You cannot use the p control chart unless the probability of each shipment during the month being on time is the same for all the shipments. The point to remember is that it is three standard deviations of the binomial distribution - not the standard deviation you get from calculating the standard deviation using something like Excels STDEV function.įor example, some people use the p control chart to monitor on-time delivery on a monthly basis.Ī p control chart is the same as the np control chart, but the subgroup size does not have to be constant. The average and standard deviation of the binomial distribution are given below. The binomial distribution is a distribution that is based on the total number of events (np) rather than each individual outcome. The control limits equations for the p and np control charts are based on the assumption that you have a binomial distribution. This means you must have 20 participants each time, or you may take a random sample that is the same each time. This means that sometimes you can have 20 participants, another time 22, another time 18 and so on. In this example, p npn 220.10 or 10 of the participants did not meet the requirements.Īs an instructor, you can track this data for each workshop. The number of participants in the workshop who do not complete the requirements is denoted by np. You have implemented a process that requires each participant to pass a written exam as well as complete a project in order to be given the title of green belt.Įither a participant completes the requirement or does not complete the requirement.Ī defective participant is one who does not complete the requirements. The type of data you have determines the type of control chart you use.Įach item inspected is either defective (i.e., it does not meet the specifications) or is not defective (i.e., it meets specifications). There are two basic types of attributes data: yesno type data and counting data.
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