After some number of repetitions, Ebbinghaus would attempt to recall the items on the list. It turned out that his ability to recall the items improved as the number of repetitions went up, rapidly at first and then more slowly, until finally the list was mastered. This was the world's first learning curve.
To test retention, Ebbinghaus practiced a list until he was able to repeat the items correctly two times in a row. He then waited varying lengths of time before testing himself again. Forgetting turned out to occur most rapidly soon after the end of practice, but the rate of forgetting slowed as time went on and fewer items could be recalled. This curve represented the the first forgetting curve.
One of the important memory phenomena discovered by Ebbinghaus is the overlearning effect. You can of course continue to practice memorizing a list beyond that required to produce two perfect recalls. For example, if it required 10 repetitions to memorize the list, then you might continue for an additional ten repetitions -- this would be "100% overlearning." The effect of overlearning is to make the information more resistant to disruption or loss. For example, the forgetting curve for overlearned material is shallower, requiring more time to forget a given amount of the material.
Ebbinghaus invented several tests of retention, as listed and described below:
Ebbinghaus was the first to discover the serial position curve -- the relation between the serial position of an item (its place in the list) and the ability to recall it. Items near the beginning of the list are easier to recall than those in the middle (the primacy effect). Those near the end of the list are also earier to recall than those in the middle (the recency effect.) These two effects together yield a curve that is roughly U - shaped.
The normal serial position curve shows that items in the middle of a list are the most difficult to commit to memory. However, this disadvantage can be reduced or eliminated by making the item distinctive, so that it stands out from the other middle-list items. For example, the item could be printed in red when the rest of the items are printed in black. The contrasting color draws attention to the item, and it receives more processing. Consequently, it is memorized more easily than its mere serial position would dictate. In addition, items on either side of the distinctive item may also benefit somewhat. The improved memory for distinctive items in the middle of a list is known as the Von Restorff effect, after its discoverer.