c chart

c chart is a type of control chart used in statistical process control to monitor the count of defects in a process over time, where the sample size is constant.

Map the data fields

• Subgroup/Lot: This field represents the different lots or batches of items being inspected.
• Process variable: This field contains the count of defects found 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 c-chart calculations are performed as follows:

Average defect count (c̄)

c̄ = Total Defects / Number of Samples

Standard Deviation

Standard Deviation = √c̄

Control Limits

For each data point:

Upper Control Limit (UCL):

UCL = c̄ + (Z-Score × Standard Deviation)

Lower Control Limit (LCL):

LCL = c̄ - (Z-Score × Standard Deviation)

Center Line (CL):

CL = c̄ (average defect count)

Defect Count for Each Sample:

Defect Count = Number of defects in each lot

Notes:

• If custom limits are provided, they override the calculated values
• The Z-Score determines the confidence level (typically 3 for 99.7% confidence)
• The actual defect count is plotted directly for each lot
• Control limits remain constant since sample size is fixed
• No sample size field is required as the inspection area/unit is assumed constant