Improvement Insights Blog

A customer called today confused about her data. She wanted to draw a control chart and thought the data might have a binomial or poisson distribution. She thought it was attribute data. She’d used the QI Macros Control Chart Wizard to create a control chart of her data and it chose an XmR chart. She wasn’t sure that was right. When I asked her what kind of data she had, she said, “write-offs”.

Write-offs are money, plain and simple. Money is variable (a.k.a. continuous or measured) data.

I explained that to her and suggested she stop worrying about what kind of distribution she has and just look at her data.

• Decimal data has to be variable/continuous/measured (e.g., 7.45, $27.95, 9.5%)
  If you have a single column or row of decimal data use the XmR Chart.
• Integer data can be attribute (defects) or variable (days). Examples: 12 defects or 12 days.
  If it’s time, use the XmR Chart.
• Integers with a Numerator/Denominator means that you will need either a p or a u chart. Example: 2/100 widgets.
  If the data is good/bad (binomial) use a p chart.
  If you can have more than one defect per unit use a u chart.
  If the denominator is a constant size, use an np chart.
  You can also covert the numerator/denominator to a ratio (e.g., 2/100 = 0.02 = 2%) and plot it as an XmR Chart. I personally find it easier to explain an XmR chart to a management team than a p or u chart with varying control limits. They get lost in why the limits vary and stop looking at the data.
• Integer data of unlikely events (e.g., injuries) would be a c chart.

You don’t need to know anything about distributions to start drawing your first control charts. You just need to know if it is decimals or integers.

Let the QI Macros Control Chart Wizard to do this for you auomatically.