memory. After the learning phase, subjects are told that the letter strings presented during the learning phase were based on specific rules, but are not explicitly told what the rules are. During a test phase, the subjects are instructed to categorize new letter strings as "ruleful" or "unruleful". The dependent variable usually measured is the percentage of correctly categorized strings. Implicit learning is considered to be successful when the percentage of correctly sorted strings is significantly higher than chance level. If this significant difference is found, it indicates the existence of a learning process that is more involved than memorizing the presented letter strings. Bayesian learning The mechanism behind the implicit learning that is hypothesized to occur while people engage in artificial grammar learning is statistical learning or, more specifically, Bayesian learning. Bayesian learning takes into account types of biases or "prior probability distributions" individuals have that contribute to the outcome of implicit learning tasks. These biases can be thought of as a probability distribution that contains the probability that each possible hypothesis is likely to be correct. Due to the structure of the Bayesian model, the inferences output by the model are in the form of a probability distribution rather than a single most probable event. This output distribution is a "posterior probability distribution". The posterior probability of each hypothesis in the original distribution is the probability of the hypothesis being true given the data and the probability of data given the hypothesis is true. This Bayesian model for learning is fundamental for understanding the pattern detection process involved in implicit learning and, therefore, the mechanisms that underlie the acquisition of artificial grammar learning rules. It is hypothesized that the implicit learning of grammar involves predicting co-occurrences of certain words in a certain order. For example, "the dog chased the ball" is a sentence that can be learned as grammatically correct on an implicit level due to the high co-occurrence of "chase" being one of the words to follow "dog". A sentence like "the dog cat the ball" is implicitly recognized as grammatically incorrect due to the lack of utterances that contain those words paired in that specific order. This process is important for teasing apart thematic roles and parts of speech in grammatical processing (see grammar). While the labeling of the thematic roles and parts of speech is explicit, the identification of words and parts of speech is