It takes several years of study to master the basic knowledge shared by linguists. Beyond this shared knowledge, most academic linguists have a specialist knowledge of particular fields, such as grammar, phonology, or sociolinguistics. Under the Evidence Act 1995 (New South Wales) the field of expertise rule has been changed to allow evidence that derives from "specialized knowledge based on the person's training, study or experience." Based on this criterion, there is little doubt about the admissibility of evidence from linguists. The expertise that forms the basis of linguistic evidence should also be sufficient to meet any reasonable interpretation of the common knowledge rule of evidence, which still prevails in some jurisdictions. The types of information that linguistics can offer as evidence in court and in criminal investigations are in two main categories: issues of authorship (speaker identification), namely, whether a particular person said or wrote something; and problems of meaning and communication. These two main categories of linguistic evidence can involve analysis at a range of language levels, including pronunciation, grammar, vocabulary, discourse, and social variation in language (known as sociolinguistics). 21 notes