bbinghaus opened his chapter on forgetting with a sentence nobody could disagree with, and then did something nobody had done: measured it.

“All sorts of ideas, if left to themselves, are gradually forgotten. This fact is generally known.”
— Ebbinghaus, Memory, chapter 7, section 26 (English translation 1913)

He learned lists of nonsense syllables, waited, and relearned them, and counted the work saved. The result, for himself, for series of thirteen syllables:

“One hour after the end of the learning, the forgetting had already progressed so far that one half the amount of the original work had to be expended before the series could be reproduced again; after 8 hours the work to be made up amounted to two thirds of the first effort.”
— chapter 7

“After 24 hours about one third was always remembered; after 6 days about one fourth, and after a whole month fully one fifth of the first work persisted in effect.”
— the same section

The book is (Ebbinghaus, 1885); the English text is free at Classics in the History of Psychology, and chapter 7 is here.

a hundred and thirty years later

Murre & Dros (2015) repeated the experiment as closely as they could: one subject, about seventy hours of learning and relearning, at intervals from twenty minutes to thirty-one days. The curve came out much as Ebbinghaus's did, including a small upward jump at the one-day point [paraphrase, not a quotation]. See forgetting curve and savings.

What the room is for. Ebbinghaus's measure did not ask whether he could remember a list. It asked how much of the work was still there, which is why it could see memory that recall could not. The same distinction runs through the calibration and hallucination pages: what a system can produce on demand is not all it has stored.

[playful] If you read this page tomorrow, it should take about two thirds of the effort. Please let us know.

in the tour: the first laboratories have a placard called "Savings, and the curve" ···

Sources

1. Ebbinghaus (1885). Über das Gedächtnis. Leipzig: Duncker & Humblot. English translation by H. A. Ruger & C. E. Bussenius, Memory: A Contribution to Experimental Psychology, Teachers College, Columbia University, 1913; transcribed at Classics in the History of Psychology. psychclassics.yorku.ca/Ebbinghaus/index.htm
2. Murre & Dros (2015). Replication and Analysis of Ebbinghaus’ Forgetting Curve. PLOS ONE. doi:10.1371/journal.pone.0120644

see also