msa in brief
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8/18/2019 MSA in Brief
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Understand MSA in brief before getting in the depths
What is MSA? A Measurement system is a process by which we assign a number to a characteristic of a product or service.The_ first step in assessing a system is to understand this process, and determine whethe_r it will satisfy our requirements.
Measurement System Analysis, often referred to as MSA, is used to assess the_ statistical properties of process measurement systems.
• MSA primarily deals with analyzing the_ effect of the_ measurement system on the_ measured value in
quantifiable terms
• mphasis is on the_ effect due to equipment and personnel
• !e test the_ system to determine the_ numerical values of its statistical properties and compare the_m to
accepted standards. "t is a process used to standardize the_ methods of analysis to ensure anddemonstrate that your measuring systems have adequate resolution, do not e#cessively bias results, andpossess a small variability in comparison with specified tolerances. "t helps you assess, monitor, andreduce measurement system variation. The_ ob$ective of a measurement system analysis study is toma%e sure that your measurement system & gages, methods, and procedures are stable and capable ofmeasuring data, before continuing with your process improvement efforts. "t ensures that your measuringprocedures and systems provide'
• Adequate resolution
• (esults that are not unduly biased, and
• )ittle variability in comparison with specified tolerances
Fundamental IssuesTo evaluate a measurement system determine'
• "f it has adequate discrimination
• "f it is statistically stable over time
• "f statistical properties are consistent over the_ e#pected range and acceptable for process analysis or
control
• "f the_ sum of all variables is an acceptable level of measurement uncertainty
Overall Objective of MSA*ncertainty of Measurement
"t is the_ range within which the_ true value of a characteristic is estimated to lie. Such data can be e#pressedas the_ statistical distribution of a series of measurements, standard deviations, probability, percentages, anderror as the_ difference between actual value minus the_ true value, as points on a control chart or diagram.
Bestin!lass Approach• +etermining the_se fundamental issues are most meaningful if made relative to process variation
• (eporting measurement error as only percent of tolerance is inadequate for the_ worldwide mar%et where
emphasis is on continual process improvement
Use of "ata in Measurement S#stem StudiesThe_ data collected using a measurement system is used'
•
To control process• "n estimating the_ e#istence of relationship between variables that can affect the_ outcome of a process
• To conduct analytical studies to increase the_ %nowledge about the_ system of causes and its effect on
processes
• To focus on measurement systems where readings can be repeated on each part, and reproduced by
different operators. A measurement is not always e#act. Measurement system variation affects individual measurements anddecisions based on data. Measurement system errors are classified into five categories' bias, repeatability,reproducibility, stability, and linearity. ou need to %now the_ e#tent of variation before deciding on the_ following applications.
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MSA Applications• stablish criteria for suitability and acceptability of new measuring equipment
• -ompare one measuring device against anothe_r
• valuate suspect equipment
• -ompare the_ performance of an equipment before and after its repair
• -alculate measurement system variation
•
stablish acceptability of manufacturing process• Manage improve the_ measurement process
Where to start?• valuate the_ components of the_ measuring system, and control the_ variation in the_m as much as
possible to ensure that an item of measuring equipment complies with the_ requirements for its intendeduse
• #pand your consideration of Measurement /rocess 0ariation to Measurement System Statistical /roperties
Measurement *ncertainty.
• 1ollow the_ basics of S/-.
$rocess Accurac# Measurements
Stabilit#
Stability 2or drift3 is total variation in measurements obtained with a measurement system on same master or parts when measuring a single characteristic over an e#tended time period 2a time period is days, not hours3.Stability is the_ %ey to predictability. Stable processes are those that are free from special cause variation. "nterms of measuring equipment, stability is determined by using a control chart. As measurements are ta%en,points within the_ limits indicate that the_ process has not changed and the_ prediction is made that it is notli%ely to change in the_ future. Statistical process control 2S/-3, scatter plots, or othe_r forms of statisticalanalysis are used to measure process stability.
%inearit#)inearity is the_ difference in the_ accuracy values through the_ e#pected operating range of the_ equipment.Selecting the_ parts throughout the_ operating range of the_ instrument can determine the_ linearity. The_ accuracy of the_se parts is determined by the_ difference between the_ master measurement and the_ observed average measurement.
Bias
+ifference between observed average of measurements and reference value. The_ reference value, also%nown as accepted reference value or master value, is a value that serves as an agreed upon reference for measured values. A reference value can be determined by averaging several measurements with a higher levelof measuring equipment.
$rocess $recision Measurements
&age '('4age (( statistically isolates different types of variation in the_ measurement process. The_se types of variation include'
• (epeatability 5 equipment variation 5 within variation
• (eproducibility 5 appraiser variation 5 between variation
• (esidual or pure error
• 0ariation due to interaction effects. 1or e#ample, out of several inspectors, one might have a tendency to
read one gage differently than othe_rs.4age ((& 4age (epeatability and (eproducibility can be applied to any %ind of measurement 2attribute or variables, indeterminate or determinate3. The_ two most common methods used and supported by statisticalsoftware are the_ A670A method 2Analysis 7f 0ariance3 and the_ average and range method.
(epeatability refers to the_ variation in measurements obtained with one measurement instrument when usedseveral times by one assessor while measuring the_ identical characteristics on the_ same part.
(eproducibility refers to the_ variation in the_ average of measurements made by different assessors usingthe_ same measuring instrument while measuring the_ identical characteristics on the_ same part.
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(( is the_ combination of repeatability and reproducibility variation, and is considered as the_ totalmeasurement variation e#cluding within part variation and variation in central location.
'eference Material A material or substance with one or more properties, which are sufficiently well established to be used for the_ calibration of an apparatus, assessment of a measurement method, or for assigning values to materials.
Measurement Uncertaint#• Measurement *ncertainty is the_ sum of all the_ probabilities 2percents3 assigned to the_ variables that
ma%e up the_ measurement system.
• The_ total of the_se probabilities should be weighed, and carry importance in proportion to the_
seriousness, and criticality of the_ measurements being made.
• +ecisions resulting from measurement system analysis include'
o*sing the_ system as is, ta%ing into account its uncertainty.o "mproving the_ system to control the_ variation in the_ contributing factors.o-onsidering othe_r measurement systems of higher levels of discrimination and capability.
/roduct and process conformance are determined by the_ measurements ta%en by a measurement system. "f the_ measuring process is changing over time, the_ ability to use the_ data gathe_red in ma%ing decisions isreduced. "n essence, establishing the_ adequacy of your measurement system using a measurement systemanalysis process is fundamental to measuring your own business process capability, and meeting the_ needs of your customer.