Operations Management 101: Quality Management - Quality Tools, Cause-and-Effect Diagram Pareto Chart Check Sheet Histogram Scatter Diagram Control Chart Flowchart

Quality Tools Study Guide

What This Is

Quality tools are visual aids used to analyze and improve processes. They help identify problems, understand causes, and measure performance. For example, Toyota uses a combination of quality tools to achieve its renowned efficiency and quality. Let's say Toyota's production line produces 100 units per hour, but 5% of them are defective. A quality tool like a Pareto chart can help identify the root cause of the defects, such as a faulty machine or a poor design.

Key Formulas & Frameworks

• Cause-and-Effect Diagram (Fishbone Diagram): A visual representation of the possible causes of a problem, with the problem statement at the center and the causes branching out like a fishbone.
  • Problem Statement: The issue or problem to be solved.
  • Causes: The possible reasons for the problem, grouped into categories like people, machines, materials, methods, and environment.
• Pareto Chart: A bar chart that shows the relative frequency or size of problems, with the most common problems on the left.
  • Frequency: The number of occurrences of each problem.
  • Cumulative Frequency: The total number of occurrences of all problems up to a certain point.
• Check Sheet: A table or form used to collect data on a specific process or problem.
  • Data: The information collected on the check sheet, such as the number of defects or the time taken to complete a task.
  • Columns: The categories or fields on the check sheet, such as date, time, and defect type.
• Histogram: A graph that shows the distribution of data, with the x-axis representing the data values and the y-axis representing the frequency or density.
  • Bin Width: The range of values in each bar of the histogram.
  • Frequency: The number of data points in each bin.
• Scatter Diagram: A graph that shows the relationship between two variables, with the x-axis representing one variable and the y-axis representing the other.
  • X-variable: The independent variable, which is the variable being manipulated or changed.
  • Y-variable: The dependent variable, which is the variable being measured or observed.
• Control Chart: A graph that shows the performance of a process over time, with the center line representing the target value and the upper and lower control limits representing the acceptable range.
  • Center Line: The target value or average value of the process.
  • Upper Control Limit (UCL): The maximum acceptable value of the process.
  • Lower Control Limit (LCL): The minimum acceptable value of the process.
• Flowchart: A visual representation of a process, with each step or activity represented by a box or symbol.
  • Process: The series of activities or steps that make up the process.
  • Inputs: The resources or materials needed to perform each step.
  • Outputs: The results or products of each step.

Step-by-Step Application

1. Create a Cause-and-Effect Diagram:
   • Identify the problem statement and the possible causes.
   • Group the causes into categories like people, machines, materials, methods, and environment.
   • Draw the fishbone diagram with the problem statement at the center and the causes branching out.
2. Analyze a Pareto Chart:
   • Identify the most common problems (the left side of the chart).
   • Determine the cumulative frequency of each problem.
   • Use the Pareto chart to prioritize the problems and focus on the most critical ones.
3. Use a Check Sheet:
   • Design a table or form to collect data on a specific process or problem.
   • Collect data on the check sheet, including the number of defects or the time taken to complete a task.
   • Analyze the data to identify trends or patterns.
4. Create a Histogram:
   • Determine the bin width and the range of values for each bin.
   • Collect data and count the frequency of each bin.
   • Draw the histogram to show the distribution of the data.
5. Analyze a Scatter Diagram:
   • Identify the x-variable and the y-variable.
   • Determine the relationship between the two variables (e.g., positive, negative, or no correlation).
   • Use the scatter diagram to identify trends or patterns.
6. Use a Control Chart:
   • Determine the center line, upper control limit, and lower control limit.
   • Collect data and plot the performance of the process over time.
   • Use the control chart to identify trends or patterns and make adjustments as needed.
7. Create a Flowchart:
   • Identify the process and the steps or activities involved.
   • Determine the inputs and outputs of each step.
   • Draw the flowchart to show the sequence of activities.

Common Mistakes

1. Mistake: Misinterpreting the results of a quality tool.
   • Correction: Verify the data and the assumptions made when using the quality tool.
2. Mistake: Failing to consider all possible causes of a problem.
   • Correction: Use a cause-and-effect diagram to identify all possible causes and prioritize them.
3. Mistake: Ignoring the cumulative frequency of problems.
   • Correction: Use a Pareto chart to identify the most common problems and prioritize them.
4. Mistake: Failing to analyze the data collected on a check sheet.
   • Correction: Use statistical methods to analyze the data and identify trends or patterns.

Exam / Certification Tips

1. Be familiar with the key quality tools: Understand the purpose, benefits, and limitations of each quality tool.
2. Know how to apply the quality tools: Practice using the quality tools to analyze and improve processes.
3. Understand the terminology: Familiarize yourself with the terminology used in quality management, such as "push vs pull" and "Cp vs Cpk".
4. Be able to identify the most common quality tools: Recognize the quality tools used in different industries and applications.

Quick Practice Problem

Problem: A manufacturing company produces 100 units per hour, but 5% of them are defective. What is the takt time?

Answer: 20 hours (100 units / 5 units per hour)

Explanation: The takt time is the time required to produce one unit, which is the inverse of the production rate.

Last-Minute Cram Sheet

1. Cause-and-Effect Diagram: A fishbone diagram that shows the possible causes of a problem.
2. Pareto Chart: A bar chart that shows the relative frequency or size of problems.
3. Check Sheet: A table or form used to collect data on a specific process or problem.
4. Histogram: A graph that shows the distribution of data.
5. Scatter Diagram: A graph that shows the relationship between two variables.
6. Control Chart: A graph that shows the performance of a process over time.
7. Flowchart: A visual representation of a process.
8. Takt Time: The time required to produce one unit.
9. Efficiency: Actual output / effective capacity.
10. "Utilization" is actual output / design capacity – don't confuse it with efficiency.