Design Variables in Service Systems
a) Model design variables include the set of parameters that can be changed by the designer to enhance system performance.
b) In general, the service system design is in control of the entities’ acceptance or admission to the system, the entities’ waiting and classification rules, the service-providing process, the logic of entity flow between servers, and the rules of system departure.
c) Entities’ arrival rate to the system is typically not within the control of system designers.
d) Adjusting such processes may be translated into providing settings of key model control factors, such as:
1) Percent or rate of entities admitted to the service system.
2) Capacity of the waiting area or line.
3) Waiting discipline and rules of selecting customers to receive service. First-come-first-served is the most common waiting discipline in service systems. A preemptive method can also be used to expedite or select customers to service.
4) Number of servers in the service system and their configuration.
5) Service time at each server.
6) Percentages used to route entity flow among servers.
7) Rules of system departure (if applicable).
Time-Based CTQs in Service Systems
A set of measures can be used to assess the performance of a service system as well as to compare performance of several system designs. Quantified performance measures should be used to assess service system performance. Such measures can be estimated from model accumulated statistics, or a special code may be necessary to compute the measures’ values. Examples of performance measures in a service system include:
1) Waiting time per customer
2) Number of customers left without receiving the service (in case there is no capacity or the waiting time was too long)
3) Time-in-system (the total time a customer spends in the service system; includes waiting time, transfer time, and service time)
4) Average and maximum size of the queue (length of the waiting line)
5) Server utilization (percent idle and percent busy)
6) Service system throughput (number of entities processed per time unit, such as the number of customers served per day)
7) Service level (number of customers who finished the service without waiting or with less than 5 minutes of waiting time)
The following points are general guidelines for 3S project selection:
a. 3S projects should possess the 3S application criteria.
b. 3S projects should have identifiable and measurable business benefits. Expected benefits from 3S projects typically include:
1) Financial benefits:reduced cost and increased sales, market share, revenues, and profit
2) Operational benefits:increased productivity, utilization, on-time deliveries, quality, and efficiency; reduced lead time, downtime, delivery tardiness and shortages, and number of defects
3) Organizational benefits:increased competitiveness, market reputation, and employee satisfaction
4) To utilize the power of six-sigma, selected 3S projects should be approached from the perspective of understanding the variation in process inputs, controlling them, and eliminating waste and defects.
5) The project selected should be amenable to 3S-DFSS or a 3S-LSS.
