Higher National Certificate in Engineering Unit 36 – LO4.1 Out of Control Conditions

Learning Outcome 4.1 Be able to analyse types of variation within a process and record information on that variation

How good is your Process? There is more to interpreting control charts than just looking to see it a point on the x-bar or R chart is over the control limit. Indeed the individual who analyses the chart needs to be aware that even when the data is within the control limits there are circumstances that portray the fact that the process is out-of-control.

How good is your Process? A trend of 7-continusouly up o 7-continuously down would statistically be impossible. This would indicate that a special cause is ‘showing’ its presence. * *

How good is your Process? A trend of 7-above x-bar-bar or r-bar or 7-below x-bar-bar or r-bar would again indicate an out of control condition. This is because for a normal distribution we would expect equal distribution either side of x-bar-bar. * *

How good is your Process? 2/3 and 1/3: i.e. you would expect 2/3 of the plotted points to be within the middle 1/3 of the chart. This is because within ± 1-σ of a normal distribution, you would expect 68% of the plotted points to be found. If less that 2/3 are within the middle 1/3 then the process is not in control and the control limits would need to be recalculated or the process brought back into control.

How good is your Process? *

How good is your Process? 2/3 and 1/3: indeed if all the points are within the middle 1/3, then again the process is out-of-control – this time to the good. Something has happened that has improved the process. The control limits can therefore be recalculated as the process can be controlled within tighter control limits.

How good is your Process? *

How good is your Process? A cyclical pattern: if a pattern can be determined with any regularity from the plotted points within the control limits then the presence of a special cause is portraying itself and the cause needs investigation. *