What if an article has no DOI?

Related questions

1. What is PMID number?
2. Where do I find DOI numbers?
3. Does APA Style require DOI?
4. What format is DOI?
5. What does PMID mean?
6. How do I find a DOI?
7. What is the difference between PMID and Pmcid?
8. What if there is no DOI?
9. What is the PubMed unique identifier PMID?
10. How do you find the PMID of an article?

Tags

1. #whether
2. #uncertain
3. #assigned
4. #article
5. #there
6. #direct
7. #entry
8. #electronic
9. #means
10. #often
11. #document
12. #reference
13. #number
14. #given
15. #online
16. #searching
17. #homepage
18. #journal