interaction scenarios into use cases. Give each use case a basic scenario and several alternate scenarios.

4. Flesh out the interaction scenario descriptions.

5. Use the personas to identify the actors. (This sounds counterintuitive, but in practice you start by identifying target users [ideally by interviewing real end-users of the system] and then grouping them into different classifications or roles, thus identifying actors.)

6. Use the interaction scenarios to write the “real” use case scenarios.

The first step is also the most important. The personas drive the entire product design, so it’s vital to get this step right. As you work your way through the scenarios and get a better understanding of how the product needs to work (from the user’s point of view), you may well need to revisit the personas or even rewrite them altogether.

In many cases, steps 2 and 3 could be reversed. In other words, you might identify the scenarios first and then group them into use cases, or you might identify the use cases first and then identify further alternate scenarios for each use case. The examples we gave earlier used the latter approach. In a large project, it’s likely that you would use both approaches combined, identifying the problem domain and filling in the details as you go. Discovery of behavioral requirements is an organic, evolutionary process.

Steps 1 through 3 should take place in parallel—you’re identifying the personas as you identify the interaction scenarios. In fact, the whole process is highly iterative. While it’s important to get the personas right as early as possible, they’re never set in stone.

Step 4 should be left until you have most of the actors and use cases identified. It can be hard work producing the scenario descriptions, so it’s better to do this after they’ve all been identified.