Jacobs used a sample of 443 female students (aged from 8-19) from the North London Collegiate School. Participants were presented with a sequence of numbers or letters to recall immediately, gradually increasing in length until they could no longer remember the order. He found that the mean span for digits was found to be 9.3 items, and for letters, it was 7.3 in the correct order for his female participants. This supports the idea that STM has a limited capacity of 7+/-2 items.
