Errors are generally divided into two types:

• Errors of Omission – Things you should have done but didn't.
• Errors of Comission – Things you did that you shouldn't have done, or at least you should have done differently.

That's fine in terms of describing what happened but it doesn't tell you why it happened. Why was the step left out or the task not done? Why did you take an incorrect action?

In terms of interpersonal interaction, all types of causes can be at fault from emotion through pure forgetfulness. In doing a task, it's at little different.

Some of the causes are related to your internal environment. If you are pre-occupied with something you cannot put your full mind to what you are doing. If your intention is to stop by the store on the way home and all of a sudden you find yourself having passed the turn-off it may be pre-occupation.

The human mind can only hold a certain amount of data in short term memory. That's why telephone numbers start at seven digits. The actual number of information bits is between five and nine (seven plus or minus 2). This has been measured scientifically.

Working memory is a somewhat different concept psychologically, but has the same constraints – seven to nine items.

The amount of information can be increased by using techniques such as 'information chunking' or dividing the information into sets and remembering the set instead of the indiviual numbers. A phone number can be area code–3digits–4digits. Other information can be chunked in other ways but the basic constraint is seven to nine items. For chunks, the limit is usually four.

I had thought that with all the practice modern society gives us in multi-tasking we must have increased our capacity so I jumped at the chance to test this in one of my classes in scientific method.

The subjects were all college students. The experiment involved several types of information and involved immediate recall as well as recall after a 5 minute activity of anaother kind. Much to my surprise, in all but one case the seven to nine item rule held true. The exceptional person remembered 10 items.

If you mind is crowed with a few other things, the fact that you wanted to go to the store on the way home can easily be pushed out of your short term memory. AND, if you have several things to pick up at the store you're better off to take a list.

What does this have to do with Ergonomics?

Ergonomists often evaluate tasks to improve work flow and efficiency and to decrease error. Knowing the reason for errors can assist in job design and in set up of work-flow. As well, it may tell us where the break down is in a system and guide us in creating solutions. Maybe the person just has too many things to remember.

There are ways to combat this limitation. Other than chunking data, training is often used. This can move items from short term to long term memory. Different rules apply once it reaches that area of the brain.

Have a pencil ready. For each trial, don't start writing until the letters disappear and you are give the directions to write them down. HAVE FUN.