I WANT TO BE A CONTROL FREAK ! The ultimate objective of a process is to make product which conform to specifications. Its the role of process control to get the most out of these processes by running them at well aimed and consistent levels of performance. A Control Chart is a graphical comparison of how the process is performing. Its data usually consists of groups of measurements selected at regular intervals. The Control Chart helps us distinguish between 'random' and 'assignable' causes of variation through its choice of Control Lines which have been calculated from the laws of probability. Control Lines are placed on the chart either side of the mean which has been determined from the preceding process data. A Control Charts prime use is to detect 'assignable causes', in other words, causes we can find.

COMMON CAUSE v SPECIAL CAUSE VARIATION COMMON CAUSE VARIATION. Natural variation within the process. Permanent unless drastic action is taken, cannot be dealt with by the setter. It's the effect of many factors. It's known as 'capability' and can be estimated before bulk production. SPECIAL CAUSE VARIATION. Due to a real change in the process. Can be dealt with by the setter. Due to one factor. Makes the process 'out of control'.

ANALYSIS OF CHARTS AND TRENDS All the points are 'normally distributed'.

IT'S CROSSED THE LINE All the points are NOT 'normally distributed'. Easy to see All the points are NOT 'normally distributed'.

What's in a trend?

TWO ON THE TROT Easy to see The chance of two points on the control line is unlikely (about 700 to 1).

7 ABOVE or 7 BELOW Easy to see Will we get better tool life? Can we save raw material if we leave it here? The 'chance' of this happening normally is very slim. The 'mean' has shifted on a permanent basis.

7 UP 7 DOWN This is not a random pattern - it's not normal. Easy to see It's all down hill from here. Are the tools wearing out suddenly? Does the m/c warm up? This is not a random pattern - it's not normal.

Is it raw material or batch rotation? IT'S DOING IT AGAIN. May be difficult to see Is it raw material or batch rotation? Is it two operators? This is not a random pattern - it's too repetitive. The process is 'cycling'.

THE MIDDLE BIT'S MISSING. Difficult to see Are the control lines in the right place? Are the operators measuring the same? This is not a random pattern - less than 40% of the points lie in the middle 1/3 of the control lines. We have a 'mixture'.

LOOK - NO VARIATION! Can be difficult to see Are the control lines in the right place? If this is real, what's made it improve? This pattern is not normal - more than 90% of the points lie in the middle 1/3 of the control lines. It's known as 'stratification'.

WHAT ARE WE GOING TO DO NOW? The customer wants as little variation in the product as possible. Removing common causes may be difficult and costly. Special causes are "one off's". Special causes can be corrected by the setter. We may want to 'engineer in' some special causes.