Deciding what constitutes a hate or extremist website is a difficult and subjective task. To develop our list, we sought counsel from the leading organizations that track extremism in the United States — The Southern Poverty Law Center and the Anti-Defamation League.

We relied in part on the 2016 list of “Active Hate Groups” published by the SPLC in February. It tracks ten types of hate groups: Ku Klux Klan, Neo-Nazi, White Nationalist, Racist Skinhead, Christian Identity, Neo-Confederate, Black Separatist, Anti-LGBT, Anti-Muslim and General Hate.

The SPLC list is controversial in some circles, but the group does provide detailed public explanations for many of its designations. For instance, the SPLC documented its decision to include the Family Research Council, an evangelical lobbying group, on its list by citing the group’s reliance on discredited science and unsubstantiated attacks on gay and lesbian people.

We supplemented the SPLC list with a list of top extremist websites provided to us by the Anti-Defamation League. The ADL does not publish this list and supplied it to us for research purposes.

We located websites associated with the SPLC hate groups and combined it with the ADL’s list of hate sites. We then compared the combined SPLC/ADL lists with the Alexa’s Top Million websites and filtered our list to use only websites that had enough traffic to appear in the top million sites worldwide.

We then wrote software to automatically browse to each website and collect a list of external domains contacted by each website. In the wake of Charlottesville, some popular white nationalist websites, such as The Daily Stormer, were shut down and we removed them from our list. Others, such as Richard Spencer’s National Policy Initiative, were shut down after we finished collecting data and so we included them in our results.

In order to identify which domains loaded advertisements or provided payment forms for the hate sites, and to eliminate domains that only provided basic functionality, we checked the external domains we found on those sites against the AdBlock Easylist. This crowdsourced list is used by ad blocking software to hide ads when users are browsing the internet.

However, just because a website contacts a third party ad network or payment network, it does not always mean an ad will appear on a webpage or a donation form will accept payments. In order to verify our results, we visited every website and clicked on the payment links to determine if a working credit card form was loaded, and we visually inspected each site to ensure that the ad networks were actually delivering ads when we loaded the page.

We then contacted all the websites and the tech companies and asked them to verify whether our results were correct. In some cases, such as Google Custom Search, the company clarified for us that although some websites were using the technology, none were being paid for its use.

Even so, our study has several limitations. For example, there are most likely many more payment processors and advertising connections to each site. We only are displaying those that we felt confident we could verify. In addition, either in public statements or interviews with ProPublica, many of the sites have disputed the characterization of them by the SPLC or ADL as extremist.