Quality control tools

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Quality control tools
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Introduction

• Seven QC tools are fundamental instruments to
  improve the quality of the product. They are used
  to analyze the production process, identify the
  major problems, control fluctuations of product
  quality, and provide solutions to avoid future
  defects. Statistical literacy is necessary to
  effectively use the seven QC tools. These tools
  use statistical techniques and knowledge to
  accumulate data and analyze them.
• Seven QC tools are utilized to organize the
  collected data in a way that is easy to
  understand and analyze. Moreover, from using the
  seven QC tools, any specific problems in a
  process are identified.

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7QC tools always include

• Check Sheet 
• Pareto Chart 
• Cause-and-Effect Diagram 
• Histogram 
• Scatter Diagram 
• Flow Chart 
• Control Chart 

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Check Sheet 

• A check sheet is a pre-designed format for
  collection of data that encourages organized
  collection and groups data into categories.  
• When to use it  To keep track of the parameters
  of an on going process.  It can be used to track
  events by such factors as timeliness, reason for
  inspection failure, etc
• How to use it Look at some preliminary data
  before developing the check sheet.  This will
  indicate what categories to use.

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• Used for the collection of quantitative or
  qualitative repetitive data. Adaptable to
  different data gathering situations. Minimal
  interpretation of results required. Easy and
  quick to use. 

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Pareto Chart 

• A simple rule, pareto, 20 issues causes 80
  results. This means, 80 if problems come from
  20 of reasons. 80 of results come from 20 of
  work. 80 of cost come from 20 of spent
  area...and so on.
• When to UseUse it when there are many problems
  or opportunities and you want to focus on the
  most important. Or when your analyzing data about
  the frequency of problems or causes in a process.

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Constructing a Pareto chart

• 1.Decide the categories to compare.
• 2.Determine the appropriate measurement.
• 3.Decide the period of time for the chart.
• 4.In a table, collect the data for each
  category. 
• 5.Arrange the categories in descending way per
  the total sums.
• 6.Construct and label bars for each category. 
• 7.Calculate the percentage for each category.
• 8.Calculate and draw cumulative percentage.
• Analysis of the Pareto Chart

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Cause-and-Effect Diagram (Fishbone Diagram)

• The Cause and Effect diagram identifies many
  possible causes for an effect or a problem.
• When to UseWhen your  figuring out the possible
  factors for a problem. When your teams thinking
  hits a roadblock.

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Procedure

• 1- Agree on the problem (effect).
• 2- Brainstorm the possible major categories of
  causes of the problem (5M and 1E).
• 3- Write the categories of causes as branches
  from the main arrow.
• 4- Brainstorm all the possible factors for each
  of the major categories. 
• 5- Again ask why does this happen? about each
  branch from step 4. 
• 6 After completion, have the team review the
  diagram

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Example
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Histogram

•  shows a bar chart of accumulated data and
  provides the easiest way to evaluate the
  distribution of data.
• This chart graphs data distributions. If you have
  numerical, variable, continuous data you can use
  the this chart. The chart organizes and sorts the
  data. It shows the data in a pictorial format.

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Construction

• Collect at least 50 data points from a process.
• Use the worksheet to set up the chart.
• After calculating width in step 2 of the
  worksheet, use your judgment and adjust it to a
  convenient round number.
• Draw x- and y-axes on graph paper. Mark and label
  the y-axis for counting the data values. Mark and
  label the x-axis with the L values from the
  worksheet.
• For each data point, locate it where it fits
  within the Ls. Mark off one count above the
  appropriate bar with an X or by shading that
  portion of the bar.

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Scatter Diagram

•  is a graphical tool that plots many data points
  and shows a pattern of correlation between two
  variables.
• Scatter Diagram Is used to investigate the
  possible relationship between two variables that
  both relate to the same "event".
• When to Use
• When you have paired numerical data.
• When trying to identify potential root causes of
  problems.
• After brainstorming causes and effects using
  a fishbone diagram, to determine objectively
  whether a particular cause and effect are
  related.
• When determining whether two effects that appear
  to be related both occur with the same cause.

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Procedure

• 1. Collect pairs of data where a relationship is
  suspected.
• 2. Draw a graph with the independent variable on
  the horizontal axis and the dependent variable on
  the vertical axis.
• 3. Look at the pattern of points to see if a
  relationship is obvious. 
• 4. Divide points on the graph into four
  quadrants. If there are X points on the graph.
• 5. Count X/2 points from top to bottom and draw a
  horizontal line.
• 6. Count X/2 points from left to right and draw a
  vertical line.
• 7. If number of points is odd, draw the line
  through the middle point.
• 8. Count the points in each quadrant.
• 9. Add the diagonally opposite quadrants.
• 10. A points in upper left points in lower
  right
• 11. B points in upper right points in lower
  left
• 12. Q the smaller of A and B
• 13. N A B
• 14. Look up the limit for N on the trend test
  table.
• 15. If Q is less than the limit, the two
  variables are related.
• 16. If Q is greater than or equal to the limit,
  the pattern could have occurred from random
  chance.

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Example
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Flow Chart

•  shows the process step by step and can sometimes
  identify an unnecessary procedure.
• When to Use 
• One of the first activities of a process
  improvement effort is constructing a flowchart.
  It provides the following benefits 
• 1- It give you and everyone a clear understanding
  of the process. 
• 2- Facilitates teamwork and communication. 
• 3- Helps to identify non-value-added operations.

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Method

•  There are many symbols used to construct a
  flowchart the more common symbols are shown
  below 
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• Identify the process steps and link them together
  with direction arrows

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Control Chart

•  provides control limits which are generally
  three standard deviations above and below
  average, whether or not our process is in
  control.
• Control charts are graphs used to study how a
  process changes over time. 
• Data is plotted in time order. A control chart
  always has a central line for the average, an
  upper line for the upper control limit and a
  lower line for the lower control limit. These
  three lines are determined from historical data. 

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• When to Use 
• When controlling ongoing processes by finding
  and correcting problems as they occur. 
• When predicting the expected range of outcomes
  from a process. 
• When determining whether a process is stable
  (in statistical control). 
• When analyzing patterns of process variation
  from special causes (non-routine events) or
  common causes (built into the process). 
• When determining whether your quality
  improvement project should aim to prevent
  specific problems or to make fundamental changes
  to the process.

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Basic Procedure

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• 1. Choose the appropriate control chart for your
  data. 
• 2. Determine the appropriate time period for
  collecting and plotting data. 
• 3. Collect data, construct your chart and analyze
  the data. 
• 4. Look for out-of-control signals on the
  control chart. When one is identified, mark it on
  the chart and investigate the cause. Document how
  you investigated, the root cause and how it was
  corrected.

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Example