![exponential distribution minitab 18 exponential distribution minitab 18](https://slidetodoc.com/presentation_image_h/c277e2f928a5a3f309e16577a31c5526/image-18.jpg)
Last month’s publication examined these data as a control chart. Second, the data must be somewhat normally distributed. First, the process must be in statistical control. To perform a Cpk calculation, two things need to be true. You have collected the data in Table 1 and now want to determine if the process is capable of meeting the specification set by management. There is no lower specification limit (LSL). This is the upper specification limit (USL) for our process. Management has set the goal that every customer must be greeted by a salesperson within six minutes of when they enter the store. Sometimes it is crowded in the store and it takes longer. Usually a customer is greeted very quickly. Suppose these data describe how long it takes for a customer to be greeted by a salesperson in a store. The scale is what determines the shape of the exponential distribution. Our data set consists of 100 random numbers that were generated for an exponential distribution with a scale = 1.5. We will use the same data set that we used last month to take a look at the impact of non-normal data on control charts. Previous Process Capability Publications.So, how can you handle these types of data when it comes to process capability? This publication examines how this is done using the exponential distribution as an example. These data are not described by a normal distribution. These types of data have many short time periods with occasional long time periods. For example, the exponential distribution is often used to describe the time it takes to answer a telephone inquiry, how long a customer has to wait in line to be served, or the time to failure for a component with a constant failure rate. There are many naturally occurring distributions. Remember, not all data are normally distributed. Cpk, applied to the raw data, is pretty much worthless as a measure of process capability. If your data are not normally distributed, then forget it. In our April publication, we explained why a Cpk value by itself is not sufficient for defining process capability – and that is if your data are somewhat normally distributed. The most common method of expressing process capability involves calculating a Cpk value, i.e., a process has a Cpk = 1.54. This month’s publication takes a look at process capability calculations and the impact non-normal data has on the results.