,
which represents the mean (or average) number of defects per sample.If you look at your calculation sheet, you will see that the first
calculation is for the value of ,
which represents the mean (or average) number of defects per sample.
The usual way to calculate the mean of a set of values is to add them up and then divide by the number of values in the set, so we could add up all the u values and divide by 20 to get the mean. However, because the u values have all been rounded already, we can get a more accurate result by dividing the total number of defects (sum c) by the total number of items sampled (sum n). These two values have already been calculated by the person who plotted the chart, and you will find them at the bottom right of the data recording section of your printed chart.
Once you have located these values on the chart, enter them in the appropriate boxes outlined in red below. (You may like to perform the calculations again yourself, just to make sure they are correct). When you are ready, click the button to check your answers.