Engineering & Technology · Industrial Engineering, Operations & Logistics

Production, service systems and operations models

A workshop reports that it handled twenty items. Its customer records show only seven completed orders. Both counts might be correct: one order can contain several items, and one item may pass through the same activity twice. Before calculating performance, define what flows and what counts as completion.

Use engineering measurement and design methods for evidence, requirements and comparisons. This chapter introduces its own rates, count balances and inequalities. All operations, times, outputs and scores are original constructed cases; no reported business or personnel performance is implied.

Back to the chapter

After reading, you can

Define an operational boundary and flow unit, then reconcile a simple account without confusing activity with completed service.

An operation transforms inputs into a useful outcome

Production systems turn materials and other inputs into products; service systems may transform information, condition or access. Both involve people, equipment, rules, information and constrained resources. A useful model starts with an operational question, such as how long an accepted repair order remains open or how many conforming kits can be completed in an hour.

Choose a boundary and a flow unit. An order, item, batch and customer visit are different units. Define the entry and exit events: receiving a request may differ from accepting a complete order; finishing assembly may differ from delivering a checked kit. A count needs the same definitions before and after an improvement.

Draw relationships that affect the answer

Activities change or assess the flow unit. Buffers hold it while it waits. Resources perform activities: one person may prepare and inspect, or a machine may handle several items together. Show who or what is required, not merely the order of boxes. A shared resource can limit two seemingly separate activities.

Information can control when a unit moves. An unavailable drawing approval or missing part identifier can hold an order even when the equipment is idle. Internal rework sends the same unit through additional work. Count those visits when estimating workload, while retaining the external order identity when counting demand and completed service.

The chosen flow unit is an accepted order. Preparation and checking are inside the boundary. A return from checking to preparation is internal rework, not a new external order. For 10 initially open orders, 6 new arrivals and 7 closures, 9 remain open under the stated no-cancellation/no-loss account.

Use a balance to check the record

For an account with no loss, cancellations or reclassification, open orders at the end equal open orders at the start plus new accepted arrivals minus closures. This is a count balance, not a model of how fast anyone works. If the record does not reconcile, investigate definitions, timing, duplicated entries or omitted exit types.

The balance also distinguishes a stock from a rate. Nine open orders is a state at a time. Seven closures in two hours is a rate of 3.5 orders/hour over that window. Neither tells us the waiting time of a particular order. Retain timestamps and the distribution of cases when those questions matter. A single total can conceal urgent, difficult or repeatedly reworked cases.

Reconcile one constructed shift

Start with 10 open orders, accept 6 new orders and close 7. The ending count is 10+6−7=9. If each closed order contains two items, 14 completed items correspond to 7 orders; they do not become 14 completed orders.

Suppose two of the closed orders required an extra preparation visit. That adds 2 activity visits to the workload account but no new external orders. The order balance still gives 9 open orders. Distinguishing those accounts prevents repeated work from being reported as additional delivered service.

Try the reasoning

Why identify the flow unit before comparing two rates?
Show the answer and reasoning
Rates of items, batches and orders can describe different amounts of service. The numerator and completion boundary must be comparable.
Does an internal rework loop add an external arrival?
Show the answer and reasoning
No. It adds activity and resource demand for an existing flow unit unless the model explicitly defines a different boundary.
Can an order-count balance explain the cause of a delay?
Show the answer and reasoning
No. It checks count consistency. Investigating delay needs timing, resource, routing and information evidence.

What to carry forward

Define the unit, boundary, events and resources before interpreting operational counts.

References

Teaching resources and research supporting this lesson’s explanations and examples.