Short-term memory
Short-term memory holds a small amount of information for a short time, about 15 to 30 seconds if you do not repeat it. It fits only a few items at once, classically 7 plus or minus 2, and closer to 4 chunks by newer estimates. Anything you want to keep has to move on to long-term memory.
In the classic multi-store model from Atkinson and Shiffrin (1968), information flows through three stages. Sensory memory catches everything for a split second. What you pay attention to enters short-term memory. What you process well there gets stored in long-term memory. Short-term memory is the narrow middle of that funnel.
Two limits define it. Duration: in a famous 1959 experiment, people forgot most of a three-letter string within about 18 seconds when they were kept from repeating it. Capacity: George Miller wrote in 1956 about the magical number seven, plus or minus two. Later work by Nelson Cowan put the real limit nearer 4 chunks once you stop people from grouping items.
Short-term memory is often confused with working memory. Short-term memory is the storage part, holding items for a moment. Working memory is the active part, holding and using them at the same time, like doing mental math. For studying, both limits mean the same thing: rereading a page loads short-term memory and then it drains away. Only retrieval, linking ideas and spaced review get it into long-term memory.
| Sensory memory | Short-term memory | Long-term memory | |
|---|---|---|---|
| How long | Under 1 to about 3 seconds | About 15 to 30 seconds without rehearsal | Minutes to a lifetime |
| How much | Large, raw sensory input | About 4 chunks (classically 7 plus or minus 2) | No known limit |
| Gets in by | Your senses, automatically | Paying attention | Processing meaning, retrieval, repetition over time |
| Lost by | Fading almost at once | Fading or being pushed out by new items | Interference, weak cues, disuse |
A professor puts a 10-step derivation on one slide and talks over it. By step 6, Mia has lost step 2. Her short-term memory is full. Next time she writes the steps in three groups with a label for each, and afterwards quizzes herself on the groups from memory. The chunks fit, and the quiz moves them into long-term memory.
- 1Group information into chunks, like 3 to 4 steps under one label instead of 10 loose items.
- 2Take in less at once. Pause the lecture or close the page after each idea.
- 3Write things down instead of juggling them in your head while you work.
- 4Within a minute, say the idea back in your own words, then test yourself again later that day.
- 5Review over several days so the item moves from short-term into long-term memory.