3. Human Computer Interaction

3.1. What’s the big picture?

People often become frustrated with computers. At some point when using these devices, you will be annoyed that the computer did something you didn’t want it to do and you can’t figure out how to get the computer to do what you want, but why is that? Humans made computers, so why are computers often so frustrating to use by humans?

Computers are becoming hundreds of times more powerful every decade, yet there is one important component of the computer system that hasn’t changed significantly in performance since the first computers were developed in the 1940s: the human. For a computer system to work really well it needs to be designed by people who understand the human part of the system well.

In this chapter we’ll look at what typically makes good and bad interfaces. The idea is to make you sensitive to the main issues so that you can critique existing interfaces, and begin to think about how you might design good interfaces.

Often software developers create a computer program or system for a device that requires the user to spend some time to learn how to use the program. These interfaces might be easy to use for the developer since they know the system really well, but a user just wants to get the job done without spending too much time learning the software (they might switch to another program if it’s too hard to use). A developer might treat the program and the user separately, however the user is part of the system, and a developer needs to create the software with the user in mind, designing a program that they will find easy to use and intuitive.

Human-computer interaction (HCI) is about trying to make programs useful, usable, and accessible to humans. It goes way beyond choosing layouts, colours, and fonts that aren’t Comics Sans for an interface. It’s strongly influenced by the psychology of how people interact with digital devices, which means understanding many issues about how people behave, how they perceive things, and how they understand things so that they feel that a system is working to help them and not hinder them. HCI “involves the study, planning, and design of the interaction between people (the users) and computers. It is often regarded as the intersection of computer science, behavioral sciences, design and several other fields of study.” By understanding HCI, developers are more likely to create software that is effective and popular. If you ask people if they have ever been frustrated using a computer system, you’ll probably get a clear message that HCI isn’t always done well.

Try out the following interactive task, and get some friends to try it:



15 seconds left

Did anyone get a wrong answer to the question even though you thought you got it right? You may have noticed that the “Yes” and “No” button sometimes swap. Inconsistency is normally a really bad thing in an interface, as it can easily fool the user into making an error.

The study of Human Computer Interaction involves a lot of psychology (how people behave) because this affects how they will use a system. As a simple example, the human short term memory only lasts for a matter of seconds (even in young people!) If a device delays a response for more than about 10 seconds, the user has to make a conscious effort to remember what they were doing, and that’s extra work for the user (which from their point of view, makes the system more tiring to use). Another example is that people get “captured” into sequences: if you start biking on a route that you take each day, you’ll soon find yourself arriving without thinking about every turn along the way, which is fine unless you were supposed to go somewhere else on the way. A similar effect occurs with confirmation dialogues; perhaps you often accidentally close a file without saving it, and the system says “Do you want to save it?”, so you press “Yes”. After you’ve done this a few times you’ll be captured into that sequence, so on the one occasion that you don’t want to overwrite your old file, you may accidentally click “Yes” anyway. A lot of people might blame themselves for such errors, but basic psychology says that this is a natural error to make, and a good system should protect users from such errors (for example, by allowing them to be undone).

Designing good interfaces is very difficult. Just when you think you’ve got a clever idea, you’ll find that a whole group of people struggle to figure out how to use it, or it backfires in some situation. Even worse, some computer developers think that their users are dummies and that any interface problems are the user’s fault and not the developer’s. But the real problem is that the developer knows the system really well, whereas the user just wants to get their job done without having to spend a lot of time learning the software – if the software is too hard to use, they’ll just find something else that’s easier. Good interfaces are worth a lot in the market.

There are many ways to evaluate and fine tune interfaces, and in this chapter we’ll look at some of these. One important principle is that one of the worst people to evaluate an interface is the person who designed and programmed it. They know all the details of how it works, they’ve probably been thinking about it for weeks, they know the bits that you’re not supposed to touch and the options that shouldn’t be selected, and of course they have a vested interest in finding out what is right with it rather than what is wrong. It’s also important that the interface should be evaluated by someone who is going to be a typical user; if you get a 12-year-old to evaluate a retirement planning system they may not know what the user will be interested in; and if you get a teacher to try out a system that students will use, they will know what the answers are and what the correct process is.

