# Miller (1957): The Magical Number 7 Plus or Minus 2

Miller's review of the evidence showed that the results of a range of studies were remarkably consistent. In digit span experiments, PPs' accurate recall of digits or other small chunks of information is always between five and nine items, and generally averages seven items.

Because of the consistency in the results, Miller concluded that the capacity of STM is fixed at seven bits of information. He suggested that STM was like a box with seven slots inside. Each slot can hold a bit of information. When every slot is full, the capacity of STM has been reached, and the addition of new information will cause some information to be lost.

Some of the studies reviewed by Miller used chunks of information. Miller knew that chunking could increase the capacity of STM, and he concluded that each slot could hold either a single bit (e.g. a digit) or a single chunk (e.g. a word) of information. Therefore, he concluded that the overall capacity of STM was 7 plus or minus 2 chunks of information.

Miller thought that a chunk of information could be any size as long as it was meaningful. However, later evidence suggested that this may not be true.