Four Sources of Variation in Job Shop Production


Variation increases complexity and is usually considered as the enemy of productivity in manufacturing systems. In some uncommon cases, known variation in process requirements across products may improve throughput if product mix is optimally planned. However, the variation usually makes production scheduling and management in job shops more difficult. Variation in job shop production consists of the following four components:

  • Type 1. Large, known variation in process requirements across distinct products ordered by customers
  • Type 2. Unpredictable and uneven demand for many products
  • Type 3. Uncertain events with significant impact on production
  • Type 4. Uncontrollable statistical variation in the system.

All four types of variation create shifting bottlenecks in production.

Four Sources of Variation in Job Shop Production

Type 1 variation is inevitable for job shops which need to make a large variety of products for meeting demand from various customers. Many job shops accept this variation to survive in the market. Different products may have different routings. Th product variety usually brings a lot of known variation in process requirements across products.

Type 2 variation is also mostly beyond the control of job shops. It comes from market. The demand cannot be smoothened because distinct products must be made by the respective due dates. High known variation in process requirements of distinct products coupled with unpredictable demand creates shifting bottlenecks. Consequently, it makes production planning and control difficult even in the absence of variation of Types 3 and 4.

Type 3 variation is caused by events like machine breakdowns, worker absenteeism and rework/rejections due to poor quality. It is relatively easier to control Type 3 variation because it happens within the factory. Preventive maintenance, worker management and quality systems are helpful to reduce this variation. Production schedule has to be revised whenever major events occur.

Type 4 variation is due to natural, random causes in production system. In high-variety production systems, it is relatively more difficult to control statistical variation in processes. Production planning must take into account this variation also and include some buffer times in the flow of work orders.

An efficient method for planning, control and management of job shop production addresses the four types of variation appropriately. Any method which does not explicitly consider these components of variation will not have discriminatory power to be efficient. No methodology like Lean Manufacturing, Six Sigma, Theory of Constraints, Factory Physics, Quick Response Manufacturing, etc. duly considers the four types of variation.

Our approach to planning, control and management of complex job shop production duly considers the four types of variation and provides a practical solution for the same purpose. Optisol scheduling software, Schedlyzer and Schedlyzer Lite enable job shops to schedule production optimally in the presence of the four types of variation. They also help with proactive capacity planning to mitigate shifting bottlenecks in advance for improving flow of work, lead times, WIP, on-time delivery and throughput. Contact us for additional information about the two software tools.