Human memory is complex and a little mysterious. Unlike electronic data storage, human memory is not perfectly reliable and predictable. In this section, we’ll take a whirlwind tour of what we know about human memory, and then we’ll think about how we can apply this knowledge to user interface design.

A model of memory

There are a number of psychological models of human memory. Most distinguish between short-term and long-term memory as separate but interrelated structures or systems in the brain. While there is no consensus on the “correct” model, one model useful for us is as follows:

• Short-term memory or working memory is a temporary store that can hold a small amount of information, such as a handful of words, numbers, or symbols, related to your current train of thought. Working memory decays very rapidly; the information can be lost when your attention is drawn to something else, and so you often have to repeat the information to yourself to avoid having it disappear. The capacity of working memory is said to be about “seven, plus or minus two” items (Miller, 1956), and it’s for this reason that North American phone numbers were chosen to be seven digits long — it’s difficult to hold more than about seven digits in your mind if you’ve heard a phone number and you’re trying to write it down.

• Middle-term memory or contextual memory holds information required for you to be able to work on your current activity, but you won’t permanently remember most of this information. For example, during a conversation, you’ll have in mind the details of what has been discussed so far. Or if you’re working on your income tax return, you’ll know where on your desk you’ve put your different papers and receipts and you’ll remember some of the key numbers and details.

• Long-term memory is a more persistent store of knowledge and memories of experiences — facts, concepts, ideas, names, images, sounds, voices, places, emotional feeling states, and so on. Long-term memory also stores procedures and skills, for both cognitive and sensory-motor tasks. Long-term memory might alternatively be called “permanent memory“, but this is misleading as information is often subject to forgetting or “false recall”.

How does memorization happen?

Memorization, or committing something from short-term memory to long-term memory, most commonly involves repetition. The more often you encounter (see or hear) something, the more likely you are to remember it later. Studying involves actively intentionally re-reading, rehearsing, and practicing.

But we also tend to remember information and experiences that are surprising, novel, or unusual without any repetition.

The exact nature of how the brain forms memories remains unknown, but it is likely that information and memories stored in long-term memory are somehow stored symbolically. That is, if you hear a professor telling you information in a lecture, you may memorize much of the information in the lecture, but you will probably not store a perfect audio recording of the professor’s lecture. You may of course remember the professor’s voice, especially if it is particularly unique, but this is separate from the information content of the lecture, which you can make use of in practical contexts without “playing back” the “audio recording” of the professor’s voice.

There are some rare people who do have a perfect “photographic” memory, though, and most people can remember music precisely enough that they can distinguish if a later performance differs by only one note.

We tend to store information in logical groupings or chunks, and memorization is most effective when a chunk is associated with other existing chunks of information in memory. Associations are logical connections or relationships between pieces of information. If you meet and get to know a new person, for instance, you’ll associate the image of their face with their name and their other personal details, like their occupation and family members.

If you are trying to learn a complex concept or process, and you feel that your understanding is incomplete or insufficient because of unanswered questions, memorization and later recall will tend not to be as reliable as when you feel that you have a complete and logical understanding of the matter.

Recall and recognition

Recall of information from long-term memory is usually triggered by some cue. Seeing someone’s face, for instance, will typically trigger you to recall that person’s name.

Successful recall is never guaranteed. The more recently the information was memorized or accessed, though, the more likely you are to be able to recall it (the recency effect).

Successful recall of some piece of information is also more likely to occur when you’ve already recalled related information. It’s as if related information is stored in adjacent locations in the brain, and by accessing information in a particular region, “lighting up” that region, recalling other related information from that region becomes easier.

Sometimes you will struggle to recall something, and the information may or may not “come to you” at a later time. Sometimes recall is inaccurate; you recall incorrect information. You might misremember a formula when you’re taking a math test, for instance. Sometimes you may have doubt about whether the recalled information is correct, but just as frequently, you may not recognize the error.

Often you may not be able to recall something, but you can recognize it when you see it. The information was in your memory, but for some reason it was “shrouded” and didn’t lend itself to being accessed at that moment.

