The u-Chart monitors the average number of defects per item in a sample (unlike another type of attribute chart, the c-Chart which records the actual number of defects in a sample).
Remember, the u-Chart is used where the sample size is not fixed, but is based on a target sample size. The target sample size is the base figure for each sample, from which any actual sample can vary by a maximum of ±20%.
Provided that the difference between each sample does not exceed this figure, the same control limits can be used in plotting the whole of the chart. If this figure is exceeded (by either a larger or a smaller sample) the control limits will have to be re-calculated for that sample.
If you had a sample size of 240 ±20%, what would be the minimum and maximum sizes of sample which would not require a re-calculation of the control limits?
(Note: A simple way to find 20% of a given number, is to divide the number by 10 and then multiply by 2.)