Design of Everyday Things [76]
• Leaving a child behind at a rest stop during a car trip. I also heard about a new mother who left her infant in the dressing room of a department store.
• Losing a pen because it was taken out to write a note or a check in some public location, then put down for a moment while doing some other task—such as giving the check to the salesperson. The pen is forgotten in the activities of putting away the checkbook, picking up the goods, talking to the salesperson or friends, and so on. Or the reverse: borrowing a pen, using it, and then putting it away in pocket or purse, even though it is someone else’s; this slip is an example of a capture error.
Forcing functions don’t always show up where they should. Sometimes their absence causes all sorts of unnecessary confusion. Read the caution statement from the game instructions shown in figure 5.6.
All those exclamation marks! And the caution is repeated throughout the instruction manual. It won’t do any good. The Nintendo Entertainment System is meant to be used by children. The instruction manual probably won’t be around. Even if it is, a group of active children, anxious to try a different game, won’t bother with it. I watched my own child follow the instruction faithfully for several days, then fail when asked to stop playing and come to dinner. I forgot on the few attempts I made to master the game. The only possible virtue of the warning is to protect the manufacturer: when the children repeatedly burn out the electronic circuits, the company can disclaim liability, asserting that the children violated the instructions.
5.6 The Nintendo Children’s Toy. This home video game set is intended for use by children. However, it has a complex safety instruction, one almost guaranteed to be ignored. To use the system, one inserts a “game pak” cartridge into the “chamber.” The power switch should be off when inserting or removing the cartridge. In the absence of any forcing function, the instruction is almost universally disregarded (if anyone even knows about it). If order is important, there should be a forcing function. If order does not matter, the instruction should be dropped. (From the Nintendo instruction manual. Nintendo® and Nintendo Entertainment System® are trademarks of Nintendo of America Inc. (c) 1986 Nintendo.)
Proper design calls for a forcing function here. There are several viable schemes. The cover over the game pack compartment could control an interlock, so that it automatically turned off the power whenever it was opened. Or the power switch could move a lever blocking the top of the game pack compartment, so that the packs could not be removed or inserted unless the lever were out of the way, turning off the power. There are other possibilities. My point is, of course, that the design should have included one; without the forcing function, failure to heed the warning is almost guaranteed.
A Design Philosophy
There are lots of ways for a designer to deal with errors.17 The critical thing, however, is to approach the topic with the proper philosophy. The designer shouldn’t think of a simple dichotomy between errors and correct behavior; rather, the entire interaction should be treated as a cooperative endeavor between person and machine, one in which misconceptions can arise on either side. This philosophy is much easier to implement on something like a computer which has the ability to make decisions on its own than on things like doors and power plants, which do not have such intelligence. But the philosophy of user-centered system design still holds. Think of the user’s point of view. Assume that every possible mishap will happen, so protect against it. Make actions reversible. Try to make them less costly. All the required principles have been thoroughly discussed in this book.• Put the required knowledge in the world. Don’t require all the knowledge to be