What are the optimal review intervals for certification exam material?
The optimal interval for any review session is just before you would forget the material -- when retrieval requires effort but is still possible. Research by Cepeda et al. (2008) found that for a 30-day retention period, the optimal single-review gap is 3-5 days. For a 90-day retention period, the optimal gap is 9-15 days. In practice, spaced repetition software calculates these intervals automatically for each card based on individual performance.
One of the most frequently asked questions from candidates new to spaced repetition is: "How often should I review?" The honest answer is that a one-size-fits-all interval schedule is less effective than an algorithm-based individualized schedule. But understanding the research on optimal intervals helps candidates design better manual review systems, interpret their SRS data, and make informed decisions about when to compress intervals (before exams) or extend them (during low-intensity periods).
This article covers the research on optimal spacing intervals, how they vary by retention goal and content difficulty, and how to apply them in both algorithmic and manual review systems.
The Optimal Gap Problem
Choosing the right spacing gap requires balancing two competing effects:
The spacing effect: Longer gaps between reviews produce stronger memory traces at encoding. A review after 7 days strengthens the memory trace more than a review after 1 day.
The forgetting effect: Wait too long, and forgetting outpaces the benefit of spacing. A review attempted after the material is completely forgotten requires essentially the same encoding effort as the original learning.
The optimal interval sits between these extremes -- long enough to require genuine retrieval effort, short enough that retrieval is still possible. This point is sometimes called the desirable difficulty interval.
"The most advantageous spacing gap is one that requires effortful retrieval but not complete relearning. The struggle of almost-forgetting-but-still-retrieving is the mechanism through which spacing produces retention gains." -- Robert Bjork, Department of Psychology, UCLA, 1994
Research-Based Interval Recommendations
Cepeda et al. (2008) conducted the most comprehensive study of optimal review gaps, testing retention over gaps ranging from 1 day to 1 year. Key findings for certification study timelines:
| Retention Goal (Time to Exam) | Optimal Initial Review Gap |
|---|---|
| 7 days | 1-2 days |
| 14 days | 2-4 days |
| 30 days | 3-6 days |
| 60 days | 7-12 days |
| 90 days | 10-18 days |
| 180 days | 20-35 days |
Note that these are optimal gaps for a single review session between initial learning and the test. For multiple review sessions (which SRS provides), gaps are shorter and grow with each successful retrieval.
For multi-review schedules appropriate to certification study:
| Review Number | Typical Gap |
|---|---|
| First review (after initial learning) | 1-3 days |
| Second review | 3-7 days |
| Third review | 7-14 days |
| Fourth review | 14-30 days |
| Fifth review and beyond | 30+ days (monthly) |
Each successful retrieval lengthens the appropriate gap. Each failure shortens it and resets the sequence.
How Anki's Algorithm Implements This
Anki implements a modified SM-2 algorithm that approximates optimal intervals based on individual performance:
Initial learning phase: New cards go through a "learning" phase with short intervals (default: 1 minute, then 10 minutes) before entering the review phase
After graduating from learning: Cards enter the review phase with a default first interval of 1-4 days
Subsequent intervals: Calculated as: New interval = Previous interval x Ease Factor x Interval Modifier
Ease factor: Starts at 250% by default. Increases when you rate a card Easy; decreases when you rate it Hard or Again. Represents the card's individual difficulty for you.
Interval modifier: A global setting that scales all intervals. Default 100%. Reducing to 90% compresses intervals (more frequent review); increasing to 110% extends them.
For most certification content, Anki's default settings produce approximately appropriate intervals. The primary adjustments needed are:
- Reducing the interval modifier to 85-90% in the final 3-4 weeks before the exam
- Lowering the easy bonus to 125-150% if cards are extending too quickly
Manual Interval Schedules (Without SRS Software)
If you prefer not to use SRS software, manual interval schedules can approximate the spacing effect with less precision:
The 1-3-7-21 schedule:
- Review new material on day 1 (immediately after learning)
- Review again on day 3 (2 days later)
- Review again on day 7 (4 days after that)
- Review again on day 21 (14 days after that)
- Subsequent reviews: monthly
This schedule is a simplified version of spaced repetition that requires only a calendar. The schedule is not individualized -- all material receives the same intervals regardless of difficulty. For a 60-card domain, each review session covers all 60 items.
