P chart LCL

Question

Hi,

I am trying to figure out differences in LCL for 2 different P charts.  For one chart I see  LCL = 0 (which does make sense) and for the other chart I see LCL varying (but above zero), which also makes sense.  For both cases I have a column with the total and another with the number of defectives, the total column is used for sample size when making both charts.  The only difference seems to be the sample sizes (total) is bigger for one chart.

blchri · Answer

Hi Reg,

See _Stig_'s response.  For more information, you can view the details section of the p-chart documentation which shows the formula's used to generate the LCLs.

Best,

Blair

unknown · Answer

The control limits are calculated based on calculations of the sample standard deviations from the mean.  Its quite possible for the mathematical calculation to result in a negative value, in which case the level is set to zero (because physically it doesn't make sense for it to be negative. Of course the value of the level will depend on the mean and sample sizes hence you might see zero or non-zero levels depending on your data, and the actual value of LSL will vary if you have non-uniform sample sizes