Seven Basic Tools of Quality
The Seven Basic Tools of Quality (also known as 7 QC
Tools) originated in Japan when the country was undergoing major quality
revolution and had become a mandatory topic as part of Japanese’s industrial
training program. These tools which comprised of simple graphical and
statistical techniques were helpful in solving critical quality related issues.
These tools were often referred as Seven Basics Tools of Quality because these
tools could be implemented by any person with very basic training in statistics
and were simple to apply to solve quality-related complex issues.
7 QC tools can be applied across any industry starting
from product development phase till delivery. 7QC tools even today owns the
same popularity and is extensively used in various phases of Six Sigma (DMAIC
or DMADV), in continuous improvement process (PDCA cycle) and Lean management
(removing wastes from process).
- Stratification (Divide and Conquer)
- Histogram
- Check Sheet (Tally Sheet)
- Cause-and-effect diagram (“fishbone” or Ishikawa diagram)
- Pareto chart (80/20 Rule)
- Scatter diagram (Shewhart Chart)
- Control chart
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1. Stratification (Divide and Conquer)
Stratification is a method of dividing data into sub-categories
and classify data based on group, division, class or levels that helps in
deriving meaningful information to understand an existing problem.
The very purpose of Stratification is to divide the
data and conquer the meaning full Information to solve a problem.
Un–stratified data (An employee reached late to office
on following dates)
5-Jan, 12-Jan,13-Jan, 19-Jan, 21-Jan, 26-Jan,27-Jan
Stratified data: (Same data classified by day of the
week )
2. Histogram
Histogram introduced by Karl Pearson is a bar graph
representing the frequency distribution on each bars.
The very purpose of Histogram is to study the density
of data in any given distribution and understand the factors or data that
repeat more often.
Histogram helps in
prioritizing factors and identify which are the areas that needs utmost
attention immediately.
3. Check sheet (Tally Sheet)
A check sheet can be metrics, structured table or form
for collecting data and analyzing them. When the information collected is
quantitative in nature, the check sheet can also be called as tally sheet.
The very purpose of checklist is to list
down the important checkpoints or events in a tabular/metrics format and keep
on updating or marking the status on their occurrence which helps in
understanding the progress, defect patterns and even causes for defects.
4. Cause-and-effect diagram. (“Fishbone”
or Ishikawa diagram)
Cause–and–effect diagram introduced by
Kaoru Ishikawa helps in identifying the various causes (or factors) leading to
an effect (or problem) and also helps in deriving meaningful relationship
between them.
The very purpose of this diagram is to
identify all root causes behind a problem.
Once a quality related problem is defined,
the factors leading to the causal of the problem are identified. We further
keep identifying the sub factors leading to the causal of identified factors
till we are able to identify the root cause of the problem. As a result we get
a diagram with branches and sub branches of causal factors resembling to a fish
bone diagram.
In manufacturing industry, to identify the
source of variation the causes are usually grouped into below major categories:
- People
- Methods
- Machines
- Material
- Measurements
- Environment
5. Pareto chart (80 – 20 Rule)
Pareto chart is named after Vilfredo Pareto. Pareto
chart revolves around the concept of 80-20 rule which underlines that in any
process, 80% of problem or failure is just caused by 20% of few major factors which
are often referred as Vital Few, whereas remaining 20% of problem or failure is
caused by 80% of many minor factors which are also referred as Trivial Many. The
very purpose of Pareto Chart is to highlight the most important factors that is
the reason for major cause of problem or failure.
Pareto chart is having bars graphs and line graphs
where individual factors are represented by a bar graph in descending order of
their impact and the cumulative total is shown by a line graph.
Pareto charts help experts in following ways:
- Distinguish between vital few and trivial many.
- Displays relative importance of causes of a problem.
- Helps to focus on causes that will have the greatest impact when solved.
6. Scatter diagram
Scatter diagram or scatter plot is basically a
statistical tool that depicts dependent variables on Y – Axis and Independent
Variable on X – axis plotted as dots on their common intersection points.
Joining these dots can highlight any existing relationship among these
variables or an equation in format Y = F(X) + C, where is C is an arbitrary
constant.
Very purpose of scatter Diagram is to establish a
relationship between problem (overall effect) and causes that are affecting.
The relationship can
be linear, curvilinear, exponential, logarithmic, quadratic, polynomial etc.
Stronger the correlation, stronger the relationship will hold true. The
variables can be positively or negatively related defined by the slope of
equation derived from the scatter diagram
7. Control Chart (Shewhart Chart)
Control chart is also called as Shewhart Chart named
after Walter A. Shewhart is basically a statistical chart which helps in
determining if an industrial process is within control and capable to meet the
customer defined specification limits.
The very purpose of control chart is to determine if
the process is stable and capable within current conditions.
In Control Chart, data are plotted against time in
X-axis. Control chart will always have a central line (average or mean), an
upper line for the upper control limit and a lower line for the lower control
limit. These lines are determined from historical data.
By comparing current
data to these lines, experts can draw conclusions about whether the process
variation is consistent (in control, affected by common causesof variation) or is unpredictable (out of control,
affected by special causes of variation). It helps in differentiating common
causes from special cause of variation.
Control charts are very popular and vastly used in
Quality Control Techniques, Six Sigma (Control Phase) and also plays an
important role in defining process capability and variations in productions.
This tool also helps in identifying how well any manufacturing process is in
line with respect to customer’s expectation.
Control chart helps
in predicting process performance, understand the various production patterns
and study how a process changes or shifts from normally specified control
limits over a period of time.
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