Statistical Process Control

Cypress-Bark-SPC

The data utilized are the weight of bags of cypress bark. There are 25 subgroups and each subgroup had 4 readings taken. The maximum specification limit is 130 kg and the minimum is 75 kg. The grouped data analysis will be calculated first then the statistical process control method will be calculated. Statistical process control is a method used to find assignable causes within a process and is a more accurate way of calculating the process capabilities.

Statistical Process Control: Text

Above is the original data given for the study. Utilizing the information given the process capability under desired specifications was calculated to be 1.11. The process capability with process centering and variability considered was calculated to be 1.132.

The histogram was constructed with weight intervals of 5. The maximum value is 120 and the minimum is 72. The range was calculated to be 48, the average is 91.94, and the standard deviation is 8.262.

Statistical Process Control: Features

The Xbar chart shows the average of each subgroup (blue line) compared to the total average (orange line), the upper control limit (gray line), and the lower control limit (yellow line). To calculate the control limits add or subtract the total average to the total range average multiplied by an xbar constant. Since the sample size for each subgroup is 4 we can find in a table for control chart constants that the constant needed is 0.729. Using this formula the upper control limit is 102.79 and the lower control limit is 81.09. Since subgroup 2 dips below the lower control limit the process is to be considered out of control. The process must be examined to find what fault produced this assignable cause so that it may be fixed in the future.

The range chart compares the range of each subgroup (blue line) to the average of the subgroup ranges (orange line) and the upper control limit for the range (gray line). To calculated the upper control limit of the range we again look at the table of control chart constant and can find the constant of 2.282 and then multiply that to the average of the subgroup ranges. This chart is meant to characterize the variability of the measurements in the process where as the xbar chart is meant to characterize the typical measurement of the process.

The individuals chart looks at each individual point in each sub group and compares them to the upper and lower tolerances originally given. This will allow us to find exact samples that are not within the specifications. It can assist in finding where in our process we need to improve.

Statistical Process Control: Features