2.1 Allow sufficient time to accommodate the slowest users

Rationale

Completing a task can require users to carry out a number of separate activities. These may include listening to or reading prompts and instructions, understanding prompts and instructions, choosing the appropriate action, recalling information and making inputs. Each of these activities will take some time. Different users will require different amounts of time, depending on their abilities and confidence.

For spoken instructions, users who have poor hearing may have to listen to them very carefully a few times before they can understand them fully. For written instructions, users who have poor reading skills or have difficulty understanding written text may have similar problems.

Choosing the appropriate action will take longer for users who have an intellectual impairment. Confusion sometimes results if the user has even minor or temporary cognitive limitations. Navigating menu systems may be particularly difficult for people with cognitive impairments.Recalling information such as PIN numbers or personal details is more difficult for many older users or people who are tired or stressed.Making inputs by selecting phone keys or pressing buttons can take much longer for users who have physical difficulties. It can be very frustrating to be constantly prompted to complete a task and the stress that this can cause makes it even more difficult for the user. Ultimately, the worst thing is to be timed out after a lengthy process and asked to start again.

Directions and Techniques

Allow up to 10 times the average response time in order to accommodate the slowest users

To accommodate the slowest users, a good rule of thumb is to allow up to 10 times the average response time for each individual activity - listening, understanding, choosing, recalling information and making inputs. Use timeouts only where necessary and reminders only where helpful.

Use a dial-out buffer

A dial-out buffer is a memory buffer that holds a number until a Dial or Send key is pressed. This enables the user to take as long as they need to enter the number and to dial it when they are ready.

Allow user-selectable settings.

Applying the previous techniques should result in an interface that suits all users. However, in some cases, what is best for one group of users is not necessarily best for all. If this is the case, it may help if the interface can be adapted by the user, or automatically for the user, to fit their individual capabilities. For example, users who need more time to read, listen, think and act could choose longer timeouts and no reminders, whilst users who are quick may prefer to have shorter timeouts for security. The choice could be made by the user selecting from a number of displayed options. Alternatively, information required to switch automatically could be encoded at the user's request in their user profile or on a smart card or SIM card.

How you could check for this:

Gather time-to-task data

During user tests or after a first release, it is possible to gather data from a broad range of users about how long they take over each activity - reading and understanding, choosing, recalling information and making inputs. This can then be used to produce more accurate rules determining how long to allow. However, this should only be done by trained statisticians and the amount of data required for statistically significant calculations will be quite large.
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