In the realm of empirical research, investigators are first and foremost concerned with the validity of their results, but validity is a multi-dimensional ideal. In this article we discuss two key dimensions of validity – internal and external validity – and underscore the natural tension that arises in choosing a research approach to maximize both types of validity. We propose that the most co...

Acknowledgments We would like to thank Amartya Sen for having inspired us to work on this issue over the past several years. We also want to thank Katarina Smutna and Andreas Beivers for excellent research assistance.

A new species of the orchid bee genus Eufriesea Cockerell (Apidae: Apinae: Euglossini) is described and figured from Islas Marías Nayarit State, México in Pacific. insularis sp. nov. , a member coerulescens group restricted to Marias. The readily recognized by its dark blue integument with purple iridescence, black pubescence, wings, clypeus green hues prominent elevated ridge along midline. kn...

Thrazarine, O-[(3R)-2-diazo-3-hydroxybutyryl)]-L-serine, is a new antitumor antibiotic produced by Streptomyces coerulescens MH802-fF5. Thrazarine was isolated from culture filtrate by Sephadex LH-20 column chromatography and reversed phase HPLC. Thrazarine induced cytolysis of tumor cell lines co-cultured with nonactivated macrophages. This effect was tumor specific because the nontumorigenic ...