7 Clusters

Unlike long term memory, the relatively small amount of information that humans are able to save in short-term memory
shows easily definable limitations.

A psychologist by the name of George A. Miller noticed a trend among his experiments on short-term memory tasks
and he came up with “the magic number 7 plus or minus 2.” He found that the amount of data that one is able to store
within short-term memory is finite, and more specifically, is thought to be limited to about 9 to 5 clusters (7 on average).  
This basically means that most people can store about 7 symbols at a time without creating the memorial associations
related to long term memory.

If you spend just a few seconds trying to memorize 7 symbols and then look away, you should be able to recall the
numbers as long as you continue to rehearse them.   Some people can remember more symbols than others, however,
very few people can hold upwards of 10 symbols in their short term memory alone.

Telephone numbers and license plates are both numbers that people are often pressured to memorize quickly. This is
part of the reason why in most countries throughout the world they both normally contain few more than 7 symbols.

Try it yourself:

Read this number out loud and try to memorize the sequence in only two seconds:     3749284
…not so hard huh?

Now try this one:  

…quite a bit more difficult right?
Organization for the Advancement of   
Interdisciplinary Learning