Question

Analyse each of the following phrases which may be used to monitor and control project quality

1- Tolerance versus control limit

2- Precision versus Accuracy

3- Attribute versus Variable

4- Sample versus population

Answer 1

1- Tolerance versus control limit

• Tolerance limit also called as specification limit it represents the voice of the customer. The control limit represents the voice of the process that is how the process is performed
• Tolerance limits are the target set for the product/process by the market or the customer . which means it is intended result on the metric that is measured. whereas the control limit is indicator of the variance in the performance of the process. means it is the actual values that the process is being operated
• tolerance limit is used to determine whether individual manufacture component is acceptable whereas the control limit used to control the manufacturing process. For the quality control limits are preferred in order to produce the good quality product

2- Precision versus Accuracy

• Precision is when the value of the repeated measurement are clustered and with little scatter. if you get the measurement as precise that does not mean that the values are very close to the target value. it shows measurements are only close to each other, it may be or may not be near to the target value. Whereas the accuracy shows how the measured values are close to the target values. if the measurement is accurate is shoes that measurements are very near to the target value. scatter is not having any significance role here.
• For quality purpose Accuracy is preferred compared to the precision.
• Accurate values can be Precise but precise values need not to be accurate. precision does not dependent on accuracy.

3- Attribute versus Variable

• Attribute sampling contains single pass/fail criteria. the product or service either pass or fail whereas in variable sampling include sliding scale criteria. the product is rated on the scale of 1-10 or class A-E or similar
• Attribute sampling either conforms or not conforms whereas the variable sampling the result is rate on the continuous scale that measures the degree of conformity.

4- Sample versus population

• it is not possible or practical to monitor and analyze the entire population , therefore the samples are obtained from part or portion of the population.
• When there is a measure to attempt over entire population , then it is called as census.. but it is costly, time consuming and destructive. so the sampling is used
• in order to monitor and measure the quality of the product then it is best to use sampling rather than population