We have provided at no charge two sample files for you to download and use with your own data sets. Each file is an Excel workbook that you can use with any version of Excel from 2003 forward. The first file is SPC.xls and is described below. The second is PCA.xls and is described on the Principal Components Analysis page.
Statistical Process Control
The SPC.xls workbook is not mathematically sophisticated. But it can produce good, solid analysis of processes over time, pointing out for you possible clues to a series of events that has gone out of control – or that might be about to.
SPC.xls performs statistical process control on data collected over a range of dates or times. You can obtain any of these:
- X-and-S charts
- X-and-R charts
- X-and-MR charts
You can also call for the so-called "Western Electric Rules" to be used, in which case the X values are flagged if they constitute a violation of one of the rules:
- Any point at least 3 sigmas from the center line
- Two of three consecutive points at least two sigmas from the center line
- Four of five consecutive points at least one sigma from the center line
- Six consecutive points on the same side of the center line
SPC.xls supports various other options, including:
- Showing or suppressing the control limits on the charts
- Choice of starting point (average, median, first point, zero, user-specified value)
- Recalculating the control limits if a Western Electric rule is violated
- Evaluating the data for sampling, should you want to determine the minimum sample size needed to detect a process shift
- Estimating the process parameters prior to the end of the data set. This option can be useful if you have implemented a change in the process at some known point.
You can download the SPC workbook here.