100-Metodos-de-Qualidade-Total

DATA COLLECTION, ANALYSIS AND DISPLAY 225
Method 98
U chart
Purpose
To identify when the number of defects in a sample of variable size is
changing over time. The U chart could be used to monitor errors in typing,
errors on drawings, marks in materials.
When to use
When monitoring a process to detect changes or, when a change has been
made to process inputs, to find out whether the number of defects also
changes. U charts are used when the sample size varies by more than + 25
per cent of the mean sample size.
How to use
There are seven simple steps involved:
1 Collect the data. Draw up a table showing the number of defects for
each lot number. The number of defects is called C. The sample size for
each lot is called n. The total number of lots is called m.
2 For each lot, calculate the number of defects per unit U by dividing C
by n.
3 Plot the data from the table onto the U control chart. The successive lot
numbers are shown on the horizontal axis, the number of defects per
unit U is shown on the vertical axis.
4 Calculate the centre line U. This is calculated as the sum of all the Cs,
divided by the sum of all the ns and is written as
U = lC / In
5 Calculate the control limits. The control limits are ± 3 s.d. about the
central line and are different for each different value of n. They are
calculated as
Upper control limit (UCL) = U + 3VUiY;;
Lower control limit (LCL) = U - 3VUiY;;
If the lower control limit is less than zero it is taken to be zero.
6 Draw the central line and the control limits on the control chart.
7 Interpret the results.