Industrial engineering focuses on improving systems that combine people, machines, materials, information, and time. Six Sigma is a structured approach for reducing defects and process variation. This cheat sheet helps students connect engineering design decisions to measurable performance, cost, quality, and efficiency.
It is useful for analyzing factories, hospitals, service systems, supply chains, and school operations.
Key Facts
- Takt time = available production time / customer demand, and it shows the pace needed to meet customer demand.
- Cycle time is the time required to complete one unit or task, and a process can meet demand when cycle time is less than or equal to takt time.
- Capacity = available time / cycle time, and the station with the lowest capacity is usually the bottleneck.
- Utilization = actual output / design capacity x 100%, and very high utilization can increase waiting time and delays.
- Process yield = good units / total units x 100%, and rolled throughput yield = yield 1 x yield 2 x yield 3 for a multi-step process.
- DPMO = defects / (units x opportunities per unit) x 1,000,000, and it measures defects per million opportunities.
- Cp = (USL - LSL) / (6 x sigma), and Cpk = minimum of (USL - mean) / (3 x sigma) and (mean - LSL) / (3 x sigma).
- DMAIC stands for Define, Measure, Analyze, Improve, and Control, which is the standard Six Sigma improvement cycle.
Vocabulary
- Industrial engineering
- A branch of engineering that designs and improves systems involving people, materials, equipment, information, energy, and time.
- Six Sigma
- A quality improvement method that uses data to reduce variation and defects in a process.
- Bottleneck
- The process step that limits the total output because it has the lowest capacity or longest effective cycle time.
- Takt time
- The maximum time allowed to produce one unit in order to match customer demand.
- Defect
- Any result, product, or service outcome that does not meet a required specification or customer expectation.
- Control chart
- A graph used to track process data over time and identify whether variation is stable or unusual.
Common Mistakes to Avoid
- Confusing takt time with cycle time is wrong because takt time comes from customer demand, while cycle time comes from the process itself.
- Ignoring the bottleneck is wrong because improving a non-bottleneck step may not increase total system output.
- Using average output only is wrong because variation, downtime, and defects can make a process unreliable even when the average looks acceptable.
- Calculating DPMO without including opportunities per unit is wrong because one unit can have more than one possible defect location.
- Assuming a process is capable just because most items pass inspection is wrong because capability compares process spread and centering to specification limits.
Practice Questions
- 1 A workstation has 420 minutes available per day and customer demand is 140 units per day. What is the takt time in minutes per unit?
- 2 A process has three stations with cycle times of 3 minutes, 5 minutes, and 4 minutes per unit. Which station is the bottleneck, and what is the process capacity in units per hour?
- 3 A factory makes 2,000 units, each with 4 defect opportunities. Inspectors find 32 defects. What is the DPMO?
- 4 A team wants to reduce delays in a hospital check-in process. Explain why measuring variation and identifying the bottleneck should come before choosing a solution.