Statistical Process Control (SPC) FAQS

Answers to the Most Common Questions Asked About Statistical Process Control

SPC FAQsHow to Choose SPC Charts

Performance Charts

What Performance Chart should I use?

The latest version of the ChartSmartXL has a Performance Chart wizard which will select the right type of chart for you. If you have an older version of the ChartSmartXL, you can buy an upgrade from our store. The right chart is based on the type of data, attribute (counted) or variable (measured), and the sample size. For attribute (i.e., integer or counted defects) data use np, p, c and u charts

If you are counting defective items (e.g. incorrect orders)
  • use a np chart if the sample size is constant
  • use a p chart if the sample size varies
If you are counting the number of defects (e.g. number of errors on an order)
  • use a c chart if the sample size is large or not available
  • use a u chart if the sample size varies
If you are using a ratio of defects/period (e.g., patient falls/1000 patient days)
  • use the XmR chart 
For variable or measured data (i.e., decimal) such as time, cost, length, weight, etc.
  • use XMR, XbarR, and XbarS Charts
  • If the sample size is one use an XMR chart If the sample size is 2-10 use an XbarR chart
  • If the sample size is 11-25 use an XbarS chart 

The ChartSmartXL pull down menu also contains a Performance Chart selector to help you choose the right chart.

Why are some lines and points red?

The ChartSmartXL performs stability analysis on your data and turns unstable points and conditions red. These red points should be investigated.

How do you calculate stability analysis?

How do I add data to a Performance Chart?

Why don't the UCL/LCL equal +3*Stdev?

Standard deviation only works for populations of "normal" data. Performance Charts use different formulas based on sample size and the type of distribution.

For a more complete explanation see our formulas page.

Why can't I change the UCL and LCL?

Control limits are calculated statistically from your raw data. They can't be specified. Don't confuse them with Specification limits which are provided by your customer.

What are the Performance Chart formulas?

Why are the sigma lines on my p, u, XbarS chart wavy instead of straight?

Varying sample sizes in p, u, and XbarS chart data will cause the sigma lines to be wavy. If you or your audience want straight lines on your u or p chart data, you can calculate a ratio (numerator/denominator) and graph the ratios using an XmR chart.

How do I run a chart for defects per 1000?

Create a new column in your Excel Spreadsheet. Create a formula = numerator/denominator*1000. Copy the formula down for all rows of your data. Select the cells in the formula column and run an XmR chart.


Other Resources

Send Us Your SPC Question

Please email any data or forms you are using to:

We also offer telephone coaching, $95 for 30 minutes. Call us today at 1-888-208-4554 to schedule a time.