BACKGROUND Focus groups are commonly used to explore participants' experiences in health and social care research. Although it is suggested that having demographically homogenous groups may help put participants at ease, the evidence is sparse.The aims of the paper are to: explore the impact of relative ethnic homogeneity and heterogeneity of focus group participants on the group discussions; i...

A social group can only be spatially coherent if its members synchronize activities such as foraging and resting. However, activity synchronization is costly to individuals if it requires them to postpone an activity that would be personally more profitable in order to do what the rest of the group is doing. Such costs will be particularly high in groups whose members belong to different age, s...

This paper studies how product differentiation affects the dynamics of US import trade. Using nonparametric and semiparametric techniques we find that trade in homogenous and reference priced products is of much shorter duration than trade in differentiated products. Differentiated products have a median survival time of five years; reference priced goods and homogenous products have a median s...

Our objective in this study was to estimate the probability that a Chlamydia trachomatis (CT) infection will cause an episode of clinical pelvic inflammatory disease (PID) and the reduction in such episodes among women with CT that could be achieved by annual screening. We reappraised evidence from randomized controlled trials of screening and controlled observational studies that followed untr...

We describe the most general homogenous, planar, light-ray-direction-changing sheet that performs one-to-one imaging between object space and image space. This is a nontrivial special case (of the sheet being homogenous) of an earlier result [Opt. Commun.282, 2480 (2009)]. Such a sheet can be realized, approximately, with generalized confocal lenslet arrays.