**Critical Ratio (CR)**: A mathematical quantity used to decide whether the size of the observed deviation from chance in a psi test is significantly greater than the expected degree of random fluctuation about the average; it is obtained by dividing the observed deviation by the standard deviation; also called the z statistic.

**References**

- Parapsychology by Michael A. Thalbourne, Republished by Puente Publications, Charlottesville, VA, USA, 2003