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What is a u-CHART

The u-chart is appropriate in the same situations as the c-chart. The differences are in the calculation of control limits and that the plotted points represent the average number of nonconformities per unit, instead of just the number of nonconformities. A u-chart must be used instead of a c-chart if the unit size of a subgroup varies from subgroup to subgroup.

The following symbols are used on a u-chart:

n — The number of units in a subgroup (the subgroup, sample, or subset size).
k — The number of subgroups in the current calculation.
c — The number of defects or nonconformities found in a subgroup.
u — The average number of defects or nonconformities per unit in a subgroup. 

These are the points that are plotted on the chart. The formula is:

The average number of defects or nonconformities per unit in a subgroup.

u-bar — The grand average number of defects or nonconformities per unit for the study period. This is drawn as the centerline on the chart. It is found by dividing the total number of defects found in all subgroups of the study period by the total number of units inspected in the study period. The formula is:

u-bar formula

How to Make and Use a u-Chart

Refer to the example in Figure 3.7.1.

 The number of dents counted on car hoods per day is shown in the table below. The number of hoods inspected varies from day to day. Note that an average for n is used

data for 3.7.1

1) Gather and record the data.

2) Calculate u-bar.


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