Throughput and Process Constraints

Throughput and Process Constraints in Six Sigma Black Belt - Measure Phase

Understanding Throughput and Process Constraints

Why This is Important

In the Six Sigma Black Belt Measure Phase, understanding throughput and process constraints is critical because:

• Throughput directly impacts business revenue and customer satisfaction
• Identifying constraints allows you to focus improvement efforts where they matter most
• Process constraints are the limiting factors preventing optimal performance
• Organizations can achieve maximum ROI by addressing the constraint before optimizing non-constraint areas
• Throughput metrics provide objective baselines for measuring improvement success

What is Throughput?

Throughput is the amount of product or service that flows through a process in a specific time period. It represents the rate at which a process produces output.

Key Definition: Throughput = Total Output / Total Time Period

Examples of throughput metrics include:

• Units produced per hour
• Customer transactions processed per day
• Loans approved per week
• Documents processed per month
• Calls handled per shift

What are Process Constraints?

Process constraints are the limiting factors that restrict throughput and prevent a process from producing more output. A constraint is any factor that prevents the process from achieving higher performance.

Types of Constraints:

• Capacity Constraints: Equipment, machines, or resources cannot handle more volume (e.g., only one printing press available)
• Resource Constraints: Insufficient personnel, skills, or labor availability
• Material Constraints: Shortage of raw materials or supplies
• Policy Constraints: Rules, regulations, or procedures that limit throughput
• Logical Constraints: Sequential dependencies where one step must complete before another begins
• Market Constraints: Demand limitations that prevent selling more product

The Theory of Constraints (TOC)

The Theory of Constraints, developed by Eliyahu Goldratt, is fundamental to understanding this concept:

1. Identify the constraint: Find the bottleneck limiting throughput
2. Exploit the constraint: Maximize the performance of the constraint with current resources
3. Subordinate other processes: Align all other processes to support the constraint
4. Elevate the constraint: Invest in improving or removing the constraint
5. Repeat the process: Once the original constraint is resolved, identify the next constraint

How to Identify Process Constraints

Step 1: Measure Current Throughput

• Establish baseline throughput metrics across the entire process
• Use data collection methods: observation, automation, logs, or work sampling
• Document throughput at each process step

Step 2: Map the Process

• Create a detailed value stream map
• Document cycle times, processing times, and wait times for each step

Step 3: Identify the Bottleneck

• The step with the longest cycle time is typically the constraint
• Look for steps with highest utilization rates (80%+ indicates constraint)
• Analyze where inventory or work-in-process (WIP) accumulates

Step 4: Quantify the Impact

• Calculate how the constraint limits overall process throughput
• Determine financial impact of the constraint on business

Calculating Throughput with Constraints

Formula: Process Throughput = Minimum Capacity of All Steps

If a process has 5 steps with the following capacities:

• Step 1: 100 units/hour
• Step 2: 85 units/hour (CONSTRAINT)
• Step 3: 120 units/hour
• Step 4: 95 units/hour
• Step 5: 110 units/hour

The process throughput is 85 units/hour because Step 2 is the constraint.

Practical Example: Manufacturing Process

A widget manufacturing facility has four steps:

• Molding: 50 units/hour capacity
• Assembly: 40 units/hour capacity (CONSTRAINT)
• Quality Check: 60 units/hour capacity
• Packaging: 55 units/hour capacity

The constraint is Assembly at 40 units/hour. This is the maximum throughput, regardless of the capacity of other steps. To improve overall throughput, the organization must either add assembly capacity or improve assembly efficiency.

Strategies to Improve Throughput by Addressing Constraints

1. Exploit (Cost-Effective Improvements)

• Eliminate idle time at the constraint
• Reduce setup time
• Improve operator training and efficiency
• Eliminate quality issues that require rework
• Optimize shift schedules

2. Elevate (Invest in Constraint)

• Purchase additional equipment
• Hire additional staff for constraint area
• Upgrade technology
• Add parallel processing capability

3. Subordinate (Support the Constraint)

• Balance input to the constraint
• Ensure other steps feed the constraint appropriately
• Prevent non-constraint steps from creating unnecessary bottlenecks upstream
• Use buffer management before constraint

