My mother, who was a tenacious historian, used to ask, when she was involved in some particularly difficult historical research: “If I were that piece of paper where would I be?” She practiced a kind of method-historical research, in which she thought about the process through which the information might have been written, collected, and stored in order to determine where the information might have ended up. As a result she found documents that no one else could find, and tracked down answers to questions that other people thought could not be answered.
Many people think that all it takes to researching and writing about a historical topic is to go into a library or archives and find the books and folders of information that are labeled conveniently with the topic of your research. So, for example, if you want to study the use of bathtubs in early Minnesota or somewhere you just go into the library and say: “Show me everything you have on bathtubs,” and the librarian will be able to pull out a folder, or a few books that give you everything you need on the topic.
Most of the time that is not how it works. Most of the information is not collected for you in advance. You have to have think about who might have been interested in writing down the information you want or collecting it later. Perhaps there are a few published reminiscences in which someone wrote about using a bathtub in Minnesota in 1849. Letters or diaries written by some of the first settlers may mention taking baths in bathtubs. Some 19th century county histories might mention bathtubs, too.
But one thing you have to keep in mind is that there may be mentions of bathtubs in early sources, but there is no guarantee that someone who indexed a book or inventoried a collection of manuscripts thought that bathtubs were something important enough to put in the index or inventory. If someone did write about bathtubs in an early county history, the words bath or tub or bathing, might still not be present in the index to the book. The whole idea of bathing might have seemed so mundane, compared to the more important topics of the first schools, the first churches, and the “first white child” born in a small town, that the publisher of the book would simply have left it out of the index, even though it was referred to in passing in the book.
This is why when I see that someone is planning a “literature search” on a particular historical topic, I am skeptical. Doing a comprehensive survey of all the available published and unpublished sources on a particular topic is certainly important. But it is only the basic first step in any kind of historical research. Even with the increasing amount of published texts available online with full text-search capability, there remain a lot of books and manuscripts that are not online and that are imperfectly cataloged or indexed. In the 1980s when I was writing about the common practice of gift-giving in the fur trade and among Indian people, I found out quickly that the terms “gift” or “present” were seldom indexed. Furthermore, people writing about gift-giving sometimes wrote about it without even using the word gift. You had read through fur-trade journals, account books, and reminiscences page-by-page to find the information that was there but not listed in the indexes.
Historical research on any topic requires a thorough knowledge of the available sources and the ability to make sense of the information you find in them. Someone who states that he or she has done a comprehensive literature search may simply have skimmed the surface, without seeing the most important or significant information buried below.