Some data on the shape of the forgetting curve

The forgetting curve is often schematically pictured like this, as on Wikipedia: Learners often take this to mean that their retention of a given fact will, over time and on average, tend to look something like that. So, for example, the Wikipedia entry on Ebbinghaus glosses it as "describ[ing] the exponential loss of information that one has learned." But: Ebbinghaus's original forgetting curve is defined in terms of "savings," a metric we tend not to use: it describes how long it takes to learn something after studying it relative to the time it takes initially to learn something. Ebbinghaus's 1885 formula is b = 100k / ((log t)^c + k), which involves an exponent and decays but is not a literal exponential curve. I've never found strong evidence that my own forgetting curves--here defined as I think it's generally understood, in terms of the probability of my getting a flashcard correct over time--are exponentially distributed. Here's my performance (LOWESS-smoothed) on my fourth…