Often interfaces are evaluated by getting typical users to try them out, and carefully noting any problems they have. There are companies that do nothing but these kinds of user tests — they will be given a prototype product, and pay groups of people to try it out. A report on the product is then produced and given to the people who are working on it. This is an expensive process, but it makes the product a lot better, and may well give it a huge advantage over its competitors. Having it evaluated by a separate company means that you avoid any bias from people in your own company who want to prove (even subconsciously) that they’ve done a good job designing it, rather than uncover any niggling problems in the software that will annoy users.

Before we look at different ways to evaluate interfaces, we need to consider what is happening with an interface.

3.2. Users and tasks

A very important consideration when designing or evaluating an interface is who the users are likely to be. For example, the typical age of a user can be significant: very young children may have difficulty reading some words and prefer images and animations, while a business person who uses an interface frequently will want it to be very fast to use, perhaps using keyboard shortcuts only.

What are some of the considerations you would make for the following user groups?

  • Senior citizens
  • Gamers
  • Casual users
  • Foreign visitors

The interface is the only part of a program that the user sees (that’s the definition of an interface!), so if the interface doesn’t work for them, the program doesn’t work.

Another important thing to do when designing and evaluating an interface is to think about what tasks it is being used for. Advertisements for digital devices often hide the complexity of a task, and simply point out the features available for doing the task. For example, suppose a smartphone is advertised as having a high resolution camera. The real task that someone might want to do is to take a photo of something they’ve just spotted, and send it to a friend. If you look at what happens in reality, the smartphone might be in their pocket or bag, and if they see something cool going past, they need to get it out, perhaps unlock it, open the camera app, adjust the lighting and other settings, press a button (is it easy to find when you’re holding the camera up?), select the photo, choose an email option, type in the friend’s address (does the system help with that?), send it (what happens if you’re out of reception range?), and then put the phone away. If any one of these steps is slow or hard to remember, the whole experience can be frustrating, and it’s possible the photo opportunity will be missed, or for some other reason the friend won’t receive the photo.

It’s very important to think about the whole context when describing a task. As an exercise, can you provide an example of a real task, including context, for a real person for each of the following:

  • set an alarm clock
  • show a slide (Powerpoint) presentation

It’s important to think through all the parts of a task when discussing an interface, as it’s the small steps in a task that make all the difference between using the interface in a real situation, compared with a demonstration of some features of the device.

3.2.1. Activity: Sending an email from multiple devices

For this activity, try sending an email from both a computer and a mobile phone. Take note of all the steps required from when you start using the device until the email is sent.

You will probably notice quite a few differences between the two interfaces.

3.2.2. Activity: Designing stovetops/remote

For this activity, you will designing the top of a cooking stove. This isn’t a computer system, but will help demonstrate some of the issues that come up. Your task is to sketch three different configurations for the stovetop which includes the arrangement of the 4 elements and the 4 control knobs.

The task is described in detail in the HCI CS Unplugged activity.

3.3. Interface Usability

Devices are often sold using catch phrases like “user friendly” and “intuitive”, but these are vague terms that are hard to pin down. In this section we will use the more technical term, Usability, which is well understood by HCI experts, and gives us some ways to evaluate how suitable an interface is for a particular task. Usability isn’t just about an interface being nice to use: poor usability can lead to serious problems, and has been the cause of major disasters, such as airplane crashes, financial disasters, and medical mishaps. It is also important because an interface that requires a lot of dexterity, quick reactions or a good memory makes it less accessible to much of the population, when accessibility can be both a moral and legal expectation.

There are many elements that can be considered in usability, and we will mention a few that you are likely to come across when evaluating everyday interfaces. Bear in mind that the interfaces might not just be a computer — any digital device such as an alarm clock, air conditioning remote control, microwave oven, or burglar alarms can suffer from usability problems.

One “golden rule” of usability is consistency. If a system keeps changing on you, it’s going to be frustrating to use. Earlier we had the example of a yes/no button pair that occasionally swapped places. A positive example is the consistent use of “control-C” and “control-V” in many different programs to copy and paste text or images. This also helps learnability: once you have learned copy and paste in one program, you know how to use it in many others. Imagine if every program used different menu commands and keystrokes for this!

