Overton indexes more than 4.7M documents from more than a thousand different policy sources, making it many times larger than similar systems.
For example in August 2021 the policy component of Dimensions/Altmetric contained ~ 2.6M citations of DOIs in policy documents from 234 sources, while Overton contained ~ 10.8M citations of DOIs from 1,504 sources.
Overton was designed specifically for working with policy rather tracking non-scholarly attention more broadly. The positive side of this trade-off is that we're able to quickly add and process new policy sources and to do a better job of matching references in free text.
It's important to note that a "policy source" in this context is a website or domain from which we are collecting documents.
Usually a website includes documents from just one organization, but this varies from country to country.
For example, in the UK a single policy source - gov.uk - hosts documents from all of the government's departments as well as many government agencies.
Conversely in Australia each government department hosts its own documents, so each one is tracked as a separate policy source.
Policy to policy citations
Uniquely Overton also tracks citations within the policy literature rather than just from policy documents to scholarly articles. There are ~ 3.2M policy to policy citations in the database. They are kept in a separate index, but are searchable alongside the ~ 10.8M policy to DOI citations.
This is a core part of our business and is how we're able to work with government agencies, think tanks and NGOs who don't always publish in academic books and journals. Our customers often have a mix of output types: for example think tanks and universities may be publishing both scholarly work and policy briefs or reports.
Looking at coverage in different countries
You can use the number of sources in each country as a very broad indicator of how good coverage is, but if you're focused on a specific geographical area then the best approach is probably to log in and use the Sources page to see directly what is and isn't included, and to use country or region filters on the Policy page to get a count for matching documents.
If you're looking more broadly then useful indicators might be the number of policy documents indexed (if you're logged in you can see the current total on the Policy page) potentially in conjunction with the proportion of policy documents that have at least one reference or citation (approx 25%).
Comparing coverage to other systems
The points above hold true when comparing Overton's index to other systems. You may also want to check subject by subject: the largest policy sources typically cover health and economics and any differences between systems will be more pronounced in areas primarily covered by smaller sources in the "long tail" like education, social services, defense or environmental protection.
Because Overton captures more of these sources it does particularly well when looking at journals or institutions that produce more social sciences outputs.