The subject-rotation schedule (for multiple domains):
- Week 1: Study Domain A, review daily
- Week 2: Study Domain B, review Domain A every 3 days + Domain B daily
- Week 3: Study Domain C, review Domain A weekly + Domain B every 3 days + Domain C daily
- Continue until all domains covered, then shift to weekly review of all domains
This approach is more practical than per-card scheduling for candidates who prefer chapter-level review over individual flashcard review.
"Manual spacing schedules, even imprecise ones, outperform massed study by substantial margins. The benefit of spacing does not require algorithmic precision -- any scheduling system that distributes review over time produces significantly better retention than no scheduling." -- Cepeda et al., Psychological Bulletin, 2006
Content-Specific Interval Adjustments
Not all certification content benefits from the same spacing intervals. The optimal interval varies by content difficulty and exam proximity:
| Content Type | Standard Initial Interval | Pre-Exam Adjustment |
|---|---|---|
| Port numbers, specifications | 3-5 days (low complexity, high specificity) | Compress to 2-3 days in final 2 weeks |
| Conceptual definitions | 5-7 days (moderate complexity) | Compress to 3-4 days |
| Framework components | 7-10 days (structural memory) | Compress to 4-6 days |
| Scenario application | 3-5 days (needs frequent practice) | Maintain -- frequent practice critical |
| Complex causal reasoning | 7-14 days (deep encoding) | Compress to 5-7 days |
Port numbers and specifications (HTTPS:443, FTP:21) can tolerate longer intervals once learned because they are high-specificity but low-complexity. Scenario application cards need shorter intervals because the reasoning process decays faster than factual recall.
Adjusting Intervals as the Exam Approaches
Standard SRS intervals are calibrated for indefinite long-term retention. As the exam approaches, adjustments are needed to ensure all material is in a fresh, accessible state:
8-6 weeks before exam: Standard intervals -- let the algorithm run normally
5-4 weeks before exam: Reduce interval modifier to 90%. This compresses all intervals by 10%, ensuring cards appear slightly more frequently without disrupting the scheduling significantly.
3-2 weeks before exam: Reduce interval modifier to 80-85%. Add a "pre-exam" filtered deck that includes all cards with intervals under 21 days for daily focused review.
Final week: Review all cards with intervals under 14 days daily. Suspend cards with intervals above 30 days (these are well-retained and do not need final-week review).
Final 2 days: Review only your lowest-rated cards (Again/Hard most frequently). Do not attempt to review all due cards -- the marginal benefit of reviewing well-known cards is low relative to the risk of study fatigue and sleep disruption.
Frequently Asked Questions
How do I set up a manual review schedule without Anki if I prefer not to use software? Use a physical index card system with date labels. Write the next review date on each card after reviewing it. Separate cards into date-labeled sections. Each day, review all cards with that day's date. This is the original pre-digital spaced repetition method and works reliably for card sets under 200 items.
Should I review cards at the same time each day? Consistent timing helps maintain the habit but is not required for the spacing effect to work. The spacing effect is produced by the interval length, not the time of day. Morning review has modest advantages for consolidation (post-sleep memory is generally accessible) but the benefit of reviewing at any consistent time outweighs any time-of-day effect.
My exam is in 10 days and I have never done spaced repetition. Is it too late to start? Starting SRS 10 days before the exam is not ideal -- there is insufficient time for multiple review cycles. Instead, use a concentrated active recall approach: review all key concepts by domain using closed-book recall, practice exam questions daily, and focus error correction on the 3-4 domains with the lowest practice scores. SRS is most valuable when started 4+ weeks before the exam.
References
- 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.
- 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.
- Bjork, R.A. (1994). Memory and metamemory considerations in the training of human beings. In J. Metcalfe & A. Shimamura (Eds.), Metacognition: Knowing about knowing (pp. 185-205). MIT Press.
- Wozniak, P.A. (1990). Optimization of learning. Masters thesis, University of Technology, Poznan.
- Roediger, H.L., & Karpicke, J.D. (2006). Test-enhanced learning: Taking memory tests improves long-term retention. Psychological Science, 17(3), 249-255.
- Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297-1317.