Memory retention changes substantially with time, study method, spacing, sleep, and the type of material being learned. Across the studies summarized here, delayed recall and recognition often favored retrieval practice or spaced learning, although results varied by task, comparison condition, and outcome measure.
Contents
- Forgetting across retention intervals
- Retrieval practice versus rereading
- Spacing and repeated retrieval
- Retention in lectures and classrooms
- Vocabulary and language memory
- Sleep, cramming, and durable recall
- Students’ use of retention strategies
Forgetting across retention intervals
One way to describe memory retention is relearning savings: the proportion of learning effort saved when previously studied material is learned again. This is different from a direct recall or recognition percentage, so it should not be treated as interchangeable with those measures.
A 2011–2012 single-subject replication used lists of 13 syllables and measured savings after several delays. Average relearning savings were 47.2% after 20 minutes, 37.3% after 1 hour, and 27.6% after 9 hours. The average was 31.7% after 1 day, 23.0% after 2 days, 16.8% after 6 days, and 4.1% after 31 days. These results describe one replication task, not a universal percentage of information remembered by every learner.
The same report presented a historical comparison from Ebbinghaus’s original series: savings were 58.2% at 20 minutes in the 1880 data. Because that figure comes from a legacy experiment reported by the replication, it should be read as a historical comparison rather than as a newly collected measurement. Source: Replication and Analysis of Ebbinghaus’ Forgetting Curve.
| Delay | Average relearning savings in the 2011–2012 replication |
|---|---|
| 20 minutes | 47.2% |
| 1 hour | 37.3% |
| 9 hours | 27.6% |
| 1 day | 31.7% |
| 2 days | 23.0% |
| 6 days | 16.8% |
| 31 days | 4.1% |
The pattern shows why a study session that feels successful immediately may not produce the same level of performance later. It also illustrates the importance of identifying the outcome: relearning savings measures the benefit of prior exposure when material is learned again, while recall measures what a learner can produce at a test.
Retrieval practice versus rereading
Retrieval practice asks learners to produce information from memory, whereas rereading or repeated study provides another exposure to the material. In a 2016 experiment, retrieval practice produced 57% correct recall after 5 minutes, compared with 31% after rereading. After 1 week, correct recall was 59% for retrieval practice and 41% for rereading. The delayed result was therefore higher for the retrieval condition in that experiment. Source: The Testing Effect and Far Transfer: The Role of Exposure to Key Information.
A 2015 study using a passage with 26 idea units found a different short-term pattern. Repeated study produced 67.5% recall after 5 minutes, while retrieval practice produced 51.2%. After 1 week, retrieval practice produced 37.5% recall of the idea units, compared with 23.6% after repeated study. The comparison demonstrates why both the delay and the material matter: repeated study was higher immediately in this passage task, while retrieval practice was higher after a week. Source: Learning psychological research and statistical concepts using retrieval-based practice.
Visual materials can also produce mixed results. In Experiment 1A of a 2023 study using visual squiggle images, retrieval-practiced items scored 63% correct on a delayed test, compared with 66% for restudied items. Baseline visual items scored 65% in one reported comparison and 54% in another reported condition. In Experiment 1B, retrieval-practiced items scored 37% correct on delayed recognition, compared with 32% for restudied items; baseline items scored 21% and 14% in the corresponding reported conditions. Source: The Testing Effect for Visual Materials Depends on Preexisting Knowledge.
These figures support a practical distinction: retrieval is often useful for durable access, but its advantage is not guaranteed for every material, test, or learner. A short quiz can be used to reveal what is available from memory, followed by feedback or targeted review.
Spacing and repeated retrieval
Spacing separates learning or retrieval attempts instead of concentrating them in one massed session. A 2019 four-participant aphasia study reported higher retention-test accuracy for spaced conditions than for massed conditions in each listed participant comparison. P1 scored 81% with spaced retrieval practice and 75% with massed retrieval practice. P2 scored 63% with spaced retrieval practice and 56% with massed retrieval practice. P3 scored 44% with spaced repetition and 21% with massed repetition. P4 scored 73% with spaced retrieval practice and 56% with massed retrieval practice. Source: Retrieval practice and spacing effects in multi-session treatment of naming impairment in aphasia.
| Participant | Spaced condition | Massed condition |
|---|---|---|
| P1 | 81% | 75% |
| P2 | 63% | 56% |
| P3 | 44% | 21% |
| P4 | 73% | 56% |
Follow-up results were lower than the retention-test results reported for the first two participants: P1’s spaced retrieval-practice accuracy was 60%, and P2’s was 44%. These values show that performance can decline between assessments even after a spaced intervention. The study involved four participants with aphasia, so its percentages should not be generalized as population-wide student estimates.
A 2014 vocabulary spacing experiment tested 40 word pairs per participant and measured retention at approximately 1 day, 6 days, and 27 days after practice. Among the tested intended intervals, 12 hours produced the best long-term retention. The study also reported that the 7-minute intended spacing condition had data from 9 of 9 participants at the final approximately 28-day test. Source: Very Similar Spacing-Effect Patterns in Very Different Learning/Practice Domains.
