Читать книгу Introduction to Statistical Process Control - Muhammad Amir Aslam - Страница 29

When the characteristics under study are measured on the continuous scale, such as the height, weight, time, and temperature, then such variables are classified as the continuous variables. The commonly used control charts are the average or the dispersion charts or the combination of both the charts. The ‐chart is the most common chart developed with the association of the R‐chart (also known as ‐ and R‐chart). The development of ‐ and S‐chart or S²‐chart is also very common. These charts have been discussed thoroughly in the latter chapters. In general the distribution of all the variable control charts under study is assumed to be normally distributed. The three‐sigma control limits are thus computed with the false alarm rate, the probability that an observation falling outside the control limits is 0.0027. The average run length of the three‐sigma control limits can be calculated as 1/0.0027 = 370.3704, which means that on average the process indicates an out‐of‐control process after 370 samples if the distribution of observations follows exactly the normal distribution. This is the most commonly used level in evaluating and comparing the process in the control chart literature. The concept of three‐sigma control limits is commonly accepted by the industry, which is easy to understand, and applied by the quality control personnel. The variable control limits can be developed using the pre‐specified constant values given in Appendix B.