The contrary "STEP" study had a religious cult that didn't believe in intercessory prayer as it's prayer group. That group ("The Unity School of Christinity") had a Christian sounding name but it was anything but Christian.
Ron Rhodes, who has a doctorate in systematic theology and who has authored some sixty theological books, noted, "The Unity School of Christianity is definitely not Christian." Probe, a respected Christian journal, calls Unity "a classic new age cult that is not Christian in any aspect of its doctrines or teachings." Even the co-founder of the cult, Charles Fillmore, once wrote, "God never performs miracles."
So, as Dr. Brown (see the OP) noted, the studies are different because (the STEP study) "has a different inclusion” criteria. She also stated, the STEP study "is instructional on how NOT to conduct a study of Christian prayer."