Human Computer Interaction
4.9. The whole story!

In this chapter we've mainly looked at how to critique interfaces, but we haven't said much about how to design good interfaces. That's a whole new problem, although being able to see what's wrong with an interface is a good basis for designing good interfaces. Many commercial systems are tested using the ideas we covered to check that people will find them easy to use; in fact, before releasing a new application, often they are tested many times with many users. Improvements are made, and then more tests need to be run to check that the improvements didn't make some other aspect of the interface worse! It's no wonder that good software can be expensive – there are many people and a lot of time involved in making sure that it's easy to use before it's released.

There are many other ideas from psychology, physiology, sociology and even anthropology that HCI experts must draw on. Things that come into play include:

• Mental models, about how someone believes a system works compared with how it actually works (these are almost never the same e.g. double clicking on an icon that only needs to be single clicked).
• Fitts's law, about how long it takes to point to objects on a screen (such as clicking on a small button).
• The Hick-Hyman law, about how long it takes to make a decision between multiple choices (such as from a menu).
• Miller's law about the number of items a person can think about at once.
• Affordances, about how properties of an object help us to perform actions on them.
• Interaction design (IxD), about creating digital devices that work for the people who will use the product.
• The NASA TLX (Task Load Index) for rating the perceived workload that a task puts on a user.

There are many more laws, observations and guidelines about designing interfaces that take account of human behaviour and how the human body functions.