The Testing Effect vs. The Spacing Effect: Which Matters More?

# The Testing Effect vs. The Spacing Effect: Which Matters More?

Two of the most well-supported findings in learning science are the testing effect and the spacing effect. Both have decades of research behind them. Both meaningfully improve long-term retention. And both are underused by the vast majority of students.

But they are not the same thing. Understanding what each does - and how they interact - will help you build a study strategy that is grounded in evidence rather than guesswork.

What Is the Testing Effect?

The testing effect (also called retrieval practice) is the finding that actively retrieving information from memory strengthens that memory more than passively reviewing it. When you quiz yourself, answer practice questions, or try to recall material without looking at your notes, you are leveraging the testing effect.

This is not intuitive. Most people assume that putting information into memory (reading, watching lectures, reviewing notes) is the primary driver of learning. But research consistently shows that pulling information out of memory - the act of retrieval itself - is what builds durable knowledge.

A landmark 2008 study by Karpicke and Roediger published in *Science* demonstrated this clearly. Students who practiced retrieval retained 80% of material a week later, compared to 36% for students who used repeated study. The retrieval group did not study more. They studied differently.

The testing effect works because retrieval strengthens the neural pathways associated with a memory. Each successful recall makes the memory more accessible in the future. Importantly, even unsuccessful retrieval attempts - where you try to remember but cannot - still produce learning benefits, as long as you review the correct answer afterward.

What Is the Spacing Effect?

The spacing effect (also called distributed practice) is the finding that spreading study sessions over time produces better long-term retention than concentrating them in a single block. Studying a topic for 30 minutes on three different days beats studying it for 90 minutes in one sitting.

This has been replicated across hundreds of studies and is one of the most robust findings in all of cognitive psychology. Hermann Ebbinghaus first documented it in 1885, and the evidence has only grown stronger since.

The spacing effect works through several mechanisms. When you return to material after a delay, you experience partial forgetting. This forgetting is not a failure - it is a feature. The effort of re-learning partially forgotten material creates stronger, more durable memories than reviewing material you still remember easily.

A 2006 meta-analysis by Cepeda and colleagues reviewed 254 studies involving over 14,000 participants and confirmed that distributed practice consistently outperformed massed practice for long-term retention, with the optimal spacing interval depending on how long you need to remember the material.

How They Differ

The testing effect and the spacing effect target different aspects of the learning process.

The testing effect is about how you study. It says: retrieve information from memory rather than passively reviewing it. The method matters.

The spacing effect is about when you study. It says: distribute your practice over time rather than concentrating it. The schedule matters.

You can use one without the other:

• Testing without spacing: You quiz yourself repeatedly in a single study session. You get the testing effect but miss the benefits of spacing.
• Spacing without testing: You re-read your notes on Monday, Wednesday, and Friday. You get the spacing effect but miss the benefits of retrieval practice.

Both approaches outperform the default student strategy of cramming (massed, passive review). But neither alone captures the full picture.

Which Matters More?

Research suggests that the testing effect produces a slightly larger boost to retention than the spacing effect alone, but the comparison is somewhat artificial because the two work best together.

A 2011 study by Karpicke and Blunt in *Science* found that retrieval practice outperformed elaborative study methods even without spacing. The act of retrieval itself was the critical ingredient. Other studies have shown that spaced re-reading, while better than massed re-reading, still underperforms spaced retrieval practice.

If you had to choose one, retrieval practice is the higher-leverage intervention. It changes the fundamental nature of what your brain does during study - from passive absorption to active reconstruction.

But in practice, you do not have to choose.

The Real Answer: Combine Them

The most effective study strategy combines both effects. You practice retrieval (testing effect) and you distribute that practice over time (spacing effect). This combination is sometimes called spaced retrieval practice, and it is the backbone of evidence-based study.

Here is what this looks like in practice:

1. Day 1: Learn new material. At the end of the session, close your notes and try to recall the key concepts. Check what you missed.
2. Day 3: Without reviewing first, try to recall the same material. Answer practice questions on the topic. Review any gaps.
3. Day 7: Test yourself again. The material will feel harder to retrieve - that difficulty is productive.
4. Day 14 and beyond: Continue testing at increasing intervals. Each successful retrieval at a longer delay strengthens the memory further.

This approach leverages both effects simultaneously. The retrieval practice builds strong memory traces. The spacing ensures those memories remain accessible over time rather than fading after a few days.

Why Most Students Use Neither

Despite the overwhelming evidence, most students default to massed, passive study - re-reading notes the night before an exam. Why?

Three reasons:

1. Fluency illusion. Re-reading feels easy, which tricks you into thinking you know the material. Retrieval practice feels hard, which makes you think it is not working. The subjective experience is the opposite of reality.
2. Short-term thinking. Cramming does work for very short-term recall - you might pass tomorrow's quiz. But the knowledge evaporates within days. Spacing and retrieval optimize for long-term retention.
3. Lack of awareness. Most students have never been taught how to study effectively. Study skills are rarely part of formal education, so people default to whatever habits they developed in middle school.

Practical Takeaways

If you want to apply these findings to your own studying:

• Replace re-reading with self-testing. After covering material, close your notes and quiz yourself. Use flashcards, practice questions, or simply try to explain the concepts from memory.
• Spread sessions out. Do not cram a topic into one sitting. Return to it across multiple days, allowing some forgetting to occur between sessions.
• Embrace difficulty. If retrieval feels effortful, that is a sign it is working. Easy retrieval means the material is already well-learned and your time is better spent on weaker areas.
• Use a system. Spaced repetition software automates the scheduling of reviews based on your performance. This removes the guesswork from spacing intervals.

The testing effect and the spacing effect are not competing strategies. They are complementary tools that, when combined, produce study outcomes that neither can achieve alone.

Explore Certin's tools built on spaced retrieval practice ->