How do you decide what to include in a dictionary?
For print dictionaries, much depends on the type and size of the dictionary being compiled. In the smaller dictionaries the main criterion is how widely a word occurs: constraints on space mean that the usefulness of all the words included has to be very carefully considered. It's usually true to say that a lexicographer will want to include more words than is physically possible. In larger dictionaries, the aim is to be as comprehensive as possible, which means that we also include rare and specialist terms that are not often encountered in everyday English.
We never leave words out of dictionaries on the grounds that they aren't ‘good English’. Similarly, if a word is used only in very informal contexts, or only by specific groups of people, or if it is offensive in some way, we make this clear in the dictionary entry. (If a dictionary is written for children at the primary school level, however, some of the more ‘adult’ words will be excluded.) New words will be added if we have enough evidence of their use in print or online sources.
Increasingly, dictionaries are being offered in electronic form. This means that issues of space will become much less important and lexicographers will be able to include even more words, phrases, and meanings.