A related issue is the *Mode error*, where the behaviour of an action depends on what mode you are in. A simple example is having the caps lock key down (particularly for entering a password, where you can’t see the effect of the mode). A classic example is in Excel spreadsheets, where the effect of clicking on a cell depends on the mode: sometimes it selects the cell, and other times it puts the name of the cell you clicked on into another cell. Modes are considered bad practice in interface design because they can easily cause the user to make the wrong action, and should be avoided if possible.

The speed at which an interface responds (its reaction time) has a significant effect on usability. This is closely related to human perception of time. If something happens fast enough, we will perceive it as being instant.

The following interactive lets you find out how fast “instant” is for you. As you click on each cell, there will sometimes be a random delay before it comes up; other cells won’t have a delay. Click on each cell, and if it seems to respond instantly, leave it as it is. However, if you perceive that there is a small delay before the image comes up, click it again (which makes the cell a little lighter). You can’t go back and change a cell, so just make a quick, gut-level decision the first time you click each one. The delay may be very short, but only make the cell gray if you are fairly sure you noticed a delay.

Once you have clicked on all the cells, click on the “Probability of perceiving delays” bar to see a graph of how often you thought there was a delay compared with how long the delay actually was. 100 ms is one tenth of a second; for most people this is where they are likely to start perceiving a delay; anything short (particularly around 50 ms) is very hard to notice. Longer delays (for example, 350 ms, which is over a third of a second) are very easy to notice.

The point of this is that any interface element (such as a button or checkbox) that takes more than 100 ms to respond is likely to be perceived by the user as not working, and they may even click it again. In the case of a checkbox, this may lead to it staying off (from the two clicks), leading the user to think that it’s not working.

Click the checkbox
Clicked 0 times

So, as you evaluate interfaces, bear in mind that even very small delays can make a system hard to use.

Another important length of time to bear in mind is our short term memory time, which is usually a matter of seconds. To remember something for longer, the user needs to rehearse it (repeat it over) or make a note of the information, such as writing it down. If a system takes some time to respond (say, 10 seconds) then chances are the user may have forgotten some detail of what they were going to do with the system. For example, if you have a phone number to type in that someone has just told you, and it takes 12 seconds before you can type it, you may forget the number, whereas if you can access the interface within a couple of seconds, you can probably enter the number without much effort. For this reason, any part of a system that takes more than about 10 seconds to respond forces the user to rehearse or write down key information, which is more tiring.

Another important usability consideration is spatial memory — our ability to remember where things are located (such as where a button or icon is). Human spatial memory has a high capacity (you can probably remember the location of many places and objects), it is long lasting (people visiting a town they grew up in can often remember the layout), and we can remember things very quickly. A very simple aspect of usability that comes from this is that the layout of an interface shouldn’t keep changing. The interactive task at the start of this chapter was deliberately set up to be frustrating by swapping the two buttons occasionally; the reason people often make a mistake in that situation is that their spatial memory takes over, so the location of the button is more important than what is written on it. Systems that aren’t consistent in their spatial placement of the “OK” and “Cancel” buttons can be frustrating.

Another place that the layout of an interface changes quickly is when a tablet or smartphone is rotated. Some devices rearrange the icons for the new orientation, which loses the spatial layout, while others keep them the same (except they may not look right in the new rotation). Try a few different devices and see which ones change the layout when rotated.

What else can cause the layout of an interface to change suddenly?

Associated with spatial memory is our muscle memory, which helps us to locate items without having to look carefully. With some practice, you can probably select a common button with a mouse just by moving your hand the same distance that you always have, rather than having to look carefully. For example, working with a new keyboard can mean having to re-learn the muscle memory that you have developed, and so may slow you down a bit.

