SPC/Control Charts

Control charts are used to differentiate & identify special causes of variation from those that are common cause related. These may be freaks/outliers, drifts, shifts, stratification, recurring patterns & systematic variation.
For variable data, use I-MR (for n=1), X(bar)-R (for n = 2 to 10) or X(bar)- s (for n>10)

For attribute data:
1. Count/proportion of defectives is estimated through binomial distribution. For constant sample size(n), estimate count of defectives using np chart, while for variable sample size, estimate proportion of defectives using p-charts.

2. Count/rate of defects is estimated through poisson distribution. For constant sample size(n), estimate count of defects using c-chart, while for variable sample size, estimate rate of defects using u-chart.