Editing Contingency table (section)

===Phi coefficient===
{{main|Phi coefficient}}
A simple measure, applicable only to the case of 2&nbsp;×&nbsp;2 contingency tables, is the [[phi coefficient]] (φ) defined by

: <math>\phi=\pm\sqrt{\frac{\chi^2}{N}},</math>

where {{math|χ<sup>2</sup>}} is computed as in [[Pearson's chi-squared test]], and ''N'' is the grand total of observations. φ varies from 0 (corresponding to no association between the variables) to 1 or −1 (complete association or complete inverse association), provided it is based on frequency data represented in 2&nbsp;×&nbsp;2 tables. Then its sign equals the sign of the product of the [[main diagonal]] elements of the table minus the product of the off–diagonal elements. φ takes on the minimum value &minus;1.0 or the maximum value of +1.0 [[if and only if]] every marginal proportion is equal to 0.5 (and two diagonal cells are empty).<ref>Ferguson, G. A. (1966). ''Statistical analysis in psychology and education''. New York: McGraw–Hill.</ref>