I am currently conducting research which seeks to establish that Charismatic Christianity, which started with Charles Parham in the early 20th century, has had the greatest influence on the church in America up into today.
One possible piece of evidence for my case rests on the total population of Charismatic Christians in America. This number is difficult to discern though for a number of couple of reasons:
Firstly, although there are denominations which represent Charismatic beliefs such as Assemblies of God, United Pentecostal Church International etc., these populations are not comprehensive in examining the totality of Charismatic influence; many non-denominational churches hold beliefs similar to Pentecostals for example.
Secondly, virtually all Protestant denominations & Catholicism have some influence of Charismatic Christianity in them today. The measure of this influence is difficult to quantify when determining how many Charismatic Christians are in actually in the population of Christians in America.
Given these difficulties listed, I am curious if any scholarly research group has conducted research to best piece together an estimate which best identifies the population of those which subscribe to the theology first promulgated by Charles Parham in the early 1900's.