8/4/2019 Overview of 7 QC Tools

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An Overview of the Seven QC ToolsSeven QC Tools Description Method of use Remarks

Pareto

diagrams

A diagram on which undersirable events or

costs associated with items such as quality

(e.g. number of defects or non-confirming

products), productivity, cost, safety and so

on are stratified according to their causes

or manifestations and plotted in order

There may be large number of undersirable

phenomena or causes or trouble. The Pareto

diagram makes it easy to see which of these

have the most serious effect on quality,

productivity, cost, safety etc. together with

their relative proportions.

Plot the "Other" category at the

far right of the diagram and

ensure that it is not too large.

Cause -and-

effect diagram

A diagram shaped like the bones of a fish

for systematically summarizing the

relationships between quality

characteristics, defects etc. and their

causes.

Useful for searching out the factors that affect

the characteristics, sorting out the

relationships between these factors (causes)

and the characteristics (the results), and

depicting these systenatucally.

Gather the opinions of as many

people as possible in order to

flush out all the relevant factors.

Graphs and

charts

Diagrams for plotting data and showing

temporal changes, statistical breakdowns

and relationships between differentquantities.

Used for organizing data. Use line graphs for

showing time trends, bar graphs for

comparing quantities and pie charts forshowing relative proportions.

Use solid lines, dotted lines,

circles and corosses skillfully for

clarity,

Checksheets Forms specially prepared to enable data to

be collected simply by making check

marks.

Use for tallying the occurrence of the defects

or causes being addressed and graphing or

charting them directly.

Claify the objective and design a

checksheet to suit it.

Histograms Prepared by dividing the data range into

subgroups and counting the number of

points in each subgroup. The number of

points (the frequency) is then plotted as a

height on the diagram.

Prepare separate, stratified histograms for

each of the 4Ms and examine the relationships

between the shapes of the distributions and

the specifications.

Use at least 30 values,

preferably aruond 100

Scatter

diagrams

Prepared by plotting paired sets of data

such as hardness and tensile strength,

temperature and yield, porosity and

insulation resistance, etc. against each

other on X and Y axes.

Collect paired sets of data on causes and

effects, and use scatter diagrams to check for

correlation between the sets of data.

Use at least 30 values, 50 if

possible.

Control charts Prepared by plotting time along the

horizontal axis and a characteristic value on

the vertical axis. Unlike line graphs, they

also show the control limit lines.

Use to check whether there are too many

chronic defets, too much variation, values

lying outside the control limits, or undesirable

trends or cycles. Control charts show whether

or not a process is in control.

Think about the best method of

stratification and pay close

attention to subgrouping.