JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Oxford University Press and The Review of Economic Studies, Ltd. are collaborating with JSTOR to digitize, preserve and extend access to The Review of Economic Studies.The conventional approach to social programme evaluation focuses on estimating mean impacts of programmes. Yet many interesting questions regarding the political economy of programmes, the distribution of programme benefits and the option values conferred on programme participants require knowledge of the distribution of impacts, or features of it. This paper presents evidence that heterogeneity in response to programmes is empirically important and that classical probability inequalities are not very informative in producing estimates or bounds on the distribution of programme impacts. We explore two methods for supplementing the information in these inequalities based on assumptions about participant decision-making processes and about the strength in dependence between outcomes in the participation and non-participation states. Dependence is produced as a consequence of rational choice by participants. We test for stochastic rationality among programme participants and present and implement methods for estimating the option values of social programmes.And the Lord said, Because the cry of Sodom and Gomorrah is great, and because their sin is very grievous; I will go down now, and see whether they have done altogether according to the cry of it, which is come unto me; and if not, I will know.. . And Abraham drew near and said, Wilt thou also destroy the righteous with the wicked? Peradventure there be fifty righteous within the city; wilt thou also destroy and not spare the place for the fifty righteous that are therein? ... And the Lord said, If Ifind in Sodom fifty righteous within the city, then I will spare all the place for their sakes. Genesis 18: Verses 20-26, King James Version 487 This content downloaded from 128.235.251.160 on Mon, 15 Dec 2014 02:47:13 AM All use subject to JSTOR Terms and Conditions 488 REVIEW OF ECONOMIC STUDIES 1. INTRODUCTION Most evaluations of social programmes focus exclusively on mean impacts. Yet, as the passage from Genesis reveals, features other than the mean are often of interest. Just as Abraham convinced the Lord to spare Sodom and Gomorrah if he could find 50 righteous persons living there, so many persons would judge programmes to be successful if enough persons, or enough of the right kinds of persons, reaped benefits from them even if the average participant did not.The case for using the mean impact to evaluate a programme rests on two key assumptions: (a) that increases in total output increase welfare; and (b) that undesirable distributional aspects of programmes a...

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Previous analyses have suggested that the database of 755 studies of the validity of the General Aptitude Test Battery (GATB) demonstrates a small but consistent positive correlation with criteria relevant to job performance. Critics have noted that some of the validity studies conducted were not included in the database and have speculated that these studies may have found negligible or even negative validities, so that the extant database is subject to selection bias. The authors use a mathematical model to estimate the magnitude of possible selection and its effects on the mean and variance of GATB validities. Although evidence of selection is found for some GATB scales and composites, the estimated effect of selection is probably too small to influence conclusions about the validity of the GATB.

Routine interpretation of liver-window scans for all abdominal CT scans has limited added utility in detecting hepatic disease.