Five Design Factors that Influence Speed
As I noted in a previous tip, there are typically
a host of factors that affect speed, and the number of required button presses
is just one of them. Many of these factors are outside of the designer's control.
For instance, average transaction speed might be affected by the physical layout
of the store, the process of getting customers through the line, the manager's
communicated expectations, and much more. But these are usually beyond the purview
of the user interface designer.
However, there's a lot the designer can do to help increase speed.
Below, I've outlined a few aspects of a usable point of sale which designers
should consider if increased speed is a requirement of the design:
- Hand movement. Obviously, the closer two points are to
each other, the shorter time it takes to get from one to the other. A usable
POS design will capitalize on this fact by ensuring that the most common tasks
require the least movement. User interface designers should also consider
the direction of motion when supporting such tasks, ensuring that
there are as few changes in direction as possible.
- Button size and spacing. Buttons that are two small or
two close together can reduce speed since users have to slow down to ensure
they are hitting the right one. It's also helpful to remember that button
size is not an absolute concept but is relative to the direction of hand motion.
So, buttons that are longer than they are wide will be easier to hit if the
hand is moving horizontally rather than vertically.
- Accessibility of commonly needed options. Buttons that
are surrounded on all sides take longer to hit than those that have one or
more sides free. This, along with visibility, is why user interface designers
should try to put the most commonly needed options at the tops, edges, and
bottoms of lists and groups.
- Logical organization. Sometimes, ordering and grouping
things in logical ways trumps the "Accessibility Rule" (see #3).
For instance, rather than move the "Medium Drink" button to the
top of a list, it might be more important to keep it between "Small"
and "Large", even if it is more commonly selected than the other
sizes. Designers must know when—and to what extent—to apply these
and other rules in order to optimally achieve the desired outcomes.
- Shortcuts. Another way to help increase speed is to design
the Point of Sale system so it will allow the user to get faster as they become
more experienced, as I've described in another tip (see Growing
With the User).
These are just some of the ways to help increase speed. Typically, research
with individual clients will reveal additional and more specific ways to achieve
significant speed gains.