How quickly do we forget? Too quickly, you might say - and you'd be right. The research on how quickly memory fades (or 'decays' as it is cheerfully termed in the research literature) dates back more than 120 years, to the work of German psychologist Hermann Ebbinghaus. He devised a methodical technique of committing meaningless strings of letters to memory (e.g. DAL; TYP; KEP; MUV etc.), and then seeing how long he was able to hold those strings in memory. His painstaking work produced a striking graph describing how memory fades over time. "The forgetting curve" as it has come to be known shows a dizzyingly steep drop over the first day or so, followed by a gradual flattening of that curve over the next month:
So we might forget something like half the information we encounter over the course of one day, but we may be able to hold on to 10 or 20% of the original information for a month. In this way, our experience of memory can be deceptive though. We think we "remember" something if we can immediately reproduce it, but just a few short hours later, we may find that it has been swallowed up by the business end of the forgetting curve.
But the forgetting curve certainly doesn't describe our relationship with all information. Though we are prone to forgetting those things which we only consider once or twice (like nonsense strings of letters, or the name of someone we just met), we are generally able to remember most things that we turn our attention to often.
Your phone number is perfect example of this. What's the difference between your phone number, and one you've never seen before? They're both just a jumble of ten numbers picked at random. But one of them feels much more familiar to you, and is certainly much easier to remember. Why? It's the number of times that you have revisited and recalled that piece of information over the years.
We tell our memories what is important to keep by how many times we practice recalling it (this also explains nicely why we forget the phone numbers of those close to use when we rely on speed dial).
So, the key to remembering information over the long term, is to revisit it and practice recollecting it often. Ebbinghaus knew this and described how the steep section of the forgetting curve could be flattened by subsequent review of information:
A nice metaphor for this is information recall as 'drops of glue'. Recalling or reconsidering something only once is rather like adding only one drop of glue to that piece information. It appears securely fastened, but the glue may lose its hold over time. However, each review or recall of material adds an additional drop of glue, further increasing the bond. Add enough glue, and you'll never forget your phone number.