7 Quality Control Tools – Graphs

They are known since heyday of Total Quality Management in the 1970s and they are still used and useful today: the seven quality control tools.

These tools have been selected to give shopfloor workers means to control, analyse and improve quality based on facts and objective data. The tools are simple enough to be used with minimum algebra.

The seven quality control tools are:

  1. Graphs
  2. Check Sheets
  3. Pareto Chart
  4. Cause & Effect diagram
  5. Scatter Diagram
  6. Histogram
  7. Control Chart

Their order may vary from one author to another but order is not that important.


In this post : Graphs

Graphs are a convenient tool to represent data in an easier understandable way. Graphs allow to communicate efficiently and easily without having to look at data tables and mentally depict the data meaning. As the saying tells, a picture is worth thousand words, so is any good graph.

Here is an example of a data table. Even not very big or complex, it’s difficult to see immediately where to focus.


Here is a graph built with some of the above data. The picture shows something and draws attention, even from some distance.

Graphs come in many types, a type is usually better fitting a specific purpose. According to the situation to analyze or information to share, the choice of the most suitable type of graph, and beyond the type, scale and other parameters of the graph will highlight or hide certain aspects.

The first type of graph, maybe the most common are line charts, lines joining plots and each plot is the graphical depiction of a pair of coordinates and those coordinates are the translation of specific parameters to check, e.g. km or miles per hour (speed), temperature over time, units per hour a day as above, etc.

Type Name Used for (examples)
Histograms Histograms are used to picture the data distribution. The bars represent the frequency of occurrence by classes of data. Histograms are upright bar charts.
Stacked histograms Stacked graphs are used to compare the parts to the whole, useful for looking at changes over time.
Combination Chart Bar and Line Graph Combination charts combine the features of the bar chart and the line chart. Useful when comparing values in different categories, e.g. expenses vs. budget
Bar chart, horizontal bar chart Display and compare the number, frequency or other measure for different discrete categories of data
Stacked Bar chart, horizontal bar chart Basically same as bar chart, but practical with long titles or with large number of different categories vs available space
Line chart Usually for tracking something over time, time series data
Area chart Comparisons, relative contribution over time
Pie chart Relative contribution of different categories to an overall total
Doughnut chart Variation of pie chart
Radar chart Displaying multivariate data on different axes starting from the same point
Scatter plot
Show the relationship between two or more sets of data
Bubble chart  Display three dimensions of data: x,y,size (four with color)

 

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