Question

Question with regards to Statistical Process Control.

Control chart systems can operate on two basic methods of measurement. State these two methods and briefly distinguish between them, giving three examples of industrial processes where each might be applied. Specify an appropriate sampling procedure in each case.

Answer 1

Control chart systems can operate on two basic methods of measurement. State these two methods and briefly distinguish between them, giving three examples of industrial processes where each might be applied. Specify an appropriate sampling procedure in each case.

ANSWER :

Control charts are also known as " Shewhart charts " Control chart is a statistical process control tool used in various areas of interests to get to know the state of control in any particular process. they are meant for process monitoring, stability and helps us a lot in taking any necessary action in case of any discrepancy found in a process. these control charts are used to find any unusual variations in the process. Let us assume that we are supposed to maintain the humidity of certain value in a room. We know the upper limit and lower limit and we can adopt a control chart for maintaining humidity readings at a particular interval of time and plot them against time to see their variations with respect to time also we get to know at what time the variation is huge or at what time humidity went out of specification etc. this chart gives us basically the variation. This variation is may be due to the common cause which is inherent in the process it may also due to the special cause.

Control charts are distinguished based on the date available

1) Continuous data-Continuous data are not restricted to defined separate values but can occupy any value over a continuous range.  something like height is continuous, but often we don't really care too much about tiny differences and instead group heights into a number of discrete bins. Humidity recording in a room with respect to time is also a continuous data. We get humidity values over a rage with respect to time.

For example, Xbar-Range chart is most commonly used chart for continuous data.

The main elements in this chart are

1 ) Chart begins with time series graph

2) Upper control limits and lower control limits and a central line is drawn for a visual

  reference .detectingshifts and also trends in the graph.

Like I said earlier for humidity control we can have X bar- range chart by specifying an upper limit, the lower limit of humidity and mean value. The X-bar chart is used to evaluate consistency in the process while R-bar chart plot the ranges of each subgroup.

2)Discrete data-Discrete data can only take particular values. There may potentially be an infinite number of those values, but each is distinct. We can think in the way that defects in castings. if we count the number of defects we have it in one casting, we get one particular number.

the above diagram is the C- chart for discrete data. C-chart is the most commonly used discrete data chart. in case of casting example, it is used for counting of defects per unit that occurred during the sampling period. here we can assign more than one defect per casting. but the condition is the number of samples for each sample should be the same.