Forgetting

The less frequently a chunk of information is accessed from long-term memory, the more likely it is to be forgotten. This is natural — things that are relevant to your daily routines will be remembered, and additionally reinforced due to the recency effect. On the other hand, facts that you studied years ago but haven’t needed since, or the names of people whom you met years ago but haven’t kept in contact with since, will tend to fade away.

But there are also many cases where letting information or skills languish for long periods of time won’t necessarily guarantee that they will be forgotten. Highly-developed motor and cognitive skills that can be done unconsciously after much practice — like riding a bicycle or speaking a foreign language — can often still be performed with surprising levels of competence after years of neglect.

What all of this means for user interface design

On the basis of this understanding of memory, memorization, recall, and forgetting, here are some guidelines to keep in mind when designing software:

• Structure your interface to reduce or eliminate the need to memorize and recall things. Donald Norman introduced the notion of “knowledge in the world versus knowledge in the head“. For example, if you present a list of options in a menu, this knowledge is “in the world”: the user can view the menu, recognize the options, and make a selection without needing to memorize or recall anything. If you were to require the user to enter commands at a command-line interface, on the other hand, this would require the user to memorize the commands — thereby storing the “knowledge in the head” — and then the user must recall the names of the commands from memory.

• If a task has a defined sequence of steps, guide the user through the task workflow step-by-step using a “wizard” interface or something similar. Don’t force the user to remember a series of commands or how to navigate to various different screens to finish the task.

• While shortcut keystrokes and command-line interfaces are often appreciated as time-savers by advanced users, you shouldn’t make these the sole means of interaction as they require memorization and recall. If you must rely on shortcuts or commands, make it is easy to refer to a quick-reference chart or other help material.

• Make icons and names easily recognizable so that they can be found easily when scanning a list or menu. Icons can be clarifying if the image represents something concrete and recognizable, but if the user has to memorize and recall what an peculiar or abstract icon really means, then it defeats the purpose of using a graphical icon. Consider showing the image and text together if the icon is abstract or its meaning is unclear.

• If the user will have to work with ID numbers such as product or customer numbers, it can be advantageous to limit these to about 7 or fewer digits or characters in length, if possible, so that it’s easier to temporarily store the numbers in working memory.

• Arbitrary names are harder to remember and recall than names that accurately describe what they represent. When names don’t match what they actually represent, not only do they become problematic to learn, but the additional memorization and recall adds to the user’s cognitive burden. Commands in the Unix operating system shell are particularly bad at violating this prinicple. For example, most Unix systems offer a command called “less” for showing the contents of a text file. The name “less” is a play on words; “less” is an improvement of another command called “more”. (“more” is a filter command that lets you view a stream of data in a page-by-page fashion.)  The name “less” doesn’t in any way communicate what that command does; it’s a dumb pun by somebody trying to be clever. “list” would be an example of a more appropriate, obvious, and memorable name.

• Offer a good online help system or other reference documentation so that, when the user has forgotten something, it can be looked up again quickly.

• In search and index systems, allow the user to use synonyms and variations in case they can’t recall the exact word or phrase (or the correct spelling of the word or phrase) needed to identify something.

• Try to use commonly accepted, well-known, standard names for things rather than inventing your own terminology. Avoid using abbreviations or acronyms if they are not immediately obvious.

• Be consistent; don’t make the user remember different ways of doing the same thing in different contexts. I’m familiar with one system where some drop-down lists had to be opened with a Ctrl+L keystroke combination, while other drop-down lists had to be opened with Alt+F11. (An absurd situation, given that in all other systems, drop-down boxes can be opened just by clicking on them.)

• Because first impressions are rarely forgotten, do everything you can to make sure your product gives a great first impression!

Security is critical in many software applications, but security measures often are seen as annoyances that negatively impact the user experience.

The essential purpose of security, however, is to protect users (and other stakeholders) from more serious negative experiences. For example, having to remember a complex password is annoying. But it reduces the chances of much worse experiences: losing access to your account, having your data stolen, deleted, or tampered with, having fraudulent transactions conducted on your behalf, or having someone impersonate you.

Thus, like so many issues in software design, there is a trade-off between security measures and usability. Let’s survey some common security mechanisms and issues and examine them from a usability perspective.

