Revision Date: 2005-09-06
| The objective of an uncertainty study is to put bounds on a measurement (x plus/minus some interval). Generally a 95% confidence interval and the t-distribution is used since there are not a lot of observations taken. To do the study use one gage, one part, one part characteristic, and one operator. The characteristic is measured on the part a number of times-10, 15, 25, etc. A standard deviation is calculated (that should be all measurement error), and the appropriate t value obtained and multiplied times the standard deviation. This becomes the plus/minus interval estimate about the measured value (uncertainty).
If you would like a bias estimate using this same data and have a reference value for the part characteristic used, you may place it in the reference block and the program will take the mean of the measurements and subtract the reference value to give an average bias value.
If you would like the bias expressed as a percent of specification or percent of process variation, you enter the USL, LSL and/or six sigmas for the spread of the process (generally obtained from an X-Bar and Range chart on the process), and the respective percents will be calculated.
I am not aware of any guide lines that have been given for these values. When you do a calibration, there is a plus/minus interval that is acceptable. This might be one source for determining acceptable values for both uncertainty and bias values. When you do an R&R, there are guidelines for the percent R&R. This may provide input into development of acceptable percentages.
Gordon Constable, Ph.D. | |
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