The word "periodical" refers to magazines, newspapers, and scholarly journals. Newspapers and popular magazines like People, or Road and Track, are familiar to everyone. Scholarly journals (also called academic, professional, or peer-reviewed journals), are written by experts for other experts. They are considered more authoritative than most other sources, because each article is written by experts and reviewed by a panel of experts from the same field before publication. In-depth research will usually require you to find scholarly journal articles on your subject.
| Popular magazines | Scholarly journals |
| Use everyday language | Use technical language |
| Written for the general reader | Written for experts by other experts |
| Do not include references to sources of information used to write the article | Include a list of references at the end of the article |
| Generally make only passing references to research, if at all | Often report on research and include descriptions of the methodology used and statistical analysis of the results (articles in the humanities, such as literature or religion, are excepted) |
| Articles are chosen and checked by editors | Articles must pass a review by experts in the same field before publication. |
Note: Scholarly journal articles are not always perfect. Every year many articles are withdrawn because of errors detected subsequent to publication, or because of ethical violations on the part of one of the authors.