Business Statistics

658 Business Statistics

17.7.7 np Chart

The np chart is used to control the actual number of defective items in a sample when the sample size

is constant. The concept of binomial probability distribution explained in Chapter 6 can be used to

find the probability of observing x defective items in a sample of size n. We also know that the mean

of binomial distribution is np, and the standard deviation of binomial distribution is

npq

, where p

is the probability of observing a defective item when the process is in control. For a large sample size,

the distribution of number of defective items observed in a sample size can be approximated by