Memory Retention Spaced Repetition
Build an optimal spaced-repetition review schedule so what you learn actually stays remembered.
Formula
Retention(t) = Initial × e^(-k×days); Intervals: 1, 3, 7, 14, 30, 60, 120 days
Worked Examples
Example 1: Medical School Anatomy
Problem:Learn 500 anatomical terms over 90 days. Medium difficulty. Target 85% retention. 45 min/day study time.
Solution:Items: 500 terms\nPeriod: 90 days\n\nNew terms per day: 500 / (90 × 0.2) = ~28/day\n\nSpaced intervals: 1, 3, 7, 14, 30, 60 days\nReviews per item: 6 reviews over 90 days\n\nTotal reviews: 500 × 6 = 3,000 reviews\nPer day: 3,000 / 90 = 33 reviews\n\nTime required:\n28 new (5 min each) = 140 min\n33 reviews (30 sec each) = 17 min\nTotal: ~157 min/day initially\n\nThis tapers to ~40 min/day after first month as fewer new items added.\n\nAt 45 min/day budget: reduce to 15-20 new terms/day\nExtend timeline to 120-150 days.
Result:500 items needs 120+ days at 45min/day | Start with 15-20 new/day
Example 2: Language Learning (1000 Words)
Problem:Learn 1,000 Spanish words over 180 days. Easy difficulty (cognates help). 20 min/day.
Solution:Items: 1,000 words\nPeriod: 180 days\n\nNew words: 1,000 / 36 (20% of period) = 28/day\n\nWith easy difficulty, retention is high:\nReviews: 1,000 × 5 (not all intervals needed) = 5,000\nPer day: 5,000 / 180 = 28 reviews\n\nTime:\n28 new × 1 min = 28 min\n28 reviews × 20 sec = 9 min\nTotal: ~37 min/day peak\n\n20 min/day budget:\nReduce to 10-15 new words/day\nOr extend timeline\n\nAlternative: 15 new/day × 180 = 2,700 words total\nAt 20 min/day sustained.
Result:15 new words/day sustainable at 20min/day | 2,700 words over 180 days
Example 3: Professional Certification
Problem:Memorize 200 key concepts for certification. Hard difficulty. 90 days. 60 min/day study time.
Solution:Items: 200 concepts\nPeriod: 90 days\n\nNew concepts: 200 / 18 = 11/day\n\nHard material needs more reviews:\nReviews: 200 × 7 = 1,400\nPer day: 1,400 / 90 = 16 reviews\n\nTime:\n11 new × 8 min (complex) = 88 min\n16 reviews × 2 min = 32 min\nTotal: ~120 min/day peak\n\n60 min/day budget:\nReduce to 5-6 new concepts/day\nExtend to 120-150 days\n\nOR accept some concepts won't be fully mastered,\nfocusing on high-yield content.
Result:60min/day supports 5-6 new concepts/day | Extend timeline or reduce scope
Frequently Asked Questions
What is spaced repetition?
Spaced repetition is a learning technique that schedules review at increasing intervals: 1 day, 3 days, 7 days, 14 days, etc. It leverages the spacing effect—memory improves when study sessions are distributed over time rather than massed. Optimal for fact-based knowledge like vocabulary, medical terms, or historical dates.
How does spaced repetition compare to cramming?
Cramming creates short-term memory that fades rapidly. Spaced repetition builds long-term retention. Research shows: after 1 week, cramming retention ~20%, spaced repetition 80%+. After 1 month, cramming near 0%, spaced repetition 70%+. Initial learning takes similar time; retention differs dramatically.
How long does spaced repetition take?
Initial learning: similar to any method. Ongoing maintenance: ~5-10 minutes per day per 100 items in mature deck. Front-loaded (more reviews early), then tapers. Lifetime maintenance is lightweight compared to re-learning from scratch.
What should I use spaced repetition for?
Ideal for: vocabulary (language learning), medical terminology, historical dates, geography, technical definitions, formulas. Less suited for: conceptual understanding, complex problem-solving, or creative skills. Best for declarative knowledge (facts).