Here is an source called ''A negative result not supportive of mediumship claims''
O'Keeffe, C. & Wiseman, R. (2005). Testing alleged mediumship: Methods and results. The British Journal of Psychology, 96(2), 165-179.
The experimental design is OK, the choice of mediums and sitters isn't...
'The five sitters (all male, age range 25–30) were either students or staff from the university. They were selected from a pool of individuals who responded to a generale-mail, circulated within the university, asking for volunteers to be involved in a scientific test of mediumship. The sitters were chosen using the following criteria; (a)they did not know one another, (b) they were the same gender, and (c) they were approximately the same age. '
Why choose such a group? Young male students or staff from a university sounds just about the most skeptical group possible for mediums to give readings to. Males are more skeptical sex, particularly younger male students (even more so if psychology students). Both O'Keefe and Wiseman will have been familiar with claims of sheep/ goat effects... this seems like adding potential bias IMHO.
(a) Young people are less likely to have close dead relatives
(b) Since this age group/sex are perhaps more likely to be cynical towards mediumship claims, any subsconsious bias against mediumship could lead to subconsciously marking statements poorly.
The sitters should have been a mix of males and females old enough to have close dead relatives. Ideally believers since then it rules out any subconscious bias to mark statements inaccurately.
And of course more care over selecting mediums rather than trusting the SNU, whose criteria over what makes a good medium isn't based upon accuracy but other criteria.
The O'Keefe/Wiseman's protocol is very much based on Dr D J Pratt's protocol. Pratt had significant results in favour of mediumship being real, he was careful to choose a good medium, Eileen Garrett, one of the most impressive and most lab tested mediums of the 1940s.