Let's say you need a detailed picture of a machine. Which would you rather use to create it, a pencil or a camera? The pencil is less expensive and you already own one. But using it for your picture requires drawing skills, trial and error, and plenty of time. With the camera, you just point and click.

Remember this analogy when you're deciding whether to use Excel or a software package specifically designed for statistical process control. Sure, you already own Excel, and you can do some types of SPC analysis with it. But do you have the necessary expertise, the patience for trial and error, and plenty of time?

Based on transcripts by Jeffery L. Cawley

Control Charts: 14 Steps vs. 4
In the October 1997 issue of Quality Digest, William Dorner laid out a 14-step operation for creating a set of control and X-bar charts in Excel. Let's compare this with NWA Quality Analyst, a commonly used SPC package.

In Dorner's Excel example, he asks you to do the following steps (abbreviated from the October 1997 article):

"Enter the following formula into cell F27: =AVERAGE(F2:F26) and then copy it from F27 to G27. Then compute the control limits and center line, which involves putting the following in cell I2: =$F$27. Consult a table of control chart constants to find that A2 is 0.577, and then [to calculate the upper control limit], in cell J2, type: I2+0.577*$G$27. Similarly, the lower control limit will [go into] cell J2; type: =I2-0.577*$G$27. Then copy cells H2:J2 to cells H3:J26. For the X-bar chart, highlight cells F1:F26. Because the control limits are not in contiguous columns, hold down the Control key as you also highlight cells H1:J26. Next use the ChartWizard to create a line chart of the four data series. Eliminate the gray background and border around the chart. Change the UCL and LCL lines to bold dashed lines with no markers. Change the center line to a lighter solid line with no markers. Change the X-bar data series to a bold solid line with large visible markers. Tweak the fonts, titles, headers, footers and margins as desired."

Here is a similar four-step operation in Quality Analyst:
Open the appropriate data set by clicking on the icon labeled Open an Existing Data Set. Double-click on the data set you want to use. (Note that the software links with many types of databases and spreadsheets, including Excel.) Select the variable you want to chart by clicking the heading of any column. Click the Individuals chart button.

The chart appears instantly, with full color and labels. Note that the chart includes both out-of-control conditions and pattern rule violations. Each out-of-control subgroup appears as a red pound sign (#), while each pattern rule violation is indicated by a yellow asterisk (*).

That's it. Four clicks of the mouse and 15 seconds later, you're done.

But what if you want to add a process capability histogram? Just click the Histogram button on the toolbar.

And if you want to compare all the charts together, a couple more clicks will give you the entire picture in high-quality graphics you can provide to senior management or your customers.

What if you want to apply your analysis to a different situation? With Excel, it's back to the 14 steps, making sure you entered everything correctly. With SPC software, it's just another point-and-click.

Why struggle when you can point and click?
Although Excel's capabilities for analyzing data are vast, most of us have limitations on the time and energy that we can spend getting our jobs done. These are resources that could otherwise be dedicated to making process improvements.

Of course, this comparison is not meant to criticize Excel, which is a powerful spreadsheet program. In fact, some SPC software is designed to connect directly with Excel to mine the data it contains. So if you need to store data in Excel or use it for a special analysis, you can make the most of both types of software. But it's not fair to compare Excel with a tool that is specifically designed to make SPC analysis painless and easy.

So why put yourself through a lot of exasperation? The do-it-yourself mentality is a trap when it comes to reinventing already-existing software applications that are high-quality, flexible and affordable. You may already own Excel, but the purchase price of SPC software is more than reasonable when you consider the reduction of errors and savings in time, training and usage. So work smarter, not harder. Put down that pencil and go get the right tool for the job.