<P> Indexes, bibliographies, concordances, and databases may not provide much textual information, but as aggregates of primary and secondary sources, they are often considered tertiary sources . </P> <P> Almanacs, travel guides, field guides, and timelines are also examples of tertiary sources . </P> <P> Survey or overview articles are usually tertiary, though review articles in peer - reviewed academic journals are secondary (not be confused with film, book, etc. reviews, which are primary - source opinions). </P> <P> Some usually primary sources, such as user guides and manuals, are secondary or tertiary (depending on the nature of the material) when written by third parties . </P>

Advantages and disadvantages of tertiary sources of data