Engel's Law expresses a negative stochastic association of the bivariate distribution of income and food share across a population. Among the many different definitions which can be found in the statistical literature four concepts are discussed and tested: Kendall's τ, quadrant dependence,...

This paper attempts to provide empirical evidence of the positive definiteness of the mean income effect matrix, a sufficient condition for market demand to satisfy the {\it law of demand} derived by H\"{a}rdle, Hildenbrand and Jerison [HHJ(1991)]. Increasing heterogeneity in spending of...