I would sugeest getting DOI for the following reasons:
A DOI is a unique alphanumeric string that provides a persistent link
to content online. We should include a DOI for every reference.
(DOIs are actually quite prevalent—publishers who use them assign one
to all their online content, even if it was published pre-Internet.)
- CrossRef.org makes DOIs easy to find. The free DOI lookup finds DOIs
one at a time. Or if you cut and paste your reference list into the
Simple Text Query form, CrossRef will find all the available DOIs at
once.
- If you publish an article using ICPSR ( Inter-university Consortium for Political and Social Research ) data and you include the DOI in the data citation as one of your references, you make it easy for other researchers to get back to the original data.
However, for online material without DOIs, include URLs or database