One common human error that an interface needs to take account of is the off by one error, where the user accidentally clicks or types on an item next to the one they intended. For example, if the “save” menu item is next to a “delete” menu item, that is risky because one small slip could cause the user to erase a file instead of saving it. A similar issue occurs on keyboards; for example, control-W might close just one window in a web browser, and control-Q might close the entire web-browser, so choosing these two adjacent keys is a problem. Of course, this can be fixed by either checking if the user quits, or by having all the windows saved so that the user just needs to open the browser again to get their work back. This can also occur in web forms, where there is a reset button next to the submit button, and the off-by-one error causes the user to lose all the data they just entered.

Another idea used by HCI designers is the principle of commensurate effort, which says that frequently done simple tasks should be easy to do, but it’s ok to require a complex procedure for a complex task. For example, in a word processor, printing a page as it is displayed should be easy, but it’s ok if some effort is required to make it double sided, two to a page, with a staple in the top left corner. In fact, sometimes more effort should be required if the command has a serious consequence, such as deleting a file, wiping a device, or closing an account. In such cases artificial tasks may be added, such as asking “Are you sure?”, or to get an extreme setting on a device (like setting a voltage for a power supply) might require pressing an “up” button many times, rather than letting the user type in an extra couple of zeroes.

These are just a few ideas from HCI that will help you to be aware of the kinds of issues that interfaces can have. In the following project you can observe these kinds of problems firsthand by watching someone else use an interface, noting any problems they have. It’s much easier to observe someone else than do this yourself, partly because it’s hard to concentrate on the interface and taking notes at the same time, and partly because you might already know the interface and have learned to overcome some of the less usable features.

3.3.1. Project: Cognitive walkthrough

The cognitive walkthrough is a technique that HCI experts use to do a quick evaluation of an interface. Details of how to do one are on the cs4fn site.

There is more information in the Wikipedia entry for Cognitive Walkthrough.

3.4. Usability Heuristics

Evaluating an interface is best done by getting feedback from having lots of potential users try it out. However, this can be expensive and time-consuming, so HCI experts have come up with some quick rules of thumb that help us spot obvious problems quickly. The formal word for a rule of thumb is a heuristic, and in this section we will look at some common heuristics that can be used to critique an interface.

There are various sets of heuristics that people have proposed for evaluating interfaces, but a Danish researcher called Jakob Nielsen has come up with a set of 10 heuristics that have become very widely used, and we will describe them in this section. If you encounter a usability problem in an interface, it is almost certainly breaking one of these heuristics, and possibly a few of them. It’s not easy to design a system that doesn’t break any of the heuristics, and sometimes you wouldn’t want to follow them strictly — that’s why they are called heuristics, and not rules.

You can find the heuristics online on Jakob Nielsen’s website; below is an explanation of each one.

3.4.1. Visibility of system status

The system should always keep users informed about what is going on, through appropriate feedback within reasonable time.

This heuristic states that a user should be able to see what the device is doing (the system’s status), at all times. This varies from the user being able to tell if the device is turned on or off, to a range of actions. A classic example is the “caps lock” key, which may not clearly show if it is on, and when typing a password the user might not know why it is being rejected; a positive example of this is when a password entry box warns you that the caps lock key is on.

One of the simplest statuses for a device is on or off, which is usually a coloured light on the outside of the computer. However, some devices take a while to show the status (for example, some DVD players take a while to respond when switched on), and the user might press the power button again or otherwise get confused about the status.

There are many tasks that users ask computers to do that require some time including copying documents, downloading files, and loading video games. In this situation, one of the most common ways to keep a user informed of the task is the progress bar.

85% Complete
An Windows busy cursor
An Apple busy cursor
The author of the Windows file copy dialog visits some friends, and struggles to decide what time he will arrive.

However, progress indicators aren’t always helpful; the spinning wheels above don’t indicate if you are going to have to wait a few seconds or a few minutes (or even hours) for the task to complete, which can be frustrating.

Giving feedback in a “reasonable time” is really important, and the “reasonable time” is often shorter than what you might think. In the section above there was an experiment to find out at what point people perceive a delayed reaction; you probably found that it was around a tenth of a second. If a computer takes longer than that to respond then it can be confusing to use. There’s more about this in the previous section.

