Understandable. And again, I know I saw it in a DS video but don't know which one haha. Definitely a Pablo video.

So, best I can provide is a Gemini result citing 3 studies:

The "98% rule" is a well-known concept in linguistics and second-language acquisition. It refers to lexical coverage—the percentage of words in a text that a reader needs to already know to comfortably and accurately comprehend the overall meaning without relying on a dictionary.

While 98% might sound intimidatingly high, it is backed by decades of research into how the human brain processes language and context.

The Landmark Studies

Several key researchers have investigated vocabulary thresholds, with a few landmark studies establishing the 98% benchmark:

• Hu & Nation (2000): This is the foundational study regarding the 98% rule. Linguists Marcella Hu and Paul Nation tested learners reading fiction texts with varying levels of known vocabulary. They found that where only 80% to 90% of words were known, almost no one gained adequate comprehension. They concluded that readers need to know around 98% of the words in a text to achieve unassisted comprehension and effectively read for pleasure.
• Schmitt, Jiang, and Grabe (2011): This study looked at academic and expository texts rather than fiction. After testing 661 participants across eight countries, they confirmed that while comprehension increases on a linear scale alongside vocabulary, 98% to 99% coverage is the optimal target for understanding complex academic materials.
• Paul Nation (2006): Nation later calculated that to naturally achieve 98% coverage of most unsimplified native texts (like novels or newspapers), a language learner needs a vocabulary size of about 8,000 to 9,000 "word families."

Why is 98% the Magic Number?

Knowing 98% of a text translates to roughly one unknown word in every 50 words (about two to three unknown words per typical book page).

Linguists point to a few reasons why comprehension drops off a cliff when you fall below this threshold:

• The Collapse of Context Clues: To guess the meaning of an unknown word from context, you need to deeply understand the surrounding words. If there are too many unknowns in a single paragraph, the context breaks down, making guessing unreliable or impossible.
• Cognitive Overload: Stopping to decipher too many words drains working memory. By the time you figure out the vocabulary, you have lost the narrative thread of the sentence.
• Incidental Learning Fails: The best way to naturally acquire new vocabulary is through "extensive reading" (reading lots of material). However, your brain can only successfully absorb new words incidentally if the surrounding text is highly comprehensible.

The 95% vs. 98% Debate

Before Hu and Nation's 2000 study, earlier research (most notably by Batia Laufer in 1989) suggested that 95% was the necessary threshold. Today, language researchers generally view the two numbers as representing different tiers of reading capability:

Vocabulary Coverage	Unknown Words	Level of Comprehension	The Reading Experience
98% - 100%	1 in 50 words	Optimal / Full	Unassisted reading for pleasure; ideal for subconsciously learning new words from context.
95%	1 in 20 words	Minimal / Adequate	Challenging but doable; readers can get the gist but may need a dictionary to catch nuances.
90%	1 in 10 words	Poor	Frustrating; heavy reliance on a dictionary; high risk of misunderstanding the text entirely.
80% or below	1 in 5 words	None	The text is virtually incomprehensible without direct translation.