A typical measure of psi phenomena is statistical deviation from chance expectation. However, critics point out that statistical deviation is, strictly speaking, only evidence of a statistical anomaly, and the cause of the deviation is not known. Hyman contends that even if psi experiments could be designed that would regularly reproduce similar deviations from chance, they would not necessarily prove psychic functioning. Critics have coined the term "The Psi Assumption" to describe "the assumption that any significant departure from the laws of chance in a test of psychic ability is evidence that something anomalous or paranormal has occurred...other words assuming what they should be proving." These critics hold that concluding the existence of psychic phenomena based on chance deviation in inadequately designed experiments is "affirming the consequent" or "begging the question."