How to Learn Effectively: Spaced Repetition, Testing Effect, and Why Highlighting Is One of the Worst Things You Can Do

Learning research is one of the areas where the gap between what the evidence supports and what most people do is widest. The methods with the strongest empirical support — retrieval practice, spaced repetition, interleaving — are rarely taught in educational settings. The methods students default to — rereading, highlighting, massed practice — are among the least effective, and in some cases actively interfere with retention.

The Testing Effect: Retrieval Practice

Retrieving information from memory is not simply a test of learning — it is itself a learning event. Every successful retrieval attempt strengthens the memory trace more than an equivalent period of re-reading the same material.

The mechanism: memory is reconstructive, not reproductive. Each time you retrieve a memory, the neural pathway used in retrieval is strengthened and the memory is re-encoded at the point of successful recall. This is why being tested after studying works better than restudying.

> 📌 Roediger & Karpicke (2006) compared retrieval practice (testing) to restudying across multiple conditions. Students who studied material once and were tested multiple times outperformed students who studied four times (and were tested once) on delayed retention tests one week later — demonstrating that testing produces durable retention gains beyond additional study time. [1]

Practical implementation: After reading a section of material, close the book and attempt to recall the key points without looking. Then check. The act of attempting retrieval — including failed attempts followed by feedback — is more effective than re-reading. Flashcards, self-testing, and practice problems exploit this effect through structured retrieval.

Spaced Repetition

Memory traces decay over time following an approximately exponential forgetting curve. Reviewing material just before it is about to be forgotten — rather than immediately after learning — produces greater consolidation than massed practice (cramming).

The spacing effect works because:

• 1. Reviewing at the point of maximal forgetting requires maximal retrieval effort, which produces maximal trace strengthening
• 2. Distributed practice allows time for consolidation processes (including sleep-dependent memory consolidation) to occur between sessions

Implementation: review new material within 24 hours. Review again in 3–5 days. Then at 1–2 weeks. Then at 1 month. Spaced repetition software (Anki, SuperMemo) automates this schedule based on individual performance — showing cards again when the algorithm predicts they're approaching the forgetting threshold.

The difference between spaced and massed practice for long-term retention is substantial — studies consistently find 50–100% improvements in test performance 1–4 weeks later in favor of spaced practice.

Interleaving

Most people practice one skill or topic exhaustively before moving to the next — blocked practice. Interleaving mixes different problems or topics within a study session.

Interleaving feels harder and produces worse performance during practice — this is the critical "desirable difficulty" finding. But it produces reliably better long-term retention and transfer (the ability to apply knowledge in novel contexts).

The reason: blocked practice allows the brain to use recently accessed information from short-term memory without fully consolidating the retrieval strategy. Interleaving forces the brain to retrieve each item from long-term memory from near-zero, strengthening the retrieval pathway more robustly.

Implementation: when solving practice problems, mix problem types within a session rather than doing all problems of one type before moving to the next. When studying multiple topics, alternate topics in a session rather than completing one fully before starting the next.

What Not to Do: The Ineffective Methods

Highlighting: produces an illusion of learning — you have processed the visual information but not engaged retrieval. Highlighted text feels familiar upon re-reading (fluency), which is mistaken for memory. Studies find highlighting produces no significant benefit over unmarked re-reading for later test performance.

Rereading: more effective than highlighting (minimally) but substantially inferior to retrieval practice for long-term retention. Rereading increases recognition (the material looks familiar) but does not improve recall (the ability to produce the information without a cue).

Massed practice: effective for passing an imminent test. Produces rapid forgetting. Material crammed the night before an exam is largely unavailable two weeks later.

---

Key Terms

• Testing effect (retrieval practice effect) — the well-replicated finding that attempting to retrieve information from memory produces greater long-term retention than an equivalent period of restudying; the most powerful single learning strategy in cognitive science research
• Spaced repetition — the practice of reviewing material at increasing intervals timed to approach the forgetting threshold; exploits the spacing effect to maximize consolidation per unit of study time
• Desirable difficulty — the principle that learning strategies that feel harder during acquisition produce better long-term retention than strategies that feel easier; the conceptual framework uniting testing effect, spacing, and interleaving
• Fluency illusion — the mistaken sense of mastery produced by recognition of familiar material (rereading, highlighting); correlates poorly with actual recall ability; the mechanism by which ineffective study methods feel productive

---

Scientific Sources

• 1. Roediger, H.L., & Karpicke, J.D. (2006). Test-enhanced learning: taking memory tests improves long-term retention. Psychological Science, 17(3), 249–255. PubMed
• 2. Kornell, N., & Bjork, R.A. (2008). Learning concepts and categories: Is spacing the 'enemy of induction'? Psychological Science, 19(6), 585–592. PubMed