Many socially important search tasks are characterized by low target prevalence, meaning that targets are rarely encountered. For example, transportation security officers (TSOs) at airport checkpoints encounter very few actual threats in carry-on bags. In laboratory-based visual search experiments, low prevalence reduces the probability of detecting targets (Wolfe, Horowitz, & Kenner, 2005). In the lab, this "prevalence effect" is caused by changes in decision and response criteria (Wolfe & Van Wert, 2010) and can be mitigated by presenting a burst of high-prevalence search with feedback (Wolfe et al., 2007). The goal of this study was to see if these effects could be replicated in the field with TSOs. A total of 125 newly trained TSOs participated in one of two experiments as part of their final evaluation following training. They searched for threats in simulated bags across five blocks. The first three blocks were low prevalence (target prevalence ≤ .05) with no feedback; the fourth block was high prevalence (.50) with full feedback; and the final block was, again, low prevalence. We found that newly trained TSOs were better at detecting targets at high compared to low prevalence, replicating the prevalence effect. Furthermore, performance was better (and response criterion was more "liberal") in the low-prevalence block that took place after the high-prevalence block than in the initial three low-prevalence blocks, suggesting that a burst of high-prevalence trials may help alleviate the prevalence effect in the field.

We discuss the kinetics of multicomponent liquidlike clusters adsorbed onto solid surfaces. In this work, the rate expressions governing the growth and decay of surface clusters coupled to a two-dimensional gas phase are derived based on previous treatments of unary systems. General expressions are presented for the equilibrium partial pressures, the monomer evaporation rates, and the monomer capture rates. The composition dependence of the rate expressions is accomplished using the theory of real solutions to represent the activity coefficients and the surface tension. Numerical results from dynamical Monte Carlo simulations are presented for a nontrivial model of two-dimensional binary clusters. A generalized Gibbs-Thomson expression for the partial pressures is found to be in excellent agreement with the numerical results even for very small cluster sizes. The species selective evaporation rates are found to obey a simple rate law that predicts strong deviations from a naïve geometric scaling behavior. The growth and decay rates of binary clusters in the simulations were accurately modeled using the analytical rate expressions.

The theory for the kinetics of coarsening of multicomponent thin films is studied. The growth and decay rates of surface clusters consisting of more than one chemically distinct species are represented through a formalism based on the theory of real solutions. Explicit expressions are presented for the time evolution of a thin film in terms of the quasichemical rate equation approach that describes the concentrations of clusters of variable size and composition. The continuum limit of the rate equations yields a partial differential equation that applies to the late stage growth. It is found that the traditional Lifshitz-Slezov-Wagner approach, which employs a first-order continuity-type equation, is inadequate to treat the compositional variation of the clusters. Instead, a second-order generalized diffusion equation should be used. A model problem of a binary film, consisting of two types of atoms interacting through nearest-neighbor and next-nearest-neighbor coupling, is studied using a dynamical Monte Carlo method. The cluster size and composition distributions are extracted from the simulations as functions of time. The film kinetics exhibits a crossover behavior between coalescencediffusion coarsening at short times and Ostwald ripening at long times. Except for very small cluster sizes, the distributions are well represented by a separable two-dimensional scaling-type solution.