Gail Rubin

A single server queue with random arrivals and balking: confidence interval estimation

This paper investigates a queueing system, which consists of Poisson input of customers, some of whom are lost to balking, and a single server working a shift of lengthL and providing a service whose duration can vary from customer to customer. If a service is in progress at the end of a shift, the server works overtime to complete the service. This process was motivated by the behavior of fishermen interviewed in the NY Great Lakes Creel Survey.We derive the distributions of the number of services (X), overtime and total server idle time (T), both unconditionally (for Poisson arrivals) and conditionally on the number (n) of arrivals per shift, assuming that the arrival times are not recorded in the data. These distributions provide the basis for estimation of the parameters from asingle realization of the queueing process during [0,L]. The conditional distributions also can be used to estimate common service time,w, when (n, X) or (n, T) are observed. Confidence intervals based onT are of shorter length, for all confidence coefficients, than the corresponding intervals based onX.

On the Estimation Problem of Mixing/Pair Formation Matrices with Applications to Models for Sexually-Transmitted Diseases

A problem of considerable importance lying at the interface of social dynamics, demography, and epidemiology is determining and modeling who is mixing with whom. In this article we describe a general approach, using nonlinear mixing matrices, for modeling the process of pair-formation in heterogeneous populations. Determining who is mixing with whom is complicated by a variety of factors, including the problem of denominators, which is, in our context, equivalent to the nonexistence of closely interacting social/sexual networks. We describe the use of a mark-recapture model for estimating the sizes of the missing link, that is, the size of the population having sexual contact with a specified population and hence at risk for sexually-transmitted diseases. The need to estimate the size of the sexually-active subset before estimating the size of the population at risk introduces extra variability into the problem. An estimator of the variance of the estimated size of the population at risk that accounts for this extra variability and an expression for the bias of such an estimator have been derived. We illustrate our results with data collected from a population of university undergraduates, and make use of our axiomatic modeling approach for mixing/pair formation to compute specific mixing matrices. Complete details of this work will be published elsewhere.

Gypsum amendment to soil can reduce selenium uptake by alfalfa grown in the presence of coal fly ash

Experiments in the field and greenhouse were conducted in the presence of coal fly ash to determine whether gypsum can reduce Se concentration in alfalfa (Medicago sativa L.). In the field experiment, conducted at a coal fly ash landfill, 11.2 t ha-1 gypsum was applied to soil as a top dressing to test the effect of gypsum in reducing selenium (Se) concentration in aboveground plant tissue. There were four treatment combinations of gypsum over a two year period, 1990 and 1991: (0, 0), (0, 11.2) (11.2, 0) and (11.2, 11.2). In 1991, the Se concentration was lower in alfalfa grown with gypsum regardless of whether the gypsum was applied in both years or in only one year, indicating that the effect of gypsum application in the first year persisted into the second year. Since there was no increase in aboveground biomass with added gypsum, differences in Se concentration reflect a competitive interaction between S and Se. In the greenhouse experiment, 12 soil treatments were tested: three levels of fly ash (0, 10 and 20%) in combination with each of four levels of gypsum (0, 2.5, 5, and 7.5%). The Se concentration in alfalfa grown in 10% fly ash declined linearly with increasing gypsum dose, resulting in a reduction in Se concentration of 0.04±0.02 μg g-1 for each 1% gypsum added for the first harvest and 0.06±0.03 μg g-1 for each 1% gypsum added in the second harvest. Based on these results, gypsum may prove useful as a management tool to reduce the uptake of Se by plants growing on coal fly ash landfills. ei]H Lambers

A Model for Estimating Daily Ozone Dose for Plants from Atmospheric Ozone Concentration and Vapor Pressure Deficit

Accurate measurement of the ozone dosages received by plants is difficult because the atmospheric ozone concentration that surrounds a plant is not the concentration that actually impinges upon the plant cells. The plants cuticle acts as a barrier to direct diffusion of ozone into cells for much of the plant surface. The primary avenue for ozone entry is via the stomata, which are adjustable pores in the epidermis. In this paper, we derive a diffusion model, consistent with Ficks first law, that uses the concurrent ambient ozone concentration and vapor pressure deficit (VPD), together with speciesspecific thresholds of ozone concentration and VPD, to predict the ozone concentration reaching the cells adjacent to the substomatal cavity at a given time. Combining this diffusion model with particular functional forms for the daily curves of atmospheric ozone concentration and VPD allows one to calculate the expected daily ozone dose that the plants cells receive and the variance of that dose. In addition, this methodology can be modified using hierarchical models to provide realistic regional estimates of the effective daily ozone dose for a species and the variance of that dose, which reflect the regional variation in the diurnal cycles of both atmospheric ozone concentration and VPD. The ozone dose and its variance predicted by this model can be used to assess ozone impact on red spruce in the northeastern United States.

Assessing trace element uptake by vegetation on a coal fly ash landfill

Improved methods are required to assess the risks posed by the uptake of potentially toxic elements such as selenium (Se), boron (B), and molybdenum (Mo) by vegetation on contaminated sites. In order to develop such methods and assess risk, vegetation was collected from two sites on a soil-capped coal fly ash landfill near Dunkirk, New York, during June of 1991 and June and August of 1992. The mean concentrations (μg g-1 dry weight) of Se and Mo in the shoots did not exceed, respectively, 0.12 and 18.7 in birds-foot trefoil (Lotus corniculatus L.), 0.06 and 12.1 in red clover (Trifolium pratense L.), 0.07 and 5.3 in timothy (Phleum pratense L.), and 0.09 and 2.2 in a mixture of grasses. These concentrations were greater than those in the same species harvested concurrently from a non-landfill site. The mean concentrations of B at the landfill ranged from 29 to 53 μg g-1 in the legumes and from 2 to 11 μg g-1 in the grasses, less than those at one non-landfill site but greater than those at another. Within the landfill, the concentration of Se in grasses was not correlated with the concentration of Se in soil and fly ash. The concentration of Se in grasses on both landfill sites was double that of grasses on the non-landfill site despite higher mean concentrations of Se in the upper soil (0–15 cm) on the non-landfill site. Therefore grass roots seem to be accessing Se from the ash by means of mass flow or other mechanisms. Based on our findings of significant variation in trace element uptake among species, harvests, and locations within sites, we recommend that contemporaneous transect sampling of at least two species be used to assess uptake of potentially toxic trace elements on landfills or other sites where contamination may occur.

Cell concentration as a factor in precursor incorporation by tobacco leaf protoplasts or separated cells

SummaryFreshly isolated protoplasts or separated cells prepared from leaves of Nicotiana tabacum L. cvs. Xanthincor Turkish Samsun exhibit substantially greater incorporation of [3H] leucine and [3H] uridine into protoplast protein and RNA, respectively, when dilute rather than concentrated suspensions are incubated. This phenomenon is considered to result from a dilution of the labeled precursors in the incubation medium by nonlabeled precursors of cellular origin.