# Why Empirical Forgetting Curves Deviate from Actual Forgetting Rates: A Distribution Model of Forgetting

## TL;DR

This paper explains why forgetting curves in memory research don't match actual forgetting rates, using a model based on memory strength distribution.

## Contribution

The paper introduces a distribution model of memory that clarifies the discrepancy between empirical forgetting curves and item forgetting rates.

## Key findings

- Forgetting curves are shaped by the distribution of memory strengths relative to a recall threshold.
- The model predicts linear or concave forgetting curves when percent correct is high, supported by experimental evidence.
- Memories just above the recall threshold help short-term performance but do not form lasting memories.

## Abstract

For over a century, forgetting research has shown that recall decreases along a power or exponential function over time. It is tempting to assume that empirical forgetting curves are equivalent to the rate at which individual memories are forgotten. This assumption would be erroneous, because forgetting curves are influenced by an often-neglected factor: the distribution of memory strengths relative to a recall threshold. For example, if memories with normally distributed initial strengths were forgotten at a linear rate, percent correct would not be linear, it would decrease rapidly when the peak of the distribution was crossing the recall threshold and slowly when one of the tails was crossing the threshold. We describe a distribution model of memory that explains the divergence between forgetting curves and item forgetting rates. The model predicts that forgetting curves can be approximately linear (or even concave, like the right side of a frown) when percent correct is high. This prediction was supported by previous evidence and an experiment where participants learned word pairs to a criterion. Beyond its theoretical implications, the distribution model also has implications for education: Creating memories that are just above the threshold helps on short-term tests but does not form lasting memories.