Situation
Meredith has been hired as production manager for a company that manufactures overhead conveyor systems. The assembly of these systems is critical and each component part must meet specifications on several characteristics. During her orientation, Meredith was told that the company rarely has any difficulty meeting specifications because they use process control on all characteristics of the components. After several weeks on the job, Meredith realized that no one ever spoke of using the process control information, but there were discussions about several incidents of finding components out of specification for thickness.
She asked to see the data and charts being used for process control on the thickness of Component A. The quality manager, Kevin, showed her recent thickness data and the charts currently in use.
Answers to December Brain Teaser
As the newly appointed quality manager at a glass container manufacturing plant, Xavier reports on key characteristics of the containers during production meetings. These reported values are used for making decisions and taking action on the production process. Internal burst pressure is critical because some containers are filled with carbonated beverages. The previous quality manager reported only one value, and as long as it was between 380 and 400 it was accepted as OK. However, Xavier is concerned that one value does not give enough information to ensure that pressure will meet customer requirements.
Q: If Xavier decides to report the average pressure value at the production meeting, will one number (one value) satisfy both production lines?A: The two production lines have predictable process behavior with different average pressure values. See the graph, “Production Lines 1 and 2 Pressure.” Production Line 1 has an average pressure of 397.2 psi, while Production Line 2 has an average of 379.2. These two average values are essentially within the range familiar to those in attendance at the production meetings and might be considered “close enough” to report only one average value. However, the average does not tell the whole story. Production Line 1 has the higher average, but the standard deviation for Production Line 1 is 95.4 psi. This gives natural process limits of 111 psi to 683 psi. Production Line 1 does not meet the minimum specification of 180 psi. Production Line 2 has the lower average with a standard deviation of 52.6 psi. The natural process limits are 222 psi to 537 psi. Thus, Production Line 2 easily meets the minimum specification.
Q: Do the current data indicate that the pressure values are all in spec?A: There is one pressure value from Production Line 1 that is below the minimum specification. None of the pressure values from Production Line 2 are below specification.
Q: What should Xavier report at the production meeting in order to give complete information about pressure so the other managers can make appropriate decisions?A: In order to give information that is complete and useful for making decisions about the production process as well as decisions regarding the finished glass containers, Xavier needs to report the average, standard deviation and natural process limits for each production line. He also needs to report the process behavior for each production line. If the rest of the people who attend the production meeting are unfamiliar with standard deviations and natural process limits, Xavier can present a histogram showing the potential for meeting specification. See “Histogram of Production Line 1 Pressure.” All of the information is present on the histogram and is easy for people to read, interpret and make decisions.
