On Sampling Error in Market Share Estimates*

The relative error in the usual estimator of a brand's market share is reformulated in terms of marketing parameters. Such error is shown to be influenced in an important way by market penetration, as well as by variation in brand and product category volume. Of particular interest is the result that the relative error does not depend on the actual share level. Using data from a marketing research firm that supplies share estimates to the health products industry, we find that the relative error may be substantial even when a large sample is available. An upper bound on this relative error is obtained using marketing parameters that can frequently be measured using industry data and a company's internal records, thus reducing the level of judgmental input required in the planning of sample surveys.     

Comment: there may be compositional effects,but they do not work that way*

This analysis joins the debate on how declines in marriage have shifted the composition of the unmarried and married populations in the United States, and how compositional shifts have affected nonmarital birth rates. Gray, Stockard, and Stone (2006) presented one model for compositional effects that Ermisch (2009) challenged with alternative statistical tests. I propose an alternative model for compositional shifts based not on theory but on observed marriage and fertility patterns. The results from this alternative model are consistent with Ermisch’s findings yet support Gray et al.’s general case that compositional effects have had an important infiuence on nonmarital birth rates.     

DNA COLLECTION IN A RANDOMIZED SOCIAL SCIENCE STUDY OF COLLEGE PEER EFFECTS

We describe the DNA collection processes of an initial pilot and full study, which is designed to investigate joint peer and genetic effects on health behaviors and attitudes in a college campus setting. In the main study, 2664 (79.5%) students completed a Web survey and 2080 (78.7% of the survey completers after adjusting for the ineligible) provided a saliva DNA sample. The response rate for completing both the survey and the DNA portion of the study is 62.5%. Our DNA yields are of high quality. Overall, our experiences and results demonstrate that genetic data can be successfully collected as a part of traditional social science survey research projects. To aid others in doing so, we provide extensive details of our data collection experiences and offer recommendations to future researchers seeking to do or evaluate similar work.     

Relationship between urbanization and bat community structure in national parks of the southeastern U.S.

Urbanization and development are predicted to increase considerably in the United States over the next several decades, and this is expected to result in large-scale habitat loss, fragmentation and loss of wildlife species. Thus, natural parks and preserves are becomingly increasingly important in the conservation of regional biodiversity. We used mist-nets and AnabatII acoustic detectors to survey bats in 10 national parks in the southeastern U.S. and examined the relationship between bat community structure and development in the surrounding 5 km. We predicted that species richness would increase with park size and that species richness and evenness would decrease with development. Species richness was not related to development or any other landscape characteristics including park size. In contrast, species evenness declined with increasing development. Percent Developed land in the surrounding 5 km area was the only variable that entered into the stepwise regression model. The decrease in species evenness in the urban parks was due to the dominance of big brown bats (Eptesicus fuscus) in these parks. The percentage of big brown bats in our captures was positively related to percent Developed land in the surrounding area. Our data suggest that urban parks may be important for conserving regional bat biodiversity. However, the low species evenness in these parks suggests that some bat species may be susceptible to the effects of urbanization and may be extirpated over time. Thus, management of urban as well as rural parks should strive to conserve as much bat roosting and foraging habitat as possible.     

Young people’s perceptions of power and influence as a basis for understanding contemporary citizenship

Persistent simplistic binary discourses of young people’s citizenship portray them either as civically deficit and disengaged citizens or the creators of new democratic modes and approaches. This paper draws on field research with two groups of young people in Australia to better recognise the nuance of young people’s experiences of citizenship, power and influence. The study investigated the extent to which different groups of young people believe that they have the power to influence society; the ways in which they seek this influence; the current barriers to their influence; and what would enable them to have greater influence. Our analysis in this paper draws on Lukes’ concepts of power [2005. Power: A Radical View. 2nd ed. London: Palgrave Macmillan] and Arvanitakis’ framework of citizenship engagement and empowerment [in Arvanitakis, J., and E. Sidoti. 2011. “The Politics of Change: Where to for Young People and Politics.” In Their Own Hands: Can Young People Change Australia?, edited by L. Walsh and R. Black, 11–20. Melbourne: ACER Press], but also builds on an emerging scholarship concerned with the geographic dimensions of young people’s citizenship engagement and action, as well as with the affective, relational and temporal dimensions of this engagement and action. Our findings suggest that power works in different ways to both constrain and liberate young people as citizens – sometimes at the same time. The paper concludes with an argument for the continuing need to understand young people’s lived and located experiences of engagement, power and influence in more nuanced and sophisticated ways. This includes reframing the discussion about young people’s experiences in terms of the nature of their democratic engagement and action rather than simply their citizenship.     

Estimates of subgroup treatment effects in overall nonsignificant trials: To what extent should we believe in them?

In drug development, it sometimes occurs that a new drug does not demonstrate effectiveness for the full study population but appears to be beneficial in a relevant subgroup. In case the subgroup of interest was not part of a confirmatory testing strategy, the inflation of the overall type I error is substantial and therefore such a subgroup analysis finding can only be seen as exploratory at best. To support such exploratory findings, an appropriate replication of the subgroup finding should be undertaken in a new trial. We should, however, be reasonably confident in the observed treatment effect size to be able to use this estimate in a replication trial in the subpopulation of interest. We were therefore interested in evaluating the bias of the estimate of the subgroup treatment effect, after selection based on significance for the subgroup in an overall “failed” trial. Different scenarios, involving continuous as well as dichotomous outcomes, were investigated via simulation studies. It is shown that the bias associated with subgroup findings in overall nonsignificant clinical trials is on average large and varies substantially across plausible scenarios. This renders the subgroup treatment estimate from the original trial of limited value to design the replication trial. An empirical Bayesian shrinkage method is suggested to minimize this overestimation. The proposed estimator appears to offer either a good or a conservative correction to the observed subgroup treatment effect hence provides a more reliable subgroup treatment effect estimate for adequate planning of future studies.