Diane P. Bischak

Using Simulation Modeling to Improve Patient Flow at an Outpatient Orthopedic Clinic

We report on the use of discrete event simulation modeling to support process improvements at an orthopedic outpatient clinic. The clinic was effective in treating patients, but waiting time and congestion in the clinic created patient dissatisfaction and staff morale issues. The modeling helped to identify improvement alternatives including optimized staffing levels, better patient scheduling, and an emphasis on staff arriving promptly. Quantitative results from the modeling provided motivation to implement the improvements. Statistical analysis of data taken before and after the implementation indicate that waiting time measures were significantly improved and overall patient time in the clinic was reduced.

The Alaska Salmon Enhancement Program: A Cost/Benefit Analysis

In May 1991, the Alaska Senates Special Committee on Domestic and International Commercial Fisheries initiated the first review of the states salmon enhancement program since its inception 20 years ago. As part of this review, a cost/benefit analysis of the States enhancement program for salmon was performed with cooperation from the Fisheries Research Enhancement Division of the Alaska Department of Fish and Game. The main results are that the additional producers surplus generated by the pink and sockeye hatchery programs are estimated to be less than the costs of running these programs. Eliminating the entire pink or sockeye salmon programs is estimated to increase net benefits by about 8% and 6%, respectively. A 15% increase in either program is estimated to result in a reduction in net benefits and a 15% decrease in either program is estimated to result in a slight increase in net benefits. Estimates of the confidence intervals for net benefits suggest that the gains from the elimination of either the pink program or the sockeye program are statistically different from zero. However, changes of plus or minus 15% of current hatchery production are found not to statistically affect net benefits.

Determining the Reorder Point and Order-Up-To-Level in a Periodic Review System So As to Achieve a Desired Fill Rate and a Desired Average Time Between Replenishments

In this paper we consider a periodic review, reorder point, order-up-to-level system, a type commonly used in practice. Motivated by a specific practical context, we present a novel approach to determining the reorder point and order-up-to-level (for a given review interval) so as to target desired values of (i) customer fill rate and (ii) average time between consecutive replenishments. Specifically, by using a diffusion model (producing normally distributed demand) we convert a periodic review, constant lead time setting into one having continuous review and a random lead time. The method is simple to implement and produces quite reasonable results.

Using patient flow simulation to improve access at a multidisciplinary sleep centre

The lack of timely access to diagnosis and treatment for sleep disorders is well described, but little attention has been paid to understanding how multiple system constraints contribute to long waiting times. The objectives of this study were to identify system constraints leading to long waiting times at a multidisciplinary sleep centre, and to use patient flow simulation modelling to test solutions that could improve access. Discrete‐event simulation models of patient flow were constructed using historical data from 150 patients referred to the sleep centre, and used to both examine reasons for access delays and to test alternative system configurations that were predicted by administrators to reduce waiting times. Four possible solutions were modelled and compared with baseline, including addition of capacity to different areas at the sleep centre and elimination of prioritization by urgency. Within the model, adding physician capacity improved time from patient referral to initial physician appointment, but worsened time from polysomnography requisition to test completion, and had no effect on time from patient referral to treatment initiation. Adding respiratory therapist did not improve model performance compared with baseline. Eliminating triage prioritization worsened time to physician assessment and treatment initiation for urgent patients without improving waiting times overall. This study demonstrates that discrete‐event simulation can identify multiple constraints in access‐limited healthcare systems and allow suggested solutions to be tested before implementation. The model of this sleep centre predicted that investments in capacity expansion proposed by administrators would not reduce the time to a clinically meaningful patient outcome.

Learning by Doing, Knowledge Spillovers, and Technological and Organizational Change in High-Altitude Mountaineering

We present an analysis of microlevel data from mountaineering on the 14 peaks over 8,000 m in height during the period 1895-1998. Prior to 1950, no expedition was successful in making an ascent and almost half of expeditions experienced a death, frostbite, or altitude sickness. By the 1990s, however, over half of the expeditions would successfully make an ascent and only about one in seven would experience an adverse outcome. Our objective is to distinguish between the effects of learning by doing and knowledge spillovers versus the effects of changes in technology or economic organization in explaining these results. As we can identify each climber by name and nationality, as well as each expedition teams methods and outcomes, we are able to disentangle the effects of learning at the individual, national, and international levels from effects due to improvements in climbing technology or changes in organizational methods and objectives. We find evidence that both individual learning by doing and learning through knowledge spillovers have contributed to the observed increase in ascent rates and to the decrease in death, frostbite, and altitude sickness rates.