Key Metrics for Measuring Throughput and Constraints

• Throughput Rate (TR): Units produced per time period
• Cycle Time (CT): Time required to complete one unit from start to finish
• Takt Time: Production rate required to meet customer demand (Demand / Available Time)
• Utilization Rate: Actual output / Maximum capacity (expressed as percentage)
• Constraint Capacity Utilization: How fully the constraint is being used
• Queue Time: Time work waits before being processed
• Process Lead Time: Total time from start to finish including all waits

How to Answer Exam Questions on Throughput and Process Constraints

Common Question Types

Type 1: Identification Questions

Question Example: "In a process with step capacities of 120, 95, 110, and 105 units per hour, which step is the constraint?"

Answer Approach:

• Look for the step with the lowest capacity
• The constraint is the bottleneck with minimum throughput
• Answer: Step 2 (95 units/hour) is the constraint

Type 2: Throughput Calculation Questions

Question Example: "If a process has steps with capacities of 80, 60, 90, and 75 units/hour, what is the maximum throughput?"

Answer Approach:

• Find the minimum capacity across all steps
• That minimum is the maximum process throughput
• Answer: 60 units/hour is the maximum throughput

Type 3: Improvement Strategy Questions

Question Example: "Your process is constrained at the packaging step. What is the first action you should take according to TOC?"

Answer Approach:

• Follow TOC sequence: Identify → Exploit → Subordinate → Elevate → Repeat
• First step is to exploit the constraint (maximize current performance)
• Only invest in elevating the constraint after exploiting it
• Answer: Eliminate idle time and inefficiencies at packaging first

Type 4: Business Impact Questions

Question Example: "Improving Step 1 capacity from 100 to 120 units/hour will increase process throughput. Is this statement true or false?"

Answer Approach:

• Determine what the current constraint is
• If Step 1 is NOT the constraint, improving it won't increase throughput
• Only improvements to the constraint affect overall throughput
• Answer: Depends on whether Step 1 is the constraint (likely False)

Type 5: Scenario Analysis Questions

Question Example: "You have a choice: invest $50,000 to increase constraint capacity by 20%, or invest $50,000 to increase non-constraint capacity by 40%. Which should you choose?"

Answer Approach:

• Always prioritize improving the constraint first
• Increasing non-constraint capacity yields no throughput improvement
• Answer: Invest in the constraint, even with smaller capacity gain

Exam Tips: Answering Questions on Throughput and Process Constraints

Tip 1: Remember the Hierarchy of Process Performance

Process throughput is always limited by the lowest capacity step. This is not negotiable. Always identify the constraint first before considering solutions.

Tip 2: Follow TOC Sequence Religiously

Exam questions often test whether you know the correct order of constraint management:

1. Identify
2. Exploit
3. Subordinate
4. Elevate
5. Repeat

If asked "what should you do first," the answer is usually to exploit the current constraint before investing money in elevating it.

Tip 3: Distinguish Between Constraint and Non-Constraint

This is critical. Questions will try to confuse you by asking about improvements to non-constraint steps. Remember:

• Improvements to the constraint → Increase overall throughput
• Improvements to non-constraints → Do NOT increase overall throughput (resources are wasted)

Tip 4: Calculate Throughput Accurately

When given multiple step capacities, use this formula:

Maximum Throughput = Minimum(Capacity of Step 1, Capacity of Step 2, ... Capacity of Step N)

Always show your work by listing each step's capacity and circling the minimum.

Tip 5: Understand Utilization Rates

A utilization rate above 80% typically indicates a constraint. If a step is running at 95% utilization while others run at 40%, that high-utilization step is likely your constraint. Low utilization steps have excess capacity and are not constraints.

