Last Updated: Jul 23, 2018 Views: 62
Not every article will have a DOI. DOI (Digital Object Identifier) or other unique identifiers assigned to articles have only come into use relatively recently. The usage of DOIs have only come into wide spread use within the last 5 to 10 years. Older articles may or may not have a DOI, The older the article is the less likely it will have a DOI. The more recent the article the more likely it will have a DOI.
Some databases such as PubMed now include the DOI in the citation to the article. There is also a free DOI lookup tool: