A comparison of procedures to correct for base-line differences in the analysis of continuous longitudinal data: a case-study

Summary.  The main advantage of longitudinal studies is that they can distinguish changes over time within individuals (longitudinal effects) from differences between subjects at the start of the study (base-line characteristics; cross-sectional effects). Often, especially in observational studies, subjects are very heterogeneous at base-line, and one may want to correct for this, when doing inferences for the longitudinal trends. Three procedures for base-line correction are compared in the context of linear mixed models for continuous longitudinal data. All procedures are illustrated extensively by using data from an experiment which aimed at studying the relationship between the post-operative evolution of the functional status of elderly hip fracture patients and their preoperative neurocognitive status.     

Impact of introduced honeybees, Apis mellifera , upon native bee communities in the Bonin (Ogasawara) Islands

The Bonin (Ogasawara) Islands are oceanic islands located in the northwest Pacific, and have ten native (nine endemic) bee species, all of which are nonsocial. The European honeybee (Apis mellifera), which was introduced to the islands for apiculture in the 1880s, became naturalized in a few islands shortly after introduction. To detect the impact of the honeybees upon native bee diversity, we analyzed pollen harvest by honeybees and surveyed the relative abundance of honeybees and native bees on flowers on several islands. Both hived and feral honeybee colonies were active throughout the year, harvesting pollen of both native and alien flowers and from both entomophilous and anemophilous flowers. Honeybees strongly depended on the alien plants, especially during winter to spring when native melittophilous flowers were rare. From June to November, honeybees exhaustively utilized native flowers, which had originally been utilized and pollinated by native bees. On Chichi and Haha Islands, where human disturbance of forests has been severe, both native and alien flowers were dominated by honeybees, and native bees were rare or extinct even in well-conserved forests. In contrast, on Ani Island and Haha's satellite islands where primary forests were well conserved and honeybees were still uncommon or absent, native bees remained dominant. These results suggest that competition for nectar and pollen of the native flowers between honeybees and native bees favors honeybees on the disturbed islands, which are thoroughly invaded by alien nectariferous, sometimes aggressive, weedy plants. Received: May 8, 1998 / Accepted: May 6, 1999     

Toward general impossibility theorems in pure exchange economies

We study the possibility of strategy-proof and efficient mechanisms in pure exchange economies. In his remarkable paper, Zhou (1991) establishes an elegant impossibility result: there is no strategy-proof, efficient, and non-dictatorial mechanism in the two-agent case. He conjectures that there is no strategy-proof, efficient, and “non-inversely-dictatorial” mechanism in the case of three or more agents. However, we discover some counterexamples to his conjecture in the case of four or more agents. We present a new interesting open question: Is there any strategy-proof, efficient, and “non-alternately-dictatorial” mechanism? Received: 17 October 2000/Accepted: 20 April 2001     

Exchange rates and the Swiss economy

Safe haven capital inflows and other factors have caused the Swiss franc to appreciate and posed challenges for policymakers. We find that these exchange rate changes do not affect the volume of exports from Switzerland’s most advanced sectors, pharmaceuticals and watches, but do matter for exports of medium-high-technology products such as capital goods and machinery. We also report that appreciations do not affect stock prices and goods prices for the pharmaceutical and watch industries but cause both stock and goods prices to tumble for the capital goods and machinery sectors. We draw many policy lessons from these findings for Switzerland and for the rest of the world.