GUEST POST: Retrieval Strength Vs. Storage Strength

GUEST POST: Retrieval Strength Vs. Storage Strength

By: Veronica Yan

Picture from Twitter

Picture from Twitter

Veronica Yan @EdScientists is a Postdoctoral researcher at the University of California, Los Angeles (advisors: Drs. Robert and Elizabeth Bjork), and the University of Southern California (advisor: Dr. Daphna Oyserman); and she is also the Chair of the Learning Advisory Board of ScreenTimeSolutions.

“Forgetting focuses remembering and fosters learning; remembering generates learning and causes forgetting; learning causes forgetting, begets remembering, and supports new learning.” – Bjork, 2011, p 16.

Solomon Shereshevsky, a Russian mnemonist active in the 1920s, had an extremely strong version of synesthesia: His senses were crossed with each other such that hearing a sound would trigger also the experience of seeing color, or being touched would trigger also the experience of a taste. As a result of this much more elaborate encoding experience, Shereshevsky seemed unable to forget information he learned. His apparently perfect memory, however, was in fact a hindrance:

“Inasmuch as S.'s images were particularly vivid and stable, and recurred thousands of times, they soon became the dominant element in his awareness, uncontrollably coming to the surface whenever he touched upon something that was linked to them even in the most general way.” (Luria, 1968, p. 114)  

Without the ability to forget or inhibit information, his world became increasingly confusing. In other words, forgetting is not the opposite of remembering; rather, we have to forget some things in order to remember others. And, as it turns out, we need to forget in order to learn.

Two Types of Memory “Strength”

Our memories do not exist on a single spectrum, ranging from completely forgotten to very well remembered. Rather, how memorable something is can be indexed in two ways, as retrieval strength and as storage strength. This theory of memory—and how the two indices relate to each other) is referred to as the New Theory of Disuse (2).

  • Retrieval strength (RS) is a measure of how easily recalled something is currently, given what is relevant to the present situation (does it come to mind now?).
    • The retrieval strength of a given piece of information can be high or low, and can fluctuate back and forth between these values.

    • Retrieval strength is measured by current performance (e.g., answering questions in class, on a test).

  • Storage strength (SS) is a measure of whether information is deeply embedded or well learned (is it likely to be recalled later?).

    • Barring organic brain damage, storage strength cannot decrease; rather, it is presumed to only accumulate

    • Storage strength cannot be directly measured, but must be inferred: Is that information easily recalled in the future? Or, if you forget that information, does it become faster to relearn it the next time?

For example, if you are on holiday, your hotel room number will be easy to remember while you are staying at that hotel (high retrieval strength), but it is not actually well learned in the sense that you will remember it a long time from now (low storage strength). Or conversely, your childhood home telephone number that your parents made you memorize may be very well learned (high storage strength), but difficult to recall in this moment because it has been a long time since you have had to think about that number (low retrieval strength).

The diagram below shows how different types of memories might rate in terms of retrieval strength and storage strength:

Storage strength, therefore, underlies the main goal of learning and teaching: We want our students to learn information, not so that they can parrot it back to us immediately, but so that this information is easily recalled in the future, in whatever situation it is needed.

The Dynamics between Retrieval Strength and Storage Strength

Retrieval strength and storage strength are related in a peculiar way with respect to learning.

  • When we study information, both retrieval strength and storage strength increase.
  • After studying that information, the higher the storage strength, the slower the loss of retrieval strength (i.e., slower forgetting).
  • When restudying information, how much storage strength increases is inversely related to the current retrieval strength: the lower the retrieval strength at that moment, the greater the boost in storage strength (i.e., learning).

What this last point underlines is that improving storage strength (or ‘learning’) is therefore not just a straightforward matter of spending more time studying.

This second (animated!) diagram illustrates how retrieval strength and storage strength change depending on the starting point (represented by the dot) of a given piece of information. The movement of the arrow to the right represents gains in retrieval strength from study: These gains in retrieval strength are greater when initial retrieval strength is low, as one might expect (if it is already high, there isn’t very much more that can be gained). The movement of the arrow upwards represents the gains in storage strength from study: These gains in storage strength are greater when initial storage strength is low (as might be expected), but also, perhaps surprisingly, when initial retrieval strength is low.

How does this relate to effective learning strategies?

When learning feels easy, this is often because retrieval strength is high. But retrieval strength may be high because:

1)    This information is already very well learned, and therefore, there isn’t much more to be gained with respect to storage strength (why focus study on something that you already know very well?); or,

2)    Cues in your present environment have brought that information to mind (e.g., if you just studied it). In this case, even relatively new information gains much storage strength.

In other words, when retrieval strength is high, regardless of the underlying storage strength, relatively little learning is happening. When retrieval strength is low, however, there is the greatest potential for making learning gains.

I’ll now discuss how two well-known effects – spacing and testing – interplay with retrieval and storage strength.

The Spacing Effect: Spacing, or distributing learning (as opposed to cramming or massing) is one way to reduce retrieval strength and boost storage strength. For example, after we have just read a textbook chapter, that information is fresh in our minds (high retrieval strength). Rereading that chapter immediately is therefore not going to lead to substantial gains in learning (storage strength). If, however, after reading that textbook chapter once, we put it aside and focus on something else, and allow ourselves “forget” that information (i.e., lower retrieval strength), now when we come back to rereading that chapter, we will see much larger gains in storage strength.

The Testing Effect: We can also think about retrieval strength and storage strength in the context of the benefits of retrieval practice. The effort required to retrieve information from memory (e.g., self-quizzing) leads to large gains in storage strength, even if it takes longer to bring that information to mind (i.e., lower retrieval strength). On the other hand, rereading is relatively passive. Having all the answers in front of you increases retrieval strength, circumvents the necessity of retrieval, and therefore leads only to small gains in storage strength.

And putting these two things together: Delayed Testing! When we test ourselves (or our students) on a particular lesson, we should not do so immediately after having just studied (or taught) it, when retrieval strength is still very high. Rather, we should wait some time after study before quizzing, when retrieval strength is lower. This delayed quiz will result in a) a more potent learning event (i.e., greater storage strength), and b) a more accurate indication of real learning!

The take-away message: Forgetting is a critical part of how we learn!


Bjork, R. A. (2011). On the symbiosis of learning, remembering, and forgetting. In A. S. Benjamin (Ed.), Successful remembering and successful forgetting: a Festschrift in honor of Robert A. Bjork (pp. 1-22). London, UK: Psychology Press.

Bjork, R. A., & Bjork, E. L. (1992). A new theory of disuse and an old theory of stimulus fluctuation. In A. Healy, S. Kosslyn, & R. Shiffrin (Eds.), From learning processes to cognitive processes: Essays in honor of William K. Estes (Vol. 2, pp. 35-67). Hillsdale, NJ: Erlbaum.

Luria, A.R. (1986). The Mind of a Mnemonist.  Translated by Lynn Solotaroff. New York/London: Basic Books Inc, Publishers.