Authentication

Authentication refers to the process of identifying the user and verifying that they are who they say there are.

To gain access to a system, a user must prove that they are somebody who has been granted permission to access that system, and this is done by providing proof of identity. Most computer systems implement this by challenging the user to provide a username and password. The username can exist only if an account has been granted to a particular approved person, and only that person should know the password for the account. And so by providing the correct password, the user is “proving” that they are who they say they are.

Since passwords are so easily guessed or stolen, they are a weak means of identification, but passwords persist because they are cheap and easy to implement. Physical tokens like ID badges or keycards, or biometric identification using fingerprints or retinal scans, are alternative but costlier means of identification. High-security installations tend to combine physical tokens or biometric identification together with a password, so that if one half is compromised (e.g., a keycard is stolen), an intruder still cannot gain access.

Short passwords can be easily guessed or “cracked” using brute-force algorithms, so requiring passwords to be of a certain minimum length, and requiring passwords to include characters from an extended character set (numbers, punctuation symbols, etc.) can greatly reduce the chances of hackers gaining illicit access. Complex passwords come at the cost of irritating users, though, who quite understandably have a hard time remembering the passwords for all of the systems and websites they interact with.

Likewise, requiring users to change their passwords every few months is recommended by security experts, but users tend to resent this added memorization burden.

The strictness of the security requirements should match what is at stake. An Internet discussion forum does not require the same security as a banking website, and a banking website does not require the same measures needed for a classified military installation.

Security violations like intrusions and leaked passwords are highly embarrassing, and so your product’s technical architects must ensure that they are employing current best practices for implementing the security system. Security audits are worthwhile if your organization can afford them.

In enterprise information systems, applications are often constructed by cobbling together several off-the-shelf products. It is highly unpleasant if the user has to log in with separate usernames and passwords every day to access, say, the Customer Relationship Management application, the workflow application, the reporting application, and so on. A single sign-on solution is recommended for such systems so that a single login grants the user access to all of the component applications.

Automatically logging out the user after a period of inactivity is a measure designed to reduce somewhat the chances of a malicious user taking over a user’s session when the user leaves the machine unattended. To reduce annoyance, however, the timeout period should not be so short that the user will be logged out during normal operation, and when the user re-authenticates himself, the system should resume where it left off.

Authorization (access control)

Authorization, also known as access control, refers to controlling what each particular user is allowed to access. Which functions are permitted, and which functions are prohibited? What information can be viewed and edited, and what is off-limits?

Rather than controlling access to individual users, most systems define roles. Users can then be assigned to one or more roles. An access control matrix can then be created to define what functions members of each role are permitted to access:

Access control is often overlooked when user interface designers produce initial sketches and mockups, and it tends to be haphazardly added later. A key decision to be made involves the presentation of the menu items and buttons for functions that the current user is prohibited from accessing. Should these controls be hidden completely, or should they be shown in a disabled state (i.e., greyed out)?

With greyed-out controls, it might not always be obvious to the user that the reason why the control is greyed out is because of an access control restriction. Users in some cases might think that the application is simply in the wrong mode, for instance, and might waste time trying to figure out actions needed to re-enable the controls.

Hiding controls can be an attractive option because it produces less clutter, but it can also cause confusion if a user is looking for something they’ve seen before (perhaps while watching a coworker use the application) and cannot find it. Hiding controls can also cause layout issues; if the layout is static, there will be empty gaps where the controls used to be, and if the layout is dynamic, controls that were nicely aligned may no longer be nicely aligned once some of them disappear. Additionally, the same screen will appear differently when users with different roles log in, and so if you include screenshots in the documentation and help content, and if the user’s permissions differ, the screenshots may not match what the user is looking at.

Fraud prevention

Enterprise systems that deal with financial transactions often must consider fraud prevention. If a user can issue a payment, for instance, what is stopping her from issuing a few payments to herself? If the system is structured so that payment transactions must be reviewed and approved by a peer coworker or a manager (this is sometimes called the two-man rule or the four-eyes principle), and if all transactions are permanently recorded and logged, then this kind of fraud is less likely to occur, as there is a risk of getting caught and reported if someone were to seek out a collaborator.