Abstract— Outpatient department has become an essential part of the hospital due to the fact that it is the first step of the treatment system. This leads to the long waiting times especially in public hospitals in Thailand. Patients always have long waiting times for a treatment followed by short consultations. In this study, Operation Research is applied to improve the system. We found out that the reason for long waiting times is that there are too many patients who come in at the same period of time. A discrete event simulation of the outpatient department was developed to examine the system by applying an appointment system to reduce the congestion of patients. The results identified the best appointment system suited for the outpatient department which has more walk-in patients and a high variability of consultation time. This could reduce the waiting times of patients without adding more resources. Keywords— Appointment system, Health system analysis, Outpatient department, Simulation I. INTRODUCTION HE case study hospital is a public hospital under Ministry of Public Health located in Chonburi, Thailand. Public hospitals in Thailand faced the same problem in an increasing of patients while there are few doctors available. We collected the data from the hospital’s database. There are about 200 patients in the outpatient department per day. 30 percent of patients are patients with appointments. The average total waiting time of patients is 2 hours. The arrival pattern of patients mostly congested around 7 to 8 AM. This caused the patients who came earlier had more waiting times than the ones who came afterwards. There are different reasons for long waiting times but the major reason in this study is the imbalance of the amount of patients in each period. The objective of this study is to apply the appointment system for appointment patients through the simulation model in order to reduce the patients’ waiting times. There are many researchers who study about Appointment System. The effective Appointment System can increase doctors’ utilization and also reduce patients’ waiting times. There are two components of an Appointment System: Natchaya Lailomthong 1 is with the Department of Industrial Engineering, Chulalongkorn University, Bangkok, 10330 Thailand. (corresponding author’s phone: +6681-625-5512; e-mail: natchaya.l@hotmail.com). Seeronk Prichanont 2 is with the Department of Industrial Engineering, Chulalongkorn University, Bangkok, 10330 Thailand. (e-mail: seeronk@gmail.com). Sponsored by Thai Health Promotion Foundation, Bangkok, 10120 Thailand. (website: http://en.thaihealth.or.th) Appointment Rules and Sequencing Rules [1]. The appointment rule consists of number of patients appointed in one period (block size), the number of patients at the beginning of the clinic (initial block), and the interval between the appointments. With the combination of these variables, there are several appointment rules. The single-block system (Fig. 1) is the appointment rule which mostly used in the public hospitals in Thailand due to the convenience and the less of doctor’s idle time. However, all patients come simultaneously causing the congestion and exceeding waiting time [2]. Fig. 1 Single-block System The individual-block/fixed interval (Fig. 2) calls patients one by one with fixed time. Reference [3] specified interval equal to the doctors’ average service time whereas [4] set interval less than the average service time. This system is suitable for the clinics with a high service time and low variance of service time. Fig. 2 Individual-block/Fixed interval System In multiple-block/fixed-interval (Fig. 3), many patients are appointed to the clinic. Reference [3]-[5] examined this system assigning the interval two times of average service time. Reference [6] offered to extend the interval in case of there are walk-in patients and emergency patients. Patient’s Waiting Time Reduction in Outpatient Department Natchaya Lailomthong 1 , Seeronk Prichanont 2 T