What is spaced repetition? Why you forget 80% of vocabulary in a week

You sit down and study 50 IELTS vocabulary words tonight. A week later, how many do you remember? According to Hermann Ebbinghaus's 1885 research — confirmed by hundreds of modern experiments — the answer is usually 20–30%. The rest has fallen into the abyss of short-term memory.

That's not your fault. It's the fault of the method. Spaced repetition is the way out, and it's the principle behind every modern flashcard app from Anki to RemNote to Mnemo.

This article explains spaced repetition cleanly: what it is, why it works, the main algorithms (SM-2, FSRS), and how to apply it to daily IELTS vocabulary practice.

The forgetting curve — why we forget

In 1885, the German psychologist Hermann Ebbinghaus ran experiments on himself: he memorised nonsense syllables, then measured how quickly he forgot them. The results were sobering:

After 20 minutes: forgot ~40%
After 1 hour: forgot ~55%
After 1 day: forgot ~65%
After 6 days: forgot ~75%
After 31 days: forgot ~80%

The curve is steepest in the first hour. If you don't intervene, knowledge plummets right after a study session.

This is why "cramming" — reading the same 50 words for two hours then moving on — fails for IELTS vocabulary. You feel like you've learned them (because they're still in working memory), but a week later when you sit a mock test, more than half are gone.

What is spaced repetition? A simple definition

Spaced repetition is a review technique where you revisit information right before you would forget it, and each successful recall expands the next interval exponentially.

The basic loop:

Learn a new word today.
Review after 1 day.
If you remember → review after 3 days.
If you remember → review after 7 days.
If you remember → review after 2 weeks, then 1 month, then 3 months...

Each successful recall strengthens the memory trace in your brain. The expanding interval is deliberate — review too early (while the memory is fresh) and your brain doesn't bother consolidating. Review too late (after total forgetting) and you have to start from scratch.

This sweet spot — what cognitive scientist Robert Bjork calls desirable difficulty — is where spaced repetition does its best work.

Why spaced repetition is 3–5× more effective than cramming

There are three scientific mechanisms behind it:

1. The spacing effect

When you review the same item across sessions separated in time, your brain forms multiple memory traces in different contexts. Each retrieval in a new context (morning vs. night, home vs. coffee shop, after a different activity) reinforces the trace differently, making the memory more durable.

2. The testing effect

Spaced repetition doesn't just make you re-read — it makes you retrieve by pulling the answer from memory before checking it. Karpicke & Roediger (2008) showed active retrieval is roughly 50% more effective than passive re-reading for long-term retention.

3. Optimised study time

Because the schedule is tuned to your individual memory (each card has its own next-review date), you don't waste time on cards you already know cold. A learner using smart SRS can maintain 5,000 IELTS-level words on 15–20 minutes a day — impossible with traditional methods.

The main spaced-repetition algorithms

SM-2 — the classic SuperMemo algorithm (1987)

SM-2 was the first widely-used SRS algorithm, developed by Piotr Wozniak for SuperMemo and the default in Anki for many years. It's simple:

You rate your recall on a 0–5 scale (0 = total blank, 5 = effortless).
The system updates an "ease factor" and computes the next interval.
Formula: next_interval = previous_interval × ease_factor.

Strengths: simple, no training data needed, easy to debug. Weaknesses: assumes per-card difficulty is fixed — wrong in practice. "Bad-information" cards that you keep forgetting don't trigger SM-2's attention, so it keeps stretching their intervals despite repeated lapses.

FSRS — the modern algorithm (2022–2024)

FSRS (Free Spaced Repetition Scheduler) was developed by Jarrett Ye based on the DSR model (Difficulty, Stability, Retrievability):

Difficulty — the card's intrinsic hardness, updated from recent performance.
Stability — how long the memory currently lasts, updated each review.
Retrievability — the probability you'd recall the card if tested right now.

FSRS is trained on millions of real review sessions to learn optimal parameters. The result: significantly more accurate scheduling than SM-2 — published research shows 20–30% fewer reviews needed to reach the same retention rate.

Anki added FSRS as an option in 2024, and Mnemo uses FSRS v5 by default — meaning you don't have to think about scheduler configuration, the system just runs the modern algorithm out of the box.

How to apply spaced repetition to IELTS vocabulary

Theory is one thing — what should you actually do every day? Here's the practical loop:

Step 1 — Build a word inbox, not a word list

Common mistake: print a 1,000-word IELTS list and try to memorise it in two months. Brains don't work linearly like that.

Instead, collect vocabulary from sources you actually read — Cambridge Reading passages, BBC 6 Minute English podcasts, The Economist, science papers. Every word you encounter in a real context is far easier to remember than the same word in a sterile list.

Mnemo's Chrome extension lets you highlight a word on any webpage and save it to your inbox in one click — exactly to support this kind of intentional collection.

Step 2 — Build cards with context

Each card should have:

Front: word + the example sentence (taken from where you encountered it)
Back: definition, IPA, and 1–2 common collocations

A good card:

Front: "The new policy aims to mitigate the impact of climate change." — word: mitigate
Back: to make less severe, /ˈmɪtɪɡeɪt/, collocations: mitigate risk, mitigate damage

Avoid bare-bones cards like "mitigate = make less". Without context, you're rote-memorising and won't recall it under pressure.

Step 3 — Review every day, briefly

15–20 minutes daily beats 2 hours on a weekend. Your brain needs sleep between sessions to consolidate memory (consolidation happens during REM sleep). Skipping a day puts the schedule slightly off — fine, FSRS adapts. But skip a week and you'll find a mountain of "due" cards stacked up, and you'll quit out of overwhelm.

Step 4 — Rate honestly

When a card appears, always try to retrieve before flipping. Then rate honestly:

Again — you blanked, couldn't retrieve.
Hard — you got it, but it was a struggle.
Good — you retrieved it naturally.
Easy — instant, no effort.

Don't tap "Good" on a card that was actually a struggle — you're lying to the algorithm, and it'll punish you by stretching the next interval too far, leading to real forgetting.

When spaced repetition is NOT the right tool

To be fair, SRS isn't a silver bullet. It's weak for:

Motor skills (instrument, swimming, driving) — these need continuous repetition, not spacing.
Conceptual understanding (system design, philosophy) — reading, discussing, and synthesising work better.
Heavy theoretical lectures — read holistically first to know what's worth carding.

SRS shines on discrete factual knowledge that needs precise recall: vocabulary, dates, formulas, names, definitions.

Summary — where to start

If you're prepping for IELTS and consistently lose words after a few days, here's the first move:

Install the Mnemo Chrome extension or create a free Mnemo account.
When reading any English text (news, blogs, papers), highlight unfamiliar words → Save.
Each morning, spend 15 minutes reviewing due cards. Rate honestly.
After 2 weeks of consistency, you'll find: same study hours, the number of words you "actually know" jumps 3–5× compared to list-based study.

This isn't marketing — it's how millions of Anki users (and now Mnemo users with FSRS v5) operate. You don't need to be smarter; you just need the right method.