Figure 2.

Lock and key metaphor for maximum likelihood estimation. (A) Two dimensions of the probability space lie along the axes of aphasia severity (s) and heuristic acceptance probability (h). These axes are depicted here as columns and rows of locked boxes. (B) Actual scores on sentence comprehension tests by an aphasic individual are depicted here as teeth on a key. The key has varying degrees of fit with the pins inside the locks. The pins represent estimates of scores based on a given sentence complexity metric. (C) A binomial probability mass function (PMF) describing the probabilities of different scores on a given subtest of the aphasia evaluation. The pins inside the lock are actually PMFs. Given a list of actual scores, these PMFs can be used to calculate the likelihood of those scores. By testing a given key in every available box, the values of the s and h parameters that provide the best fit can be determined.