There are some other important delay periods in interface evaluation: a delay of around 1 second is where natural dialogues start to get awkward, and around 10 seconds puts a lot of load on the user to remember what they were doing. Nielsen has an article about the importance of these time periods. If you want to test these ideas, try having a conversation with someone where you wait 3 seconds before each response; or put random 10 second delays in when you’re working on a task!

Getting computers to respond quickly often depends on the algorithms used (covered in the chapter on algorithms), and can also depend on the design of a program (such as whether it stores data on disk or waits for a network response before continuing). It is particularly noticeable on small devices like smartphones, which have limited computing power, and might take a second or two to open an app or respond to some input. It’s not too hard to find these sorts of delays in systems when you’re evaluating them.

3.4.2. Match between system and the real world

The system should speak the users’ language, with words, phrases and concepts familiar to the user, rather than system-oriented terms. Follow real-world conventions, making information appear in a natural and logical order.

The language, colours and notation in an interface should match the user’s world, and while this seems obvious and sensible, it’s often something that is overlooked. Take for example the following two buttons — can you see what is confusing about them?

The following interface is from a bank system for paying another person. Suppose you get an email asking someone to pay you $1699.50 dollars for a used car; try entering “$1699.50” into the box.


The notation “$1699.50” is a common way to express a dollar amount, but this system forces you to follow its own conventions (probably to make things easier for the programmer who wrote the system).

Are there other ways of expressing dollar amounts that are valid, but this system rejects? This system should be flexible with the inputted text to prevent errors (we will talk more about this later on).

3.4.3. User control and freedom

Users often choose system functions by mistake and will need a clearly marked “emergency exit” to leave the unwanted state without having to go through an extended dialogue. Support undo and redo.

It is very frustrating to make a mistake and not be able to get out of it. It is particularly bad if one small action can wipe a lot of work that can’t be recovered (the reset button on some web forms is infamous for this — it is often next to the submit button, and you can wipe all your data with an off-by-one error.) A common way to provide user freedom is an “undo” feature, which means that not only can mistakes be fixed easily, but the user is encouraged to experiment, trying out features of the interface secure in the knowledge that they can just “undo” to get back to how things were, instead of worrying that they’ll end up in a state that they can’t fix. If “redo” is also available, they can flick back and forth, deciding which is best. (In fact, redo is really an undo for undo!)

Here’s an example of a button that doesn’t provide user control; if you press it, you’ll lose this whole page and have to find your way back (we warned you!)

Do you wish to close this window?

Sometimes the interface can force the user into doing something the user does not want to do. For example, it is quite common for operating systems or programs to perform updates automatically that require a restart. Sometimes the interface may not give them the opportunity to cancel or delay this, and restart nevertheless. This is bad if it happens when the user is just about to give a presentation.

Another common form of this problem is not being able to quit a system. A positive example is the “home” button on smartphones, which almost always stops the current app that is in use.

3.4.4. Consistency and standards

Users should not have to wonder whether different words, situations, or actions mean the same thing. Follow platform conventions.

Consistency (something being the same every time) is extremely useful for people using interfaces, and is sometimes called the “golden rule of HCI”. If an interface is consistent with other interfaces then learning in one interface transfers directly to another. One of the biggest examples of consistency in computer programs is copy and paste, which works the same way in most software, so users only have to learn the concept once. The shortcut keys for copy and paste are also fairly consistent between programs. But in some software, copy/paste behaves differently, and this can be confusing for users.

An example of inconsistency is generally found within spreadsheet programs, where the result of pushing “control-A” (select all) depends on whether you are editing a cell or just have the cell selected (this particular problem is a ‘mode’ problem). While this may make sense to a user experienced with spreadsheets, a new user can be very confused when the same action causes a different response.

A lack of consistency is often the reason behind people not liking a new system. It is particularly noticeable between Mac and Windows user; someone who has only used one system can find the other very frustrating to use because so many things are different (consider the window controls for a start, which are in a different place and have different icons). An experienced user of one interface will think that it is “obvious”, and can’t understand why the other person finds it frustrating, which can lead to discussions of religious fervour on which interface is best. Similar problems can occur when a radically different version of an operating system comes out (such as Windows 8); a lot of the learning that has been done on the previous system needs to be undone, and the lack of consistency (i.e. losing prior learning) is frustrating.

