Tertiary sources consist of information which is a collection of primary and secondary sources. Tertiary sources are good starting points for research projects because they often distill large amounts of information.
Even more difficult in discerning the difference between a primary and secondary source is reviewing tertiary sources. Some writers don't make the distinction between tertiary and secondary because both types of materials do not represent original works (primary sources). However, for the purposes of reviewing the literature, it is important to understand how tertiary sources can contribute to your overall search for relevant information for your paper.
Reviewing tertiary source material can be of value in improving your overall research paper because they:
Often compile factual information in one place and to search for the data in multiple sources takes time (e.g., searching for names of heads of state in an almanac),
Lead the reader to additional sources. For example, rather than citing in your literature review a long list of additional sources on a topic, you can simply cite a comprehensive bibliography compiled by another researcher,
Distil large quantities of closely related information or data (e.g., a statistical compendium),
Often contain references and bibliographies that can point you to key primary and secondary sources.
Examples of tertiary sources you could review as part of your overall study include:
Tertiary sources also include user-contributed online resources such as Wikipedia.