p chart

p chart is a type of control chart used in statistical process control to monitor the proportion of defective items in a process over time.

Map the data fields

• Subgroup/Lot: This field represents the different lots or batches of items being inspected.
• Process variable: This field contains the number of defective items in each lot.
• Sample Size: This field indicates the number of items inspected in each lot.
• UCL: Optional field for custom upper control limits.
• LCL: Optional field for custom lower control limits.
• Central line: Optional field for custom center line values.

Calculation Methodology

The p-chart calculations are performed as follows:

Proportion defective (p̄)

p̄ = Total Defectives / Total Samples

Standard Error for Each Sample

Standard Error = √(p̄ × (1 - p̄) / Sample Size)

Control Limits

For each data point:

Upper Control Limit (UCL):

UCL = p̄ + (Z-Score × Standard Error)

Lower Control Limit (LCL):

LCL = p̄ - (Z-Score × Standard Error)

Center Line (CL):

CL = p̄ (overall proportion)

Proportion for Each Sample:

Proportion = Defectives / Sample Size

Notes:

• If custom limits are provided, they override the calculated values
• The Z-Score determines the confidence level (typically 3 for 99.7% confidence)
• Each sample can have a different sample size, resulting in variable control limits
• The actual proportion plotted is the number of defectives divided by the sample size for each lot