3.4.5. Error prevention

Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone conditions or check for them and present users with a confirmation option before they commit to the action.

A computer program shouldn’t make it easy for people to make serious errors. An example of error prevention found in many programs is a menu item on a toolbar or dropdown being ‘greyed out’ or deactivated. It stops the user from using a function that shouldn’t be used in that situation, like trying to copy when nothing is selected. A good program would also inform the user why an item is not available (for example in a tooltip).

Below is a date picker; can you see what errors can be produced with it?


A related problem with dates is when a user needs to pick a start and end date (for example, booking flights or a hotel room); the system should prevent a date prior to the first date being selected for the second date.

Any time a dialogue box comes up that says you weren’t allowed to do a certain action, the system has failed to prevent an error. Of course, it may be difficult to do that because the error can depend on so many user choices, but it is ideal that the system doesn’t offer something that it can’t do.

3.4.6. Recognition rather than recall

Minimize the user’s memory load by making objects, actions, and options visible. The user should not have to remember information from one part of the dialogue to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.

Humans are generally very good at recognising items, but computers are good at remembering them accurately. A good example of this is a menu system; if you click on the “Edit” menu in a piece of software, it will remind you of all the editing tasks available, and you can choose the appropriate one easily. If instead you had to type in a command from memory, that would put more load on the user. In general it’s good for the computer to “remember” details, and the user to be presented with options rather than having to remember them.

3.4.7. Flexibility and efficiency of use

Accelerators – unseen by the novice user – may often speed up the interaction for the expert user such that the system can cater to both inexperienced and experienced users. Allow users to tailor frequent actions.

When someone is using software every day, they soon have common sequences of operations they do (such as “Open the file, find the next blank space, put in a record of what just happened”). It’s good to offer ways to make this quick to do, such as “macros” which do a sequence of actions from a single keystroke.

Similarly, it’s good to be able to customise software by allocating keystrokes for frequent actions (such as “file this email in the ‘pending’ folder”). Common tasks like copy and paste usually have keystrokes added to them, and these allow experienced users to perform the task without having to reach for a mouse.

3.4.8. Aesthetic and minimalist design

Dialogues should not contain information which is irrelevant or rarely needed. Every extra unit of information in a dialogue competes with the relevant units of information and diminishes their relative visibility.

Software can contain many features, and if they are all visible at the same time (for example, on a toolbar) this can be overwhelming for a new user.

3.4.9. Help users recognize, diagnose, and recover from errors

Error messages should be expressed in plain language (no codes), precisely indicate the problem, and constructively suggest a solution.

It’s not hard to find error messages that don’t really tell you what’s wrong! The most common examples are messages like “Misc error”, “Error number -2431”, or “Error in one of the input values”. These force the user to go on a debugging mission to find out what went wrong, which could be anything from a disconnected cable or unfixable compatibility issue, to a missing digit in a number.

A variant of this is a message that gives two alternatives, such as “File may not exist, or it may already be in use”. A better message would save the user having to figure out which of these is the problem.

3.4.10. Help and documentation

Even though it is better if the system can be used without documentation, it may be necessary to provide help and documentation. Any such information should be easy to search, focused on the user’s task, list concrete steps to be carried out, and not be too large.

Often help is an afterthought, and tends to be feature-centred (e.g. a catalogue of menu items), rather than task-centred (the series of actions needed to complete typical tasks, which is more useful for the user). When a user needs help, they typically have a task to complete (such as upload photos from a camera), and good documentation should explain how to do common tasks, rather than explain each feature (such as “Setting the camera to USB mode”).

3.5. 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 key idea. Many commercial systems are tested using the ideas above 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.

We also haven’t talked much about accessibility: making a system usable for someone with a disability such as impaired vision or impaired use of their limbs provides extra challenges, but is also very worthwhile because the device itself may improve accessibility (such as software to browse the web using an audio interface, or a computer-controlled wheelchair that can avoid colliding with obstacles).

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

3.6. Further reading