 Some have suggested, though, that these kinds of studies are flawed from the outset (see Brümmer 2008 and Davison 2017, chapter 5). It would be difficult to to ensure that some group of people is the subject of no petitionary prayers, for instance, since it is impossible to prevent people from praying for those whom they know. Also, God is typically assumed to be a free person, not a natural force that acts automatically in all similar cases, so we cannot assume that God will simply ignore those people for whom petitionary prayers have not been offered. This means that even if a study showed some statistically significant difference between the two groups of people, we could not be sure that it was due to the offering of petitionary prayers alone, as opposed to some other factor or factors. 