Human subjects decide when to sink a fixed cost "C" to seize an irreversible investment opportunity whose value "V" is governed by Brownian motion. The optimal policy is to invest when "V" first crosses a threshold "V"* = (1 + "w"*)"C", where the wait option premium "w"* depends on drift, volatility, and expiration hazard parameters. Subjects in the Low "w"* treatment on average invest at values quite close to optimum. Subjects in the two Medium and the High "w"* treatments invested at values below optimum, but with the predicted ordering, and values approached the optimum by the last block of 20 periods. Copyright Copyright © 2009 The Review of Economic Studies Limited.

This paper analyzes seller choices and outcomes in approximately 700 recent Internet auctions on eBay. The 'Buy it Now' option that is available in these auctions allows the seller to supplement or replace the auction with a posted price offer. We use a structural model to control for the endogenous conduct of the auction (e.g., number of bids and bidders) as well as product and seller characteristics. Among other results, we find that the 'Buy it Now' option was used more often by sellers with higher ratings (awarded by previous buyers) and sellers offering fewer units; and that posted prices were more prevalent for used items. Sellers obtained higher prices for unused and undamaged items overall, and especially when selling at the 'Buy it Now' price. * Acknowledgement. This research has received generous financial support from the UCSC Division of Social Sciences. We are grateful to David Reiley for lending his web crawler PERL script, and to Dan Levin for pointers to the theoretical literature. +University

In 2017, disruptions in the global supply of helium reminded consumers, distributors, and policy makers that the global helium supply chain lacks flexibility, and that attempts to increase production from the U.S. Federal Helium Reserve (the FHR) may not be able to compensate for the loss of one of the few major producers in the world. Issues with U.S. and global markets for helium include inelastic demand, economic availability of helium only as a byproduct, only 4-5 major producers, heliumÕs propensity to escape earthÕs crust, an ongoing absence of storage facilities comparable to the FHR, and a lack of consequences for the venting of helium. The complex combination of these economic, physical, and regulatory issues is unique to helium, and determining heliumÕs practical availability goes far beyond estimating the technically accessible volume of underground resources. Although most of these issues have been analyzed since helium was recognized to be a valuable mineral commodity in the early 1900s, very few economic models have been developed that adequately consider the unique characteristics of helium and helium markets. In particular, there is a notable lack of recent empirical work to estimate the responsiveness of helium demand, supply, prices, and trade patterns to the ongoing drawdown and sale of helium reserves stored in the FHR. In general, existing models of helium either do not account for an oligopoly controlling supply, or they do not evaluate potential helium extraction and storage programs based on an intertemporal maximization of the value of the resource. Such models could be of very limited use to decision makers. This review found only one working paper with a helium market model that has incorporated both of these vital considerations. That and other economic studies along similar lines could be very useful in helping inform current helium policy discussions and decisions.

a b s t r a c tWe assessed the habitat use patterns of the Amargosa vole Microtus californicus scirpensis, an endangered rodent endemic to wetland vegetation along a 3.5 km stretch of the Amargosa River in the Mojave Desert, USA. Our goals were to: (1) quantify the vole's abundance, occupancy rates and habitat selection patterns along gradients of vegetation cover and spatial scale; (2) identify the processes that likely had the greatest influence on its habitat selection patterns. We trapped voles monthly in six 1 ha grids from January to May 2012 and measured habitat structure at subgrid (225 m 2 ) and trap (1 m 2 ) scales in winter and spring seasons. Regardless of scale, analyses of density, occupancy and vegetation structure consistently indicated that voles occurred in patches of bulrush (Schoenoplectus americanus; Cyperaceae) where cover >50%. The majority of evidence indicates the vole's habitat selectivity is likely driven by bulrush providing protection from intense predation. However, a combination of selective habitat use and limited movement resulted in a high proportion of apparently suitable bulrush patches being unoccupied. This suggests the Amargosa vole's habitat selection behavior confers individual benefits but may not allow the overall population to persist in a changing environment.