LCL in a U Chart

[calpoly]

Greetings everybody,
I am putting together a U-Control Chart (Nonconformities per unit)and seem to be getting a negative LCL. I've checked my formula and seems correct. What does the negative LCL mean? Thank you in advance for your responses. It will help me a great deal.

 

[BillPSU]

What is your number of inspected in the subgroup? Mathematically you can get a negative LCL but you should substitute zero as your LCL. As you approach one as the inspected parts in a subgroup you can calculate a negative LCL.

 

[leanIE]

Here's a link that makes a good point about using LCLs at all for U charts.....I don't think there is a definitive answer to cover every application of the U-chart.

http://www.sytsma.com/tqmtools/uchart.html

Here's another that says assume a LCL of 0 whenever the data cannot possibly be lower than 0.

http://www.hanford.gov/safety/vpp/spc.htm

clear as mud?

 

[calpoly]

Thank you so much for your responses so far.