The Psychology of Quality and More
Pareto Chart: How to do it
How to do it
Break down the group into actual items to be measured. It can be useful to do this in several different ways plotting several charts, in order to find the most spiky chart and thus the most significant grouping. For example, product sales may be broken down by industry, geographic area and turnover, with a Pareto Chart being drawn for each.
If possible, aim not to have too many items in each group (about seven is a good maximum), or else bars will be too low to be of use. Thus, when breaking down by industry, you might use major groupings such as 'chemical' and 'medical' rather than finer groups such as 'acids' and 'polymers'. If a group is found to be significant, then it can be broken down
Use a measure which best reflects the key objective. For example, if the aim is to reduce cost, then measure the total failure cost of each defect type rather than the number of defects. The figure below shows how using a different measurement unit can significantly change the Pareto ordering.
Fig. 1. The effect of weighting
If the measurement unit is a simple frequency, aim for a total of 50 or more measurements.
In any repeat measurement, keep constant any variables which might distort the figures. This includes the sample size or measurement period, along with anything else which might affect the result, such as people, materials, etc. If this cannot be done, then recognize it and take it into account when interpreting the final chart.
Common sense should always be used during interpretations, as the highest bars do not always represent the best action items. For example, in producing a report, 'analysis' may be a high-value activity, so the aim may be to increase this at the expense of other activities. There may also be other selection criteria to take into account, such as the time and cost of corrective actions.
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