When several pretreatment periods are available, identification of the treatment effect in a difference-in-differences framework requires an assumption relating dynamics for controls and treated in absence of treatment. Mora and Reggio (2012, Working Paper 12-33, Universidad Carlos III de Madrid) define a family of alternative identifying assumptions and propose a model that, contrary to the usual econometric specifications, allows one to identify the treatment effect for any given assumption in the family. In this article, we introduce a command, didq, that implements the model presented in Mora and Reggio, reports the estimated effect under alternative assumptions, and performs tests for the equivalence of the estimates. We also explain how to use the command to obtain the standard difference-in-differences estimator with or without polynomial trends.
This study looks at international competitiveness of agriculture in the European Union and the United States. The most intuitive concept is that of price competitiveness. We calculate relative prices for 11 member states of the European Union and the United States for the period 1973-2002. We assume that markets are perfectly competitive and in long-run equilibrium, so that the observed price always equals average total cost, as measured by the cost dual to the production function. This assumption is used in our calculation of relative competitiveness and productivity gaps between the European Union and the United States and in our decomposition of relative price movements between changes in relative input prices and changes in relative productivity levels.
JEL classifications: Q16, Q17
Recent research (Reardon and Firebaugh, 2002, Frankel and Volij, 2009, and Mora and RuizCastillo, 2009a has shown that two entropy-based segregation indices possess an appealing mixture of basic and subsidiary but useful properties. It would appear that the only fundamental difference between the mutual information, or M index, and the Entropy, Information or H index, is that the second is a normalized version of the first. This paper introduces another normalized index in that family, the H* index that, contrary to what is often asserted in the literature, is the normalized entropy index that captures the notion of segregation as departures from evenness. More importantly, this paper shows that applied researchers do better using the M index than using either H or H* in two circumstances: (i) if they are interested in the decomposability of segregation measures for any partition of organizational units into larger clusters and of demographic groups into supergroups, and (ii) if they are interested in the invariance properties of segregation measures to changes in the marginal distributions by demographic groups and by organizational units.
In the context of educational segregation by ethnic group, it has been argued that rigorous pair wise segregation comparisons over time or across space should be invariant in two situations:when the ethnic composition of the population changes while the distribution of each ethnic group over the schools remains constant (invariance 1), or when the size distribution of schools changes while the ethnic composition of each school remains constant (invariance 2). This paper makes three contributions to this literature. First, it presents a testing strategy for choosing between the two properties. Second, it argues that both properties have strong implications, and that there are reasons to defend that the overall segregation index need not satisfy either one. In particular, the contrast between invariant segregation indices and the Mutual Information segregation index that violates both properties is illustrated with a number of examples. Third, nevertheless, it is shown that pair wise segregation comparisons using this index can be expressed in terms of (i) changes in the ethnic composition of the population, (ii) changes in the school size distribution, and (iii) changes in a third term that is invariant 1 or invariant 2. These decompositions can be used to reach the analogous ones obtained in Deutsch et al. (2006).
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