7 QC Tools

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Introduction & Why use the 7 QC Tools?

The 7 QC tools helps to analyze the data and most helpful in problem solving methods. It is the fundamental tool to improve our product and process quality by identify and analyzing the problems.

As per the Deming chain to achieve the organizational goal, we must tackle the product & process related problem and to analyze these problems we get help from 7 QC tools. This 7 QC tools gives us the analytical and statistical competence to solve the problems.

What are 7 QC tools?

7 Basic Quality techniques

  1. Pareto Charts
  2. Cause and Effect Diagrams
  3. Histograms
  4. Check sheet
  5. Scatter Diagrams
  6. Control Charts
  7. Flow Charts

Pareto Chart

Purpose:

  • Prioritize problems.
  • Pareto Charts are used to apply the 80/20 rule of Joseph Juran which states that 80% of the problems are the result of 20% of the problems. A Pareto Chart can be used to identify that 20% route causes of problem.

How is it done?

  • Create a preliminary list of problem classifications.
  • Tally the occurrences in each problem classification.
  • Arrange each classification in order from highest to lowest
  • Construct the bar chart

Benefits:

  • Pareto analysis helps graphically display results so the significant few problems emerge from the general background
  • It tells you what to work on first

To know in details What is Pareto Principle?, How to Make Pareto in Excel?

Cause & Effect Analysis

Purpose:

  • Graphical representation of the trail leading to the root cause of a problem
  • It’s a diagram that demonstrates the relationship between Effects and the categories of their causes
  • The Arrangement of the Diagram lets it look like a fishbone it is therefore also called fish-bone diagram

How is it done?

  • Decide which quality characteristic, outcome or effect you want to examine (may use Pareto chart)
  • Backbone –draw straight line
  • Ribs – categories
  • Medium size bones –secondary causes
  • Small bones – root causes

Benefits:

  • Breaks problems down into bite-size pieces to find root cause
  • Fosters teamwork
  • Common understanding of factors causing the problem
  • Road map to verify picture of the process
  • Follows brainstorming relationship

To learn in details How to create a cause and effect diagram (Fishbone diagram)?

Histogram

Purpose:

  • A Histogram is a bar graph
  • To determine the spread or variation of a set of data points in a graphical form
  • usually used to present frequency

How is it done?

  • Collect data, 50-100 data point
  • Determine the range of the data
  • Calculate the size of the class interval
  • Divide data points into classes Determine the class boundary
  • Count # of data points in each class
  • Draw the histogram

Benefits:

  • Allows you to understand at a glance the variation that exists in a process
  • The shape of the histogram will show process behavior
  •  Often, it will tell you to dig deeper for otherwise unseen causes of variation.
  • The shape and size of the dispersion will help identify otherwise hidden sources of variation
  •  Used to determine the capability of a process
  • Starting point for the improvement process

Check Sheet

Purpose:

  • Tool for collecting and organizing measured or counted data
  • Data collected can be used as input data for other quality tools

Benefits:

  • Collect data in a systematic and organized manner
  • To determine source of problem
  • To facilitate classification of data (stratification).

Scatter Diagram

Purpose:

  • To identify the correlations that might exist between a quality characteristic and a factor that might be driving it
  • A scatter diagram shows the correlation between two variables in a process.
  • These variables could be a Critical to Quality (CTQ) characteristic and a factor affecting it two factors affecting a CTQ or two related quality characteristics.
  •  Dots representing data points are scattered on the diagram.
  • The extent to which the dots cluster together in a line across the diagram shows the strength with.

Benefits:

  • Decide which paired factors you want to examine. Both factors must be measurable on some incremental linear scale.
  • Collect 30 to 100 paired data points.
  • Find the highest and lowest value for both variables.
  • Draw the vertical (y) and horizontal (x) axes of a graph.
  • Plot the data
  • Title the diagram

The shape that the cluster of dots takes will tell you something about the relationship between the two variables that you tested.

You may occasionally get scatter diagrams that look boomerang- or banana-shaped.

  • To analyze the strength of the correlation, divide the scatter plot into two sections.
  • Treat each half separately in your analysis

Benefits:

  • Helps identify and test probable causes. 
  • By knowing which elements of your process are related and how they are related, you will know what to control or what to vary to affect a quality characteristic.

Control Chart

Purpose:

  • The primary purpose of a control chart is to predict expected product outcome.
  • Statistical tool, showing whether, a process is in control or not.
  • Taking samples of a process and detect possibility of process being out of control

How does it Work?

  • Define Upper limit, lower limit and medium value
  • Draw Chart.
  • Gather values and draw them into chart

Benefits:

  • Predict process out of control and out of specification limits
  • Distinguish between specific, identifiable causes of variation
  • Can be used for statistical process control
Strategy for eliminating assignable-cause variation:
  • Get timely data so that you see the effect of the assignable cause soon after it occurs.
  • As soon as you see something that indicates that an assignable cause of variation has happened, search for the cause.
  • Change tools to compensate for the assignable cause.
Strategy for reducing common-cause variation:
  • Do not attempt to explain the difference between any of the values or data points produced by a stable system in control.
  • Reducing common-cause variation usually requires making fundamental changes in your process

Flow Chart

Purpose:

  • Visual illustration of the sequence of operations required to complete a task.
  • Schematic drawing of the process to measure or improve.
  • Starting point for process improvement
  • Potential weakness in the process are made visual.
  • Picture of process as it should be.
  • Way of representing a Procedure using simple symbols and arrows

How is it done?

  • List major steps
  • Write the process step inside each symbol
  • Connect the Symbols with arrows showing the direction of flow
  • List sub-steps under each in order they occur

Benefits:

  • Identify process improvements
  • Understand the process
  • Shows duplicated effort and other non-value-added steps
  • Clarify working relationships between people and organizations
  • Target specific steps in the process for improvement.
  • Simplest of all flowcharts
  • Used for planning new processes or examining existing one
  • Keep people focused on the whole process
  • Show what happens at each step in the process
  • Show what happens when non-standard events occur
  • Graphically display processes to identify redundancies and other wasted effort

Benefits of all – Tool-wise