Rooted in the Evangelical world, but aiming global audience, Open Doors Ministries started in 1955 from a Dutch founder.
Its department World Watch Research was set up to carry out research into the persecution of Christians on a solid foundation that aims to withstand scholarly scrutiny and accepts academic guidance. WWR has been gathering and publishing detailed data on the persecution of Christians since 1992. One of their main tools for tracking and measuring the extent of persecution in the world is the World Watch List (WWL). WWL methodology has gradually evolved since the 1990s. In 2012, the methodology was comprehensively revised in order to provide greater credibility, transparency, objectivity and academic quality. In 2013 and 2016, further refinement of the methodology took place.
Here is an uptade of the methodology they use. Link.