Sometimes it is convenient to make a control chart simply by plotting the number of nonconform- ing items X (from now called d) in the sample instead of the proportion of nonconforming P. This is particularly true if the sample size n is constant and the nonconforming proportion P is quite small. Such a chart, commonly referred to as an np chart, provides essentially the same information as the p chart. The basic difference between the p chart and np chart is a scaling of the vertical axis by a con- stant n. The upper control limit for the np chart is n times the upper control limit of the p chart, that is, UCL (np chart) = n UCL (p chart).