Bayesian estimation of the rate at which a process, monitored by an X̄ chart, goes out of control

This paper examines a process that is monitored by an X̄ chart. It is assumed that the process may go out of control due to the occurrence of any of several independent assignable causes. The time until each specific assignable cause occurs is exponential, but the distributional parameters of the various causes are unknown and are not necessarily identical. A Bayesian approach is used to estimate these parameters. The approach encompasses prior knowledge about the parameters as well as observations of the process, including when the out-of-control situation was detected and the associated assignable cause. Numerical illustrations are provided that indicate how the posterior results depend upon the choice of the parameters of the prior distributions.

Estimating the rate at which a process goes out of control in a statistical process control context

Models for the economic design of control charts have, in the past, implicitly assumed that the rate at which the process goes out of control is known or can be easily estimated. It is suggested that there are difficulties with this assumption; in particular, the most obvious estimators generally perform badly. Four estimators for this rate are developed and simulation experimental results are presented on the performance of each. A maximum likelihood estimator and an estimator that incorporates an exact expression for the expected time until the process goes out of control provide the best performance in general. However, there are some situations in which a naive estimator can perform best. Interpretation of the results provides insights for estimating the unknown rate in practice.

An observational study of the effectiveness of alternative care providers in the management of obstructive sleep apnea.

Alternative care providers have been proposed as a substitute for physician‐based management of obstructive sleep apnea. The purpose of this study was to describe the clinical course of patients with a new diagnosis of obstructive sleep apnea who were treated with continuous positive airway pressure and followed by alternative care providers at a tertiary care sleep clinic. It was hypothesized that care by alternative care providers would result in improvement of daytime sleepiness and satisfactory treatment adherence, and that a specific number of follow‐up visits could be identified after which clinical outcomes no longer improved. The Epworth Sleepiness Scale score was measured for each patient at baseline and at each alternative care provider visit. Patients were discharged when they demonstrated a significant improvement in sleepiness and were adherent to therapy. The Epworth Sleepiness Scale score decreased by 3.9 points from baseline to discharge. Patients with three or more visits required more follow‐up time to achieve the same clinical improvement as those with only two visits. Continuous positive airway pressure adherence was comparable to previous studies of physician‐led care and improved with ongoing alternative care provider follow‐up. The current results suggest that clinical care by alternative care providers leads to continued improvements in sleepiness in patients with obstructive sleep apnea who are treated with continuous positive airway pressure, and that a minority of patients require longer follow‐up to achieve a satisfactory clinical response to therapy.

Intrinsic motivation, external reward, and their effect on overall motivation and performance

ABSTRACT An unresolved debate lingers concerning the effect of performance-contingent rewards on motivation and performance. Behavioral psychology and economics suggest that performance-contingent rewards improve performance. In contrast, cognitive evaluation theory predicts that performance-contingent rewards undermine motivation and performance. We discuss the predictions of these two streams and develop an experiment that resolves the limitations of previous studies by using a new measure of intrinsic motivation: self-selection into a specific area of knowledge, as revealed by choice of academic major. Students from mathematics-related and literature-related areas were selected and randomly assigned to math and English language tests. Participants received a participation fee or a performance-contingent payment in addition to a fee. Both performance-contingent rewards and intrinsic motivation improved motivation and performance, in contrast with cognitive evaluation theory’s predictions.

Evaluating healthcare systems with insufficient capacity to meet demand

Modeling healthcare systems using discrete-event simulation (DES) provides the flexibility to analyze both their steady-state and transient performance. However, there has been little work on how best to measure healthcare system performance in cases where there is at least one unstable and lengthening queue in the system, so that traditional steady-state measures such as mean queue length or mean time in queue are meaningless. Using the example of an academic sleep disorders clinic, the authors discuss some of the challenges in constructing a DES model of a healthcare system that has a growing waiting list due to insufficient capacity in one or more areas. Specific considerations include: bottleneck identification through pre-analysis, how to determine a meaningful warm-up period, and the selection of performance measures given system instability.