3.1.1. Calculating Upper control limits
In order to establish the limits of the control chart based on the percentage of positive results for each month (also called a p-chart), the count of positive results follows a binomial distribution with parameters n and p (where n is the number of samples and p the percentage of positive results). As stated in the last section, two UCLs have been established, which leads to three different areas:
1) an area of no concern (white area): the results obtained during this period warrant no further investigation,
2) an area of potential concern (yellow area): these results and their potential causes need some form of review, and
3) an area of concern (red area): these results must be investigated in detail.
It is important to note that the control chart is used primarily for
two cases where i) npð1 pÞ>5 and 0:1