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In the following excerpt from Appendix A (page 219 of original article), there were two errors in equation A-2. Two instances of "K" should have been replaced with ellipses ( . . . ). The correct equation is as follows:Provided that the roles included in a particular structural event are events in N occurring with probability p(n 1 ), p(n 2 ), . . . p(n k ), the sampling distribution of joint structural events is given by the multinomial formula:

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.The mechanisms governing the composition offormal social groups (e.g., task groups, organizationalfounding teams) remain poorly understood, owing to (1) a lack of representative sampling from groups found in the general population, (2) a "success" bias among researchers that leads them to consider only those groups that actually emerge and survive, and (3) a restrictive focus on some theorized mechanisms of group composition (e.g., homophily) to the exclusion of others. These shortcomings are addressed by analyzing a unique, representative data set of organizational founding teams sampled from the U.S. population. Rather than simply considering the properties of those founding teams that are empirically observed, a novel quantitative methodology generates the distribution of all possible teams, based on combinations of individual and relational characteristics. This methodology permits the exploration offive mechanisms of group composition-those based on homophily, functionality, status expectations, network constraint, and ecological constraint. Findings suggest that homophily and network constraints based on strong ties have the most pronounced effect on group composition. Social isolation (i.e., exclusion from a group) is more likely to occur as a result of ecological constraints on the availability of similar alters in a locality than as a result of statusvarying membership choices. SOCIOLOGISTShave made major strides toward understanding the conditions under which new organizations and new organizational forms are created, as well as the kinds of social locations that are most likely to spawn their creators. Beginning with Max Weber's ([1904-1905] 1992) analysis of ascetic Protestantism's contributions to the entrepreneurial spirit, sociologists have offered both macro-and microlevel interpretations of entrepreneurial

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