Most people think the criteria to justify usability only based on the requirements of users. It sounds true but not in any situation. Sometimes, besides users, should take other issues into account. Design of faucet in public toilet maybe a good example to explain the factors behind the screen.

The function of public faucet is simple and straight: give out water. And the operation just contains two processes: turn on or turn off the faucet. If you think like this, you could probably design a faucet with manual switch. But I'm afraid to tell you that this is not a good design, even from the standpoint of users. Consider the following scenarios:

• A construction worker wanted to wash the dirt off his hands, he turned on the switch with his grimy hands. After washing, he tried to turn off the switch and found the dirt was sticked on it.
• A child turned on the switch, wash his hands but he forgot to turn off the switch when he left.

The above scenarios reveal the issues about hygiene as well as error correction. To the construction worker, it's ridiculous that using the cleaned hands to touch the dirty switch again. On the other hand, children usually forget to turn off the switch that would bring out the problem of wasting. In order to solve these problems, an sensor faucet will be better.

However, we all know an sensor faucet is not a good choice. Why? Did you come across this embarrassing situation? When you stand in front of the sensor faucet, stretch out your hands, wait for the water but nothing happens.  Then, you move your hands in different angles and hope the infrared indicator can detect. Or you would give up and find another switch and repeat the same actions. The demerit of sensor faucet is we hardly recognize it can operated or not. Since it gets no manual switch, how can we detect its failure by just observe its appearance? Under this situation, users may need to spend more time to observe which faucet operates normally. I usually check those sinks before washing my hands. If I find water drops in sink, the faucet probably can operate.

From the analysis of faucet in public toilet, we learned that designing usable thing is not only concerned the requirements of users. Other factors like hygiene, error correction should be considered. Sometimes, they play more important roles in the process of design. The following is the design concerns about the public faucet:

• Manual switch: easy to operate, dirt on the switch, forget to turn off
• sensor switch: not easy to operate, no dirt, automatically turn off

Users would like to operate easily, but hygiene and wasting seem more important. Then, automatic faucet is "better" design.  So, user won't win every time, something in somewhere would be the main factor which strongly influence our designs.