Tip 6: Connect Throughput to Business Metrics

Exam questions may ask about business impact. Remember:

• Throughput directly impacts revenue (more output = more sales)
• Constraint removal improves profitability
• Non-constraint improvements waste resources
• TOC focuses investments on maximum ROI

Tip 7: Recognize Hidden Constraints

Questions may describe constraints indirectly:

• "Work piles up before the packaging step" → Packaging is the constraint
• "One machine runs 24/7 while others sit idle" → That machine is the constraint
• "One person has a backlog" → That person is the constraint
• "Cannot meet customer demand despite having capacity" → Market constraint

Tip 8: Avoid Common Pitfalls

• Pitfall: Assuming improving the weakest step improves throughput (only true if it's the constraint)
• Pitfall: Ignoring the constraint and improving other areas
• Pitfall: Assuming all steps should have equal capacity (they don't need to)
• Pitfall: Confusing constraint with waste (constraints are structural; waste is inefficiency)
• Pitfall: Investing in constraint elevation without first exploiting it

Tip 9: Use Data in Your Answers

Always reference specific numbers, capacities, or cycle times when available. Vague answers will not earn full credit. For example:

• Instead of: "Step 2 is the constraint"
• Say: "Step 2 is the constraint because its capacity of 75 units/hour is lower than all other steps (85, 90, and 95 units/hour)"

Tip 10: Practice the TOC Five-Step Process

Be able to apply all five steps to any process scenario:

1. Identify: Which step has the lowest capacity? (Show calculations)
2. Exploit: How can we maximize the constraint's current performance? (List 3-5 actions)
3. Subordinate: How should other steps adjust to support the constraint?
4. Elevate: What investment should we make? (Calculate ROI)
5. Repeat: What becomes the new constraint after improvement?

Tip 11: Remember Context Matters

Different industries have different constraints:

• Manufacturing: Capacity constraints are most common
• Services: Labor and skill constraints are common
• Healthcare: Equipment and regulatory constraints are common
• Finance: Process and approval constraints are common

Use industry context if provided in exam questions.

Tip 12: Final Check Before Submitting

Before finalizing your answer, ask yourself:

• "Did I identify the actual constraint?"
• "Would my solution increase overall throughput?"
• "Am I following the TOC sequence?"
• "Is my throughput calculation correct?"
• "Would I waste resources implementing this?"

Sample Exam Questions and Answers

Question 1: "A manufacturing process has four sequential steps with the following capacities: Step A (100 units/hour), Step B (85 units/hour), Step C (110 units/hour), and Step D (95 units/hour). What is the process throughput and which step should receive improvement efforts first?"

Answer: The process throughput is 85 units per hour, limited by Step B, which is the constraint. Step B should receive improvement efforts first because it is the bottleneck limiting overall process throughput. According to the Theory of Constraints, we should first exploit the constraint by eliminating idle time and inefficiencies, rather than immediately investing in new equipment. Only after fully exploiting Step B should we consider elevation (investing in additional capacity).

Question 2: "True or False: Improving the capacity of a non-constraint step will increase overall process throughput."

Answer: False. Improving non-constraint steps does not increase overall process throughput because the process is limited by the constraint, not by non-constraint steps. Process throughput equals the capacity of the constraint. Improving non-constraint capacity only increases excess inventory and ties up resources without improving output. This is why the Theory of Constraints emphasizes focusing all improvement efforts on the constraint.

Question 3: "In your process, Step 2 is the constraint with a capacity of 60 units/hour, but it is currently running at only 40 units/hour. What is your first recommended action?"

Answer: The first recommended action is to exploit the constraint by maximizing the utilization of Step 2's available capacity. Since Step 2 can handle 60 units/hour but is only processing 40 units/hour, there is 20 units/hour of untapped capacity. Before investing in expensive elevation (buying new equipment or hiring new staff), we should investigate and eliminate the causes of underutilization: idle time, setup delays, quality issues requiring rework, or insufficient work flow from upstream steps. This exploitation phase is cost-effective and should be completed before elevation investments.

Final Summary

Success on Six Sigma Black Belt exam questions regarding throughput and process constraints depends on:

• Understanding that process throughput equals the constraint's capacity
• Mastering the Theory of Constraints five-step process
• Recognizing that only constraint improvements increase throughput
• Calculating throughput correctly by identifying the minimum capacity
• Prioritizing constraint exploitation before expensive elevation
• Applying context-appropriate solutions based on the constraint type
• Supporting answers with specific data and calculations
• Avoiding the common pitfall of improving non-constraints

With focused preparation on these concepts and systematic application of TOC principles, you will confidently answer any exam question on throughput and process constraints.