10 THE SEVEN QUALITY CONTROL (7 QC) TOOLS: AN OVERVIEW
10.9 Scatter diagram
This is a tool for charting the relationship between two variables to determine whether there is a correlation between the two which might indicate a cause-effect relationship or indicate that no cause-effect relationship exists (Juran & Godfrey, 1998).
Scatter plot illustrates the degree of correlation between two variables (also called correlation diagram). The independent variable is plotted on horizontal (x) axis and the dependent variable on the vertical (y) axis. The results, when plotted on a graph, will give what is called a scatter graph, scatter plot or scatter diagram (Dale, 2003).
An example of a scatter diagram is given in Figure 10.8.
Source: Based on ASQ Scatter Diagram Template
Figure 10.8. An example for scatter diagram 10.10 Control chart
The control charts are based on statistical data and are used to record and monitor the progress of the process over time. Its significance lies in the fact that we can intervene in a timely manner and keep the process in the right path by constantly monitoring the process.
Control charts are graphical tools to monitor the activity of a process. The control charts only record the presence of a problem. It is not possible to expect quality improvement or process stability from the control chart itself. This requires feedback, appropriate intervention to the process.
The first step in the use of SPC (Statistical Process Control) is to collect data to a plan and plot the gathered data on a graph called a control chart (see Figure 10.9). The control chart is a picture of what is happening in the process at a particular time: it is a line graph (Dale, 2003).
A control chart, then, is a graphic representation of the variation in the computed statistics being produced by the process. It has a decided advantage over presentation of the data in the form of a histogram in that it shows the sequence in which the data were produced.
It reveals the amount and nature of variation by time, indicates statistical control or lack of it, and enables pattern interpretation and detection of changes in the process (Juran & Godfrey, 1998).
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 lines are determined from historical data. By comparing current data to these lines, we can draw conclusions about whether the process variation is consistent (in control) or is unpredictable (out of control, affected by special causes of variation) (ASQ, n.d).
Source: Based on ASQ Control Chart Template
Figure 10.9. An example for control chart References
AIGPE (n.d.): 7 Basic QC Tools. Retrieved from
https://www.academia.edu/15364733/7_Basic_Quality_Control_Tools_for_successful _process_improvement (8 April 2019)
ASQ (n.d.): The 7 Basic Quality Tools for Process Improvement. Retrieved from https://asq.org/quality-resources/seven-basic-quality-tools (27 February 2019)
Dale, B. G. (2003): Tools and Techniques: An Overview (Chapter 16). In: Dale, B. G. (ed.):
Managing Quality, Fourth Edition, Blackwell Publishing, Oxford. pp. 308–348.
Farkas, G., Tóth, G. N. (2018): Folyamatjavítás, - fejlesztés I.: A minőségjavítás eszközei I. (in english: Process improvement and development I.: Tools for Quality Improvement I.
Óbudai Egyetem (Óbuda University), Budapest
Hebb, N. (n.d.): Flowchart Symbols and Their Meanings: Flowchart Symbols Defined.
Retrieved from https://www.breezetree.com/articles/excel-flowchart-shapes/ (12 May 2019)
ISO 9000:2015: Quality management systems. Fundamentals and vocabulary (3 – Terms and definitions)
Jagadeesh, R. (2015): Application of 7 QC Tools to Investigate the Rejection of Lathe Beds – Case Study of a Machine Tool Manufacturing Company. Cases in Management, 4 (1):
37–58.
Juran, J. M., Godfrey, A. B. (1998): Juran’s Quality Handbook. McGraw-Hill, New York Paliska, G., Pavletic, D., Sokovic, M. (2007): Quality tools – systematic use in process industry.
Journal of Achievements in Materials and Manufacturing Engineering, 25 (1): 79–82.
Pavletić, D.,Soković, M., Paliska, G. (2008): Practical application of quality tools. Quality Festival 2008. 2nd International Quality Conference, Kragujevac, 13-15 May, 2008.
Singh, J., Singh, P. (2015): Assessment of Procurement-Demand of Milk Plant Using Quality Control Tools: A Case Study. International Journal of Industrial and Manufacturing Engineering, 9 (10): 3350–3355.
Soković, M., Jovanović, J., Krivokapić, Z., Vujović, A. (2009): Basic Quality Tools in Continuous Improvement Process. Journal of Mechanical Engineering, 55 (5): 1–9.
REVIEW QUESTIONS
1. What are the two crucial meanings of quality? 103 2. What is improvement? 103
3. What did Ishikawa say about quality problems in relation with the 7 QC tools? 103 4. What are the 7 QC tools? 103
5. What are the roles of 7 QC tools? 106 6. How and when to use a flow chart? 107 7. How and when to use a check sheet? 108 8. How and when to use a histogram? 109 9. How and when to use a Pareto chart? 110
10. How and when to use a cause-effect diagram? 111 11. What is the role of scatter diagram? 113
12. What is a control chart? 113
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