From the Parapsychology Foundation:
Dr. Emily W. Kelly describes this presentation in her own words when she says "Mediumship holds a uniquely important place in psychical research, in that it is the only phenomenon that combines elements of spontaneous case studies, field studies, survival research, and experimental method. For the most part, mediumship develops spontaneously in a few gifted individuals, it must be studied more or less in its natural setting, and yet it is the only phenomenon directly relevant to survival research that can be produced and observed under conditions of experimental control. Mediumship therefore combines the significant emotional and psychological circumstances that often produce the strong psi effects seen in spontaneous experiences with the ability to control the conditions and thus reduce the uncertainties that too often accompany spontaneous cases about the possibility of normal sources of information."
Dr. Emily W. Kelly describes this presentation in her own words when she says "Mediumship holds a uniquely important place in psychical research, in that it is the only phenomenon that combines elements of spontaneous case studies, field studies, survival research, and experimental method. For the most part, mediumship develops spontaneously in a few gifted individuals, it must be studied more or less in its natural setting, and yet it is the only phenomenon directly relevant to survival research that can be produced and observed under conditions of experimental control. Mediumship therefore combines the significant emotional and psychological circumstances that often produce the strong psi effects seen in spontaneous experiences with the ability to control the conditions and thus reduce the uncertainties that too often accompany spontaneous cases about the possibility of normal sources of information."