LAND LEASE MARKETS AND AGRICULTURAL EFFICIENCY: THEORY AND EVIDENCE FROM ETHIOPIA

This paper develops a theoretical model of land leasing that includes transaction costs of enforcing labor effort, risk pooling motives and non-tradable capital inputs. We test the implications of this model compared to those of the "Marshallian" (unenforceable labor effort) and "New School" (costlessly enforceable effort) perspectives using data collected from four villages in Ethiopia. We find that land lease markets operate relatively efficiently in the villages studied, supporting the New School perspective. Land contract choice is found to depend upon the social relationships between landlords and tenants, but differences in contracts are not associated with significant differences in input use or output value per hectare. We find that other household and village characteristics do affect input use and output value, suggesting imperfections in other factor markets.Land Economics/Use,

Higher Moment Estimators for Linear Regression Models With Errors in the Variables

This paper proposes instrumental variable estimators for multiple linear regression models with errors in the explanatory variables, that require no extraneous information. As is very well known, the ordinary least squares estimator (OLS), which is based on the sample moments of order two, is unbiased when there are no errors in the variables, but it becomes biased and inconsistent when there are such errors [Fuller (1987)]. In contrast, the suggested estimators are based on higher sample moments and can be considered as a special type of instrumental variable estimator. They are consistent, under quite reasonable assumptions, when there are measurement errors. While most consistent estimators based on higher moments (HM) proposed previously in the literature [Geary (1942), Drion (1951), Durbin (1954), Pal (1980)] for regressions with errors in the variables seem to be quite erratic [Kendall and Stuart (1963), Malinvaud (1978)], the suggested estimators appear to perform remarkably well in many situations. Although most data do contain errors of measurement, this fact is often ignored by the analysts and statistical procedures designed for data measured without error are applied. It is shown that ignoring the presence of even small measurement errors and using traditonal OLS estimators may lead to performing standard Student t-tests with type I errors of considerably higher sizes than intended, while this is not so with the proposed HM estimators. Our experimental findings suggest also that even if the sample is not very large, when the errors in the variables are non-negligible, our estimators do perform better than the OLS estimators in terms of root mean squared errors, when the explanatory variables are strongly correlated and the multiple correlation of the regression is high. Such situations are typical of many statistical analyses based on aggregate data. When the multiple correlation coefficient is smaller and the explanatory variables are less correlated, our HM estimators will still outperform the OLS estimator if the sample is large, even if the measurement errors are not very important. Such cases are frequently encountered in analyses of survey data. Tests for the presence of errors in the variables are also described, and the power of the tests are assessed in the Monte Carlo experiments. Nous proposons, pour les modèles de régression linéaire où les variables explicatives contiennent des erreurs de mesure, des estimateurs de variables instrumentales d'un type particulier, qui n'exigent aucune information extrinsèque. On sait que l'estimateur des moindres carrés ordinaires (MCO), qui est basé sur les moments échantillonnaux d'ordre deux, est centré lorsqu'il n'y a pas d'erreurs sur les variables0501s qu'il devient biaisé et non convergent en présence de telles erreurs [Fuller (1987)]. Par ailleurs, les estimateurs que nous suggérons sont basés sur des moments d'ordres supérieurs et peuvent être vus comme des estimateurs de variables instrumentales. Sous des hypothèses très raisonnables, ces estimateurs demeurent convergents même lorsqu'il y a des erreurs de mesure. Alors que la plupart des estimateurs convergents basés sur des moments d'ordres supérieurs (MOS) proposés antérieurement [Geary(1942), Drion(1951), Durbin (1954), Pal (1980)] pour les modèles de régression avec erreurs sur les variables, semblent très erratiques [Kendall et Stuart (1963), Malinvaud (1978)], les estimateurs que nous proposons se comportent remarquablement bien, dans un grand nombre de cas. Quoique la plupart des données contiennent des erreurs de mesure, ce fait est souvent ignoré par les analystes qui appliquent, la plupart du temps, des procédures statistiques conçues pour le traitement de données mesurées sans erreur. Nous démontrons que le fait de négliger la présence d'erreurs de mesure même relativement faibles et d'utiliser les estimateurs MCO traditionnels, peut faire en sorte que les tests de Student standards comportent des erreurs de type I dont le niveau est considérablement plus élevé que le niveau désiré, alors que ce n'est pas le cas si on utilise les estimateurs MOS proposés. Même si les échantillons ne sont pas très grands, les résultats de nos expériences suggèrent également que dans les cas où les erreurs sur les variables ne sont pas négligeables, le comportement de nos estimateurs lorsqu'on l'évalue en termes de la racine carrée des écarts quadratiques moyens, est supérieur à celui des MCO, quand les variables explicatives sont fortement corrélées et que le coefficient de corrélation multiple est élevé. Ce genre de situations est typique des analyses statistiques basées sur des données agrégées. Si le coefficient de corrélation multiple est moins élevé et que les variables explicatives sont moins corrélées, nos estimateurs MOS peuvent encore s'avérer supérieurs aux estimateurs MCO lorsque les échantillons sont suffisamment grands, et cela même si les erreurs de mesure ne sont pas aussi importantes. De tels cas se rencontrent fréquemment lorsqu'on a affaire à des données d'enquêtes. Nous décrivons également des tests d'erreurs sur les variables et nous évaluons la puissance de ces tests au moyen d'expériences de Monte-Carlo.Errors in the variables; Measurement errors; Higher moment estimators; Instrumental variable estimators, Erreurs sur les variables ; Erreurs de mesure ; Variables instrumentales ; Moments d'ordres supérieurs

