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All good dictionaries operate an inclusion policy, which provides guidance on which words, meanings or expressions get into the dictionary. The best basis for this is a user profile – a description of the kinds of people you expect to use your dictionary, and an analysis of their reference needs. In some cases, creating a user-profile is straightforward. If you are writing an "advanced dictionary of oil and gas exploration", it will be aimed at professional and academic workers in this highly specialized area, and what they need is explanations of the terminology of their field (terms which most laypeople don't know, and don't need to know). At the other end of the spectrum, where you find a general English dictionary, the user-group is more varied, so creating a user profile is more difficult.
The Macmillan Dictionary falls between these two extremes. As a learner's dictionary, it has a reasonably clear user-group. It is aimed primarily at people involved in the world of English language teaching – whether as students, teachers, trainers, or researchers. This is a diverse and international group of people, who nevertheless have certain requirements in common. This user profile informs our editorial policy across the board. For example, definitions are written in simpler, less technical language than you would find in a general English dictionary. We also provide information about how words combine with one another (syntactically or collocationally) – something which is not generally included in dictionaries aimed at mother-tongue speakers of English (because those users don't need it).
The user profile also underpins our decisions about what to include in the dictionary. When the dictionary existed in printed form, there was an additional limiting factor: we couldn't include everything we wanted to, because we had only a finite amount of space. With that constraint lifted, we can now focus on what we really need to include in order to supply the reference information that our users need. In this new situation (where space is unlimited) our first question, when evaluating candidates for inclusion in the Macmillan Dictionary, is: "is this likely to be useful to the kind of people who regularly consult our dictionary?" Before the Macmillan Dictionary went online in 2009, it already provided good coverage of the core vocabulary of contemporary English. Since then, our regular updates have added thousands of words to the dictionary. Some are neologisms – words and phrases that began to be used only recently (in 2009, for example, the extensive vocabulary of social networking – words like retweet and unfriend – barely existed). Others are established words that we didn't have room for before, especially those from fields of particular relevance to our users: IT, business, linguistics, and so on.
Frequency of use is also an important criterion: a strong candidate will not only show up frequently in corpus resources, but the evidence will indicate that it is used in a wide range of text-types. Conversely, anything which we judge, on the basis of corpus evidence, to be parochial (limited to a small user-group) or ephemeral (unlikely to be around for very long) will not be accepted. It isn't always easy to make these judgements (despite the excellent linguistic evidence we now have access to) so we will occasionally make the wrong call. But if you come across a word which isn't yet covered in the Macmillan Dictionary but you think it should be, the best way to let us know is to submit it to our crowd-sourced Open Dictionary. When we are preparing updates to the main dictionary, the Open Dictionary is the first place we look for potential new entries.
Michael Rundell, Editor-in-Chief of Macmillan Dictionaries
Michael frequently writes about dictionaries and the English language on Macmillan Dictionary Blog. Here are some of his posts on the topic of what is included in a dictionary and why: