A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Sketch a brief introduction to each section and subsection
Planning a first draft
Part I. Research and writing: from planning to production

Just as your whole report needs an introduction that frames what follows, so does each of its sections. If a section is only a page or two, you need just a short paragraph; for a section several pages long, you might need to sketch in two or more paragraphs. This opening segment should introduce the key terms that are special to its section, ideally in a sentence at its end expressing its point. That point might be a reason, a response to a different point of view, or a warrant you must explain. In a section that you think will be longer than five pages or so, you might state its point both at the end of its introduction and again in a conclusion.