Land lease markets and agricultural efficiency: theory and evidence from Ethiopia

This paper develops a theoretical model of land leasing that includes transaction costs of enforcing labor effort, risk pooling motives and non-tradable productive inputs. We test the implications of this model compared to those of the “Marshallian” (unenforceable labor effort) and “New School” (costlessly enforceable effort) perspectives using data collected from four villages in Ethiopia. We find that land lease markets operate relatively efficiently in the villages studied, supporting the New School perspective relative to the other two models. Land contract choice is found to depend upon the social relationships between landlords and tenants, but differences in contracts are not associated with significant differences in input use or output value per hectare. We find that other household and village characteristics do affect input use and output value, suggesting imperfections in other factor markets. These results imply that interventions to improve the functioning of land lease markets are likely to be of little benefit for agricultural efficiency in the villages studied, whereas improvements in other factor markets may be more beneficial.Land use Ethiopia., Agriculture Economic aspects Egypt.,

Cobordism obstructions to independent vector fields

We define an invariant for the existence of r pointwise linearly independent sections in the tangent bundle of a closed manifold. For low values of r, explicit computations of the homotopy groups of certain Thom spectra combined with classical obstruction theory identifies this invariant as the top obstruction to the existence of the desired sections. In particular, this shows that the top obstruction is an invariant of the underlying manifold in these cases, which is not true in general. The invariant is related to cobordism theory and this gives rise to an identification of the invariant in terms of well-known invariants. As a corollary to the computations, we can also compute low-dimensional homotopy groups of the Thom spectra studied by Galatius, Tillmann, Madsen, and Weiss.Comment: 46 page

Revisiting The First Galaxies: The effects of Population III stars on their host galaxies

We revisit the formation and evolution of the first galaxies using new hydrodynamic cosmological simulations with the ART code. Our simulations feature a recently developed model for H2 formation and dissociation, and a star formation recipe that is based on molecular rather than atomic gas. Here, we develop and implement a recipe for the formation of metal-free Population III stars in galaxy-scale simulations that resolve primordial clouds with sufficiently high density. We base our recipe on the results of prior zoom-in simulations that resolved the protostellar collapse in pre-galactic objects. We find the epoch during which Pop III stars dominated the energy and metal budget of the first galaxies to be short-lived. Galaxies which host Pop III stars do not retain dynamical signatures of their thermal and radiative feedback for more than 10^8 yr after the lives of the stars end in pair-instability supernovae, even when we consider the maximum reasonable efficiency of the feedback. Though metals ejected by the supernovae can travel well beyond the virial radius of the host galaxy, they typically begin to fall back quickly, and do not enrich a large fraction of the intergalactic medium. Galaxies with total mass in excess of 3 x 10^6 Msun re-accrete most of their baryons and transition to metal-enriched Pop II star formation.Comment: 13 pages, 9 figures, published in Ap

Revisiting The First Galaxies: The epoch of Population III stars

We investigate the transition from primordial Population III (Pop III) star formation to normal Pop II star formation in the first galaxies using new cosmological hydrodynamic simulations. We find that while the first stars seed their host galaxies with metals, they cannot sustain significant outflows to enrich the intergalactic medium, even assuming a top-heavy initial mass function. This means that Pop III star formation could potentially continue until z~6 in different unenriched regions of the universe, before being ultimately shut off by cosmic reionization. Within an individual galaxy, the metal production and stellar feedback from Pop II stars overtake Pop III stars in 20-200 Myr, depending on galaxy mass.Comment: 9 pages, 7 figures, published in Ap