4. A hospital is using X and R charts to record the time it takes to process pat
ID: 331402 • Letter: 4
Question
4. A hospital is using X and R charts to record the time it takes to process patient account information. A sample of five applications is taken each day. The first four weeks' (20 days') data give the following values: (15 Points) Xbar - 16 Minutes and Rbar 7 Minutes If the upper and lower specifications are 21 minutes and 13 minutes, respectively, calculate and interpret the indices: Estimate of the Population Standard Deviation (Sigma), 6 Sigma, Cp.and Cpk. Interpret the results. 5. Explain what is meant by variation and what occurs when a process is in control or out of control. The text refers to this as chance or assignable cause. Give examples of an assignable cause and a chance cause. (10 points) 6. Using the data in Question 1 prepare a XmR Control Chart (20 points) a. What is the Mean and the Moving range for the data. b. Interpret the control chart 7. What do control limits represent? What do Specification limits represent? (15 points) a. Describe the three cases that compare specification limits to control limits. b. What is the impact of each on quality?Explanation / Answer
What is Variation:
In simple terms, variation is the spread or dispertion from a central point or mean. Variation indicated the amount by which a process is stretched. For example, if we are trying to cut a log of wood into 5 parts each of length 20 cm and the actual measures are 19.5, 19.6, 20.2, 20 and 20.3 then the values that are lesser than or greater than 20 indicate the variation. Variation in statistics can be measured by the following paramters; range, variance, standard deviation, etc.
Process in Control & Process Out of Control:
To Determine if a process is in control or not first UCL (upper control limit) and LCL (Lower Control Limit) are defined. UCL and LCL indicate the variation above and below the central value that is caused by natural variation in the process. A process is said to be out of control if the output from the process lies outside the range of UCL and LCL.
If all the points lie within UCL and LCL then the process is said to be in Control
Assignable Cause & Chance Cause:
An assignable cause is a non-random cause that causes variation. Assignable causes can be detected and usually few in number. An out of control process indicates the presence of non-random causes of variation that is presence of assignable cause. Assignable causes can be detected and eliminated practically.
E.g.: Untrained worker is an assignable cause. It is not a variation inherent in the system and can be eliminated easily.
A chance cause is something inherent in the process. A single chance cause adds to a small amount of variation. However, there are usually numerous chances causes present in a process. It is impractcal and very difficult to eliminate chance causes.
e.g. Vibration caused during machining process
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