Learning new words feels like progress. The counter goes up, the list gets longer, something is clearly happening. Reviewing old ones feels like standing still. You already know that word — why go back to it?
Because "know" is not a state. It is a moment, and it passes.
You cannot feel yourself forgetting
In 2008, Karpicke and Roediger published a study in Science using exactly the material you are working with: foreign-language word pairs. Students learned words until they could recall each one correctly. Then the groups split — some kept re-studying what they had learned, others kept testing themselves on it.
A week later the gap was large. Repeated studying after that first correct recall did nothing at all. Repeated testing produced a big improvement.
And the uncomfortable part of the same paper: students' predictions about their own performance were uncorrelated with how they actually did. People cannot feel which words are slipping. "I know this" and "I will recall this next week" are not the same fact.
Which is a good reason not to decide by feel what needs reviewing.
The interval matters more than the volume
Spaced practice is among the most heavily replicated findings in memory research. A 2006 meta-analysis pooled 839 assessments from 317 experiments across 184 papers: spacing reliably beats cramming.
But the spacing has a shape. In 2008 the same group showed that the best gap between reviews grows with how long you want to remember. Too soon and you spend a review on something you were never going to forget. Too late and you have already lost it — you are not strengthening a memory, you are learning the word again from scratch.
That is why every word in Fluent carries its own return date instead of one shared "revise everything on Saturday".
It holds outside the lab
The most honest evidence is not a one-hour lab session. It is a full semester.
In 2014, Lindsey and colleagues built personalised review scheduling into a real middle-school foreign-language course. On a cumulative exam after the semester had ended, personalised review produced a 16.5% gain in retention over normal classroom practice.
Not 16.5% on the day. 16.5% of what was still there months later.
What this means for you
- Testing is not the assessment, it is the learning. When Fluent asks you about a word, that question is doing the work. Recalling with effort beats reading with ease.
- Your due words are the most valuable items you own. They sit right at the edge of forgetting, which is exactly where a review buys the most.
- A new word on top of a hundred forgotten ones is not progress. The list grows. The memory does not.
If review has piled up, start your session there. Fluent will put those words first. Your list will not get longer — it will get yours.
References
- Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354–380. https://doi.org/10.1037/0033-2909.132.3.354
- Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095–1102. https://doi.org/10.1111/j.1467-9280.2008.02209.x
- Karpicke, J. D., & Roediger, H. L. (2008). The critical importance of retrieval for learning. Science, 319(5865), 966–968. https://doi.org/10.1126/science.1152408
- Roediger, H. L., & Karpicke, J. D. (2006). Test-enhanced learning: Taking memory tests improves long-term retention. Psychological Science, 17(3), 249–255. https://doi.org/10.1111/j.1467-9280.2006.01693.x
- Lindsey, R. V., Shroyer, J. D., Pashler, H., & Mozer, M. C. (2014). Improving students' long-term knowledge retention through personalized review. Psychological Science, 25(3), 639–647. https://doi.org/10.1177/0956797613504302
- Pavlik, P. I., & Anderson, J. R. (2008). Using a model to compute the optimal schedule of practice. Journal of Experimental Psychology: Applied, 14(2), 101–117. https://doi.org/10.1037/1076-898X.14.2.101