Some of the first noted research on human memory was conducted in 1890 by William James. Based on his works, James assumed memory consists out of two systems:

• primary memory, which lasts for a few seconds and holds in our consciousness the perception of events in our environment, and
• second memory, which has unlimited duration and can be brought to consciousness if wanted.

A measure for the capacity of short-term (primary) memory was first introduced even a bit earlier, in 1887, by Joseph Jacobs, who tested the span of digits his students were able to remember. Under the criterion that at least 50% of the digits need to be remembered correctly, subjects he tested mostly remembered about 7 digits. What he also noticed is that this result can be improved by grouping the digits (for example in groups of three), or reading aloud.

Famous People and Their Contributions to the Study of Memory Retrieved March 16, 2011.