The magical number 7

Back in the 1950s, a Harvard psychologist named George A. Miller wrote "The Magical number seven, plus or minus two," a landmark journal article. Miller studied short-term memory, especially the quick memorization of random sequences of numbers. He wanted to know, going into the study, how many numbers people could be reliably expected to remember a few minutes after having been told those numbers only once.

The answer - the magical number - was about 7. Some people able to remember 8 or 9 numbers and equal number of people able to remember only 5 or 6 numbers, so Miller figured that 7 (plus or minus 2) numbers accurately represented the ability range of nearly the entire population. That is why all local telephone numbers were designed as 7-digits.

Miller's concept went beyond numbers, though, to other organization of data. For example, most of us can remember about 7 recently learned pieces of similarly classified data, like names or clues in a parlor game. This is also explained the magic number 6 in my last post ( "Winner takes it all").

Source - Accidental Empires , Chapter 2.