than two levels. Tetrachoric correlation assumes that the variable underlying each dichotomous measure is normally distributed. The coefficient provides "a convenient measure of [the Pearson product-moment] correlation when graduated measurements have been reduced to two categories." The tetrachoric correlation coefficient should not be confused with the Pearson correlation coefficient computed by assigning, say, values 0.0 and 1.0 to represent the two levels of each variable (which is mathematically equivalent to the φ coefficient). Lambda coefficient The lambda coefficient is a measure of the strength of association of the cross tabulations when the variables are measured at the nominal level. Values range from 0.0 (no association) to 1.0 (the maximum possible association). Asymmetric lambda measures the percentage improvement in predicting the dependent variable. Symmetric lambda measures the percentage improvement when prediction is done in both directions. Uncertainty coefficient The uncertainty coefficient, or Theil's U, is another measure for variables at the nominal level. Its values range from −1.0 (100% negative association, or perfect inversion) to +1.0 (100% positive association, or perfect agreement). A value of 0.0 indicates the absence of association. Also, the uncertainty coefficient is conditional and an asymmetrical measure of association, which can be expressed as . This asymmetrical property can lead to insights not as evident in symmetrical measures of association. Others Gamma test: No adjustment for either table size or ties. Kendall's tau: Adjustment for ties. Tau-b: Used for square tables. Tau-c: Used for rectangular tables. See also Confusion matrix Pivot table, in spreadsheet software, cross-tabulates sampling data with counts (contingency table) and/or sums. TPL Tables is a tool for generating and printing crosstabs. The iterative proportional fitting procedure essentially manipulates contingency tables to match altered joint distributions or marginal sums. The multivariate statistics in special multivariate discrete probability distributions. Some procedures used in this context can be used in dealing with contingency tables. OLAP cube, a modern multidimensional computing form of contingency tables Panel data, multidimensional data over time References Further reading Andersen, Erling B. 1980. Discrete Statistical Models with Social Science Applications. North Holland, 1980. External links On-line analysis of contingency tables: calculator with examples Interactive cross tabulation, chi-squared independent test, and tutorial Fisher and chi-squared calculator of 2 × 2 contingency table More Correlation Coefficients Nominal Association: Phi, Contingency Coefficient, Tschuprow's T, Cramer's V, Lambda, Uncertainty Coefficient, March 24, 2008, G. David Garson, North Carolina State University CustomInsight.com Cross Tabulation The POWERMUTT Project: IV.