One of the best ways to study the usability of an existing system is to watch real users using it. The techniques are most often discussed in the context of software, but usability plays a role in designing other types of systems as well.
As one example, three confederates and I observed users operating grocery-store self-checkout systems. We found that most of the users had some familiarity with the system and those users were able to operate the system quickly. Some users, who appeared to be less experienced with the devices, took longer to complete their task. These users had to carefully read each screen to figure out what to do.
The speed with which the experienced users operated the checkout system suggests it is easy for experts to use. The need to read each screen that we observed in the novice users suggests that the system is not particularly intuitive, although it probably feels intuitive to those who are already experienced with it.
We also observed that the self-checkout took longer than going through a cashier’s checkout, even for the experienced users. At least one user also commented that they preferred the human interaction with a cashier to the automated checkout system. In their current state, the main advantage of the self-checkout lane for shoppers is that the line-up is sometimes shorter.