Statistical Process Control (SPC) FAQS

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.

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.

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.