Ergonomics – The Human Factor

By Jim Oren

(This Article was excerpted from the Antic Online’s Professional GEM Column)
Reprinted without permission from STart Magazine Volume 4 No 1 (August 1989)

A reader of a novel or science fiction story must suspend disbelief to participate in the story. If the author’s grammar or plot it clumsy, it jars the reader and breaks down the illusion., pulling the reader out of the story. Similarity, a software designer who fails to pay attention to the speed and consistency of a program’s interface will distract users from the programs functions and detract from the care that has been lavished on the function of the program.

Making an interface fast and useable is often treated as a black art.. It isn’t – there are well known methods derived from the field of ergonomics, the study of interaction between man and machine. The ergonomics of the human computer interface were studied extensively at Xerox PARC, the birthplace of the Alto, Star and smalltalk systems which led to the Macintosh and the Atari ST. The designers of these original systems knew and applied the principles of ergonomics.

What follows is a short venture into the world of ergonomics. You will find more than the usual quota of math, slated with examples of real design problems. But there is no way print can convey the vibrancy and tactile pleasure of a good interface, or the sullen boredom of a bad one.. The best way to make ergonomics real is to look for your own examples . Get out your favorite arcade game and see if you can spot some of these principles in action. Dig out the most annoying program in your reject pile and look for the mistakes. Then look at your own with a critical eye.


We’ll start right at the users fingers with the basic equation governing the positioning of the mouse, Fitt’s Law:

T = I x LOG2(D/S + 0.5)

T is the amount of time to move to a target
D is the distance of the target from the current position.
S is the size of the target, stated in equivalent units.
LOG2 is the base 2 (binary) logarithm function
I is a proportional constant, about 100 milliseconds per bit, which corresponds to the human’s “clock rate” for making incremental movements.

We can squeeze an amazing amount of information out of this formula when attempting to speed up an interface. Since motion time goes up with distance, we should arrange the screen with the usual working area near the center, so the mouse will have to move a smaller distance on average from a selected object to a menu or panel. Likewise, any items which are usually used together should be placed together. The most common operations will have the greater impact on speed, so they should be closest to the working area and perhaps larger than other icons or menu entries. If you want to have all operations take about the same time, then the targets furthest from the working area should be larger, and those closer may be proportionately smaller.

Consider the implications for dialogs. Small check boxes are out. Large buttons which are easy to hit are in. There should be ample space between selectable items to allow for positioning errors. Dangerous options should be widely separated from common selections.


If you used the ST desktop for any period of time you’ve probably noticed that your fingers know where to find the file menu. This phenomenon is sometimes called “muscle memory” and its rate of onset is given by the Power Law of Practice:

T(n) = Y(1) x n-a

T(n) is the time on the nth trial
T(1) is the time in the first trial, and is approximately 0.4.

The first thing to note about the power law is that it only works if a target stays in the same place. This should be a potent argument against rearranging icons , menus , or dialogs without some explicit request by the user. The time to hit a target which moves around arbitrarily will always be T(1).


Just as fingers are the way the user sends data to the computer so the eyes are the channel from the machine. The rate at which information may be passed to the user is determined by the “cycle time” of the user’s visual processor. Experimental results show that this time ranges between 50 and 200 milliseconds.

Events separated by 50 milliseconds or less are always perceived as a single event. Those separated by more than 200 milliseconds are always seen as separate.

Suppose that your application ‘s interface contains an icon which should be inverted when the mouse passes over it. We know that flipping it within 1/20th of a second is necessary and sufficient. Therefore if a “first cut” at the program achieves this performance, there is no need for further optimization, unless you want to interleave with other operations.

If it falls short, it will be necessary to do some assembly coding to achieve a smooth feel.

On the other hand, two actions which you want to appear distinct or convey two different pieces of information must be separated by an absolute minimum of a fifth of a second, even assuming that they occur in an identical location on which the users attention is already focused.

It should be quickly added that stimulus enhancement will only work when it unambiguously draws attention to the target. Three or four blinking objects scattered around the screen are confusing, and worse than no enhancement at all.

Short Term Memory

Both the information gathered by he eyes and movement commands on their way to the hand pass through short term memory (also called working memory). The amount of information that can be held in short term memory at any one time is limited. You can demonstrate this limit on yourself by attempting to type a sheet of random numbers by looking back and forth from the numbers to the screen. If you see like most people, you will be able to remember between five and nine numbers at a time. So universal is this finding that it is sometimes called “the magic number seven, plus or minus two”.

he shirt-term capacity sets a limit on the number of choices which the user can be expected to grasp at once. It suggests that the number of independent choices in a menu should be around seven, and never exceed nine. If this limit is exceeded then the user will have to take several glances, with pauses to think in order to make a choice.


The effective capacity of short term memory can be increased when several related items are mentally grouped as a “chunk”. A well designed interface should promote the use of chunking as a strategy by the user. One easy way is to gather together related options in a single place. This is one reason that like commands are grouped into a single menu which is hidden except for the title . If all the menu options were “in the open” the user would be overwhelmed with dozens of alternatives at once. Instead a “show info” command, for instance becomes two chunks: pick file menu, then pick show.

Sometimes the interface can accomplish the chunking for the user. Consider the difference between a slider bar in a GEM program and a three digit entry field in a text mode application. Obviously, the GEM user has fewer decisions to make in order to set the associated variable.


This article is a modest sampler from a much larger field. The Card, Moran and Newell book was the primary source for this article.

Stuart K Card, Thomas P Moran & Allen Newell :
The Psychology of Human-Computer Interaction
Lawrence Erlbaum Associates, Hillsdale, New Jersey 1983
(fundamental and indispensable. This volume of experimental results make it weighty. The good parts are at the beginning and end)

Macintosh User Interface Guidelines – Inside Macintosh
Apple Computer, Inc. 1984
(Though not everything translates this is a fine piece of principled design work)

James D Foley, Victor L Wallace and Peggy Chan
The Human Factors of Computer Graphics Interaction Techniques
IEEE Computer Graphics (CG&A) Nov 1984 pp 13-48
(A good overview, including higher level topics. Excellent bibliography)

J D Foley and A Van Dam
Fundamentals of interactive computer graphics.
Addison Wesley 1984, Chapters 5 and 6
(If you can’t get the article above, read this. If you are designing graphics applications, buy the whole book. Staggering bibliography)

Ben Shneiderman
Direct Manipulation : A step Beyond Programming Languages
IEEE Computer August 1983 pp57-69
(What do Pac-man and Visicalc have in common ? Shneiderman’s analysis is vital to creating hot interfaces.

Oren, Tim
Professional GEM
Antic Online, Compuserve
(The complete text of this article and several others on the same topic are included in Tim’s Professional GEM column in the index section of Antic Online. Log onto CumpuServe and type GO ANTIC)

Tim Oren was a member of the original GEM Team at Digital Research and designed the GEM resource Construction Set, later ported to the Atari ST. After leaving DRI he designed the GEM version of KnowledgeSet’s Graphic Knowledge Retrieval System, one of the first hypertext systems available for CD-ROM. At the same time he was the author of the Professional GEM series of online columns in Antic Online and contributor to the inaugural issues of Start. Tim is currently employed by Apple computer’s Advanced Technology Group, where he leads a project in multimedia information retrieval. (1989)