Retention in lectures and classrooms
Spacing has also been examined in lecture settings. In a 2019 medical-education study, the spaced-lecture cohort had an 86.5% participation rate on the retention test, while the traditional-lecture cohort had a 68.9% participation rate. The spaced-lecture cohort averaged 33.8% on the retention test, compared with 31.8% for the traditional-lecture cohort.
On 20 new retention-test questions, the spaced-lecture cohort averaged 36.6% correct and the traditional-lecture cohort averaged 33.6%. On nine baseline retention questions, the averages were 27.7% for the spaced-lecture cohort and 27.8% for the traditional-lecture cohort. Source: Making a Lecture Stick: the effect of spaced instruction on knowledge retention in medical education.
| Lecture measure | Spaced lecture | Traditional lecture |
|---|---|---|
| Retention-test participation | 86.5% | 68.9% |
| Overall retention-test average | 33.8% | 31.8% |
| 20 new questions | 36.6% | 33.6% |
| 9 baseline questions | 27.7% | 27.8% |
The baseline results were nearly identical, while the new-question results favored the spaced-lecture cohort. Since participation rates also differed, the classroom percentages should be interpreted together with the study design and test composition.
Vocabulary and language memory
Longer delays provide a useful test of whether learning remains accessible beyond the next study session. A 2026 L3 vocabulary study measured recognition after a 50-day delay. Covert retrieval practice produced 71.52% average recognition, repeated studying produced 60.61%, and new L3 words produced 55.15%. These are recognition results from a recent delayed vocabulary study, not general memory percentages for all language learners. Source: The Benefits of Covert Retrieval Practice on Short- and Long-Term L3 Vocabulary Retention.
The spacing experiment described above used word pairs and retention tests at approximately 1, 6, and 27 days. Its finding that a 12-hour intended study interval produced the best long-term retention among the tested intervals suggests that the timing between encounters can matter, not just the total number of encounters. The experiment’s 40 word pairs per participant and approximately 28-day final test define the scope of that result.
For vocabulary study, these findings point to two measurable targets: the learner’s ability to retrieve a word without seeing it and the learner’s ability to recognize it later. Recognition, recall, and relearning savings are distinct outcomes; a percentage from one should not be used as a direct conversion into another.
Sleep, cramming, and durable recall
Sleep and spacing can interact with how practice is distributed. A 2016 adolescent vocabulary study compared 5 hours with 9 hours of nightly time in bed while measuring recall of 40 GRE-type word pairs during a 14-day protocol. From the first study day through the final delayed test, the sleep-restricted group had 28 fewer hours of sleep opportunity than the 9-hour group. Source: Sleep Restriction Impairs Vocabulary Learning when Adolescents Cram for Exams: The Need for Sleep Study.
After equal total repetitions, spaced-item recall did not significantly differ between the 5-hour and 9-hour sleep groups across immediate, 24-hour, and 120-hour recall tests. For massed items, however, sleep restriction significantly reduced recall relative to the 9-hour sleep condition. The source reports these outcomes in figures; the supplied study summary does not provide the corresponding numeric recall values, so no additional percentages are stated here.
A 2019 post-lecture study compared review-only learning with an immediate retrieval quiz. Review-only students scored 71% on the immediate quiz, versus 51% for students who took the immediate retrieval quiz. At one week, the review-only group scored 53%, while the retrieval-quiz group scored 56%. Source: Comparison of Rewatching Class Recordings versus Retrieval Practice as Post-Lecture Learning Strategies.
The immediate and delayed results moved in different directions: review-only performance was higher immediately, while the retrieval-quiz group was slightly higher at one week. That pattern is consistent with the broader lesson that immediate fluency and durable recall are separate measures.
Students’ use of retention strategies
Measured memory performance and reported study behavior are not the same thing. A 2025 student survey found that 32% of students reported using retrieval practice often and 24% almost always. Twenty-six percent reported using retrieval practice sometimes, 13% rarely, and 5% never.
For spacing, 25% reported using it often and 16% almost always. Thirty-seven percent reported using spacing sometimes, 18% rarely, and 4% never. Source: A closer look at students’ knowledge of effective learning strategies, where they learn about them, and why they do not use them.
| Reported frequency in the 2025 survey | Retrieval practice | Spacing |
|---|---|---|
| Almost always | 24% | 16% |
| Often | 32% | 25% |
| Sometimes | 26% | 37% |
| Rarely | 13% | 18% |
| Never | 5% | 4% |
The survey describes how often students said they used each strategy; it does not measure their memory scores. Taken alongside the experimental results, it provides a behavior context for retention-supporting methods: retrieval practice and spacing were reported at different frequencies, and many students selected “sometimes” rather than the most frequent categories. These percentages should be interpreted as survey responses, not as estimates of how much students remembered.