Prepared By: Mr. Prashant S. Kshirsagar (Sr.Manager-QA dept.)

◦ Introduce the basic concepts of an attribute measurement systems analysis (MSA). ◦ Understand operational definitions for inspection and evaluation. ◦ Define attribute MSA terms. ◦ Define Procedure for conducting attribute MSA ◦ Demonstrate trial for conducting attribute MSA 2

 A measurement systems analysis is an evaluation of the efficacy of a measurement system.  The purpose of Measurement System Analysis is to qualify a measurement system for use by quantifying its accuracy, precision, and stability.  It is applicable to both continuous and attribute data.

 Most problematic measurement system issues come from measuring attribute data in terms that rely on human judgment such as good/bad, pass/fail, etc. This is because it is very difficult for all testers to apply the same operational definition of what is “good” and what is “bad.”

 When, we are not getting any measurement values then the tool used for this kind of analysis is called Attribute gage R&R.  The R&R stands for repeatability and reproducibility.  Repeatability : is the variation in measurements obtained with one measurement instrument when used several times by one appraiser while measuring the identical characteristic on the same part.  Reproducibility : It is defined as the variation in the average of the measurements made by different appraisers using the same measuring instrument when measuring the identical characteristic on the same part. .

 To evaluate product features and make accept/reject decisions.  Mandatory criteria for establishment and use of operational definitions include:  A) Criteria that can be applied to an object (or a group of objects) which precisely describes what is acceptable and unacceptable.  B) A written description of the process for collecting data, including the method in which accept/reject decisions will be made.  C) Review of the accept/reject criteria with people who will do the inspections to ensure that the requirements are understood.

 Select at least 20 parts to be evaluated during the study.  At least 5 of the parts should be defective in some way. If larger sample sizes are used, include at least 25% defective parts.  Care should be taken when selecting defective parts – If possible select parts which are slightly beyond the specification limits or acceptance standards. Label each part with proper identification.  Three inspectors will evaluate each part thrice (Three trials).  A fourth person should record the data. Note down the observations in the form of 1 or 0, 1 is OK, 0 is not ok.  The order of inspections should be randomized after each group of inspections to minimize the risk that the inspector will remember previous accept/reject decisions. The inspectors must work independently and cannot discuss their accept/reject decisions with each other.

Appraiser AAABBBCCC Trialsiiiiiiiiiiiiiiiiii The data recorder may use a table similar to the one given below. 0 Not Ok 1 Ok

 Type 1 Errors: when a good part is rejected.  Type 1 errors increase ‐  Manufacturing costs. Incremental labor and material expenses are necessary to re – inspect, repair, or dispose the suspect parts.  Type 1 errors are also called as “Producer’s Risk” or alpha errors.  Type 2 Errors: when a bad part is accepted.  Type 2 errors may occur  Perhaps the inspector was poorly trained or rushed through the inspection and inadvertently overlooked a Small defect on the part.  When Type 2 errors occur, defects slip through the containment net and are shipped to the customer.  Because Type 2 errors put the customer at risk of receiving defective parts; customer may raised the complaint!  Type 2 errors are sometimes called as “Consumer’s Risk”.  Type 2 errors are also called as “beta” errors.

What is effectiveness?  The effectiveness of an inspection process is correct call! ◦ Correct Call (Cc):- The number of times of which the operator (s) identify a good sample as a good one.  Effectiveness = number of correct evaluations number of total opportunities

What is False Alarm?  False Alarm (Fa) – The number of times of which the operator (s) identify a good sample as a bad one.  The probability of a false alarm, also known as Type I error or producer’s risk, is given by:  Fa (False Alarm) = number of false alarms number of non-defective items

What is Miss rate?  A miss is a defective item that is classified as non- defective.  Miss rate (Mr) – The number of times of which the operators identify a bad sample as a good one.  The probability of a miss, also known as Type II error or consumer’s risk, is given by: Mr (Miss rate) = number of misses number of defective items

 Acceptability criteria: If all measurement results agree, the gage is acceptable. If the measurement results do not agree, the gage can not be accepted, it must be improved and re-evaluated. EFFECTIVENESS (< 80% - Not Acceptable) MISS - RATE ( > 5% - Not Acceptable ) FALSE ALARM RATE( > 10% -Not Acceptable)

 What could have caused the poor agreement?  What should be done to improve the measurement system?  What should be done to improve consistency?  Do the Brain Storming!

 If any of the decisions disagree, the measurement system may need improvement.  Improvement actions include:  Reworking the gage,  Re ‐ training the inspectors,  Clarifying the accept/reject criteria,  Adding more lighting  After implementing the improvement actions, repeat the study. If the error cannot be eliminated,  Must take appropriate corrective actions, such as switching to a new measurement system, adding redundant inspections, or conducting a more extensive study.

Exercise