Editorialsauthors of journals give a word limit, which differs from journal to journal. It is useful to keep in mind that many databases, including Medline, cut abstracts off at 250 words.
Who is it for?Abstracts have different functions, depending on where and how they are read. When they appear as part of the paper, that is, with the full text of the article, readers use them to judge whether they are interested in reading further and also to be orientated. Abstracts are not always accompanied by the rest of the text, however. They appear widely on online databases, which are used not only by researchers and practitioners looking for articles to obtain but also by large numbers of people with an interest in public health in countries where the original article may not be available, and where abstracts serve as a substitute for the full article.To meet the requirements of all these different users, abstracts must be understandable without reference to the text. They must be informative rather than descriptive. They must accurately reflect the content of the article. Both for a reader in a country where a copy of the current issue of a scientific journal is almost unheard of and for the busy editor of the journal, phrases like "The results are discussed in the light of previous findings" stimulate feelings ranging from mild amusement to frustration. What were the results? What do they mean?Your abstract will probably be read by thousands of researchers and practitioners all over the world, many more people than will ever read the full text. The large majority (more than 80%) will not be native English speakers. What's more, the abstract will be translated into other languages − usually by translators who are not scientists. Does all this seem like a lot of responsibility for you to bear when writing an abstract? It's easy to face up to: Keep it simple! Write plain, grammatical English, using the correct technical terms but with no jargon (from a mediaeval French word meaning the warbling, twittering and chattering of birds, with the same root as 'gargle'; i.e. sounds that are meaningless). Avoid pompous sentences and don't use abbreviations unless you really have to.
When should it be written?Most people find it easiest to write the abstract when the rest of the paper has been fully constructed, each sentence agonisingly and lovingly rounded out. Then, they take the best of those sentences (they abstract, ab trahere, to pull out), distribute them artfully and sometimes come to an even more polished conclusion than they did in the paper. Perhaps, just as an exercise, it might be interesting to try at least once to write the abstract first. If you can write your abstract first, you will have done a lot of thinking before setting pen to paper or fingers to the keyboard. Once all that thinking has been done, the text of your whole article will be very much easier to write.