# Comparing Two Measurement Devices

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Comparing Two Measurement Devices
Brian Novatny 2003

Background Each measurement made by an instrument or measuring device consists of the true, unknown level of the characteristic or item measured plus an error of measurement. In practice it is important to know whether or not the variance in errors of measurement of an instrument, or the imprecision of measurement, is suitably small as compared to the variance of the characteristic or product measured, or the product variability.

Background For efficiency of the measuring process, the variance in errors of measurement should be several or many times smaller than the variability of the characteristic measured or product variance. High measurement error causes the power of most statistical tests to decrease unless compensated for by larger sample sizes Power = 1 - Beta Beta is the Type II error

Measurements are indicated by
Device 1 = B1 + Xi + Ei1 where B1 = bias for device 1 Xi = true value Ei1 = random errors for device 1 Device 2 = B2 + Xi + Ei2

Regression Approach Y = b0 +b1*X
Device 1 = Intercept + Slope * Device 2 Intercept value should be zero, if not, it indicates bias of the two devices Slope term should be around 1 indicating “Similarity” Mean Square Error estimate “Precision” R-Square term estimates some measure of strength

Problems with Regression
The X variable (independent variable) should be measured without error this is never the case, but errors in the X variable should be small, and it won’t be when comparing devices they will be on equal footing Asymmetry - specifically designate one device to predict the other prefer symmetric approach where is doesn’t matter which variable is the input and which is the output Inverse regression can pose several problems when trying to resolve the asymmetry problem

Problems with Regression
What should the R-square value be? No objective justification How to handle the case when there are multiple measuring devices? Pairwise comparisons multiple testing error problem

Some Solutions Grubbs model
uses sums and differences Pittman and Morgan along with Maloney and Rastogi provide proofs and refinements to Grubbs model Blackwood and Bradley multivariate test on bias and precision Tan and Iglewicz modify the standard regression approach based on Mandel’s work of Errors in Variables

Grubbs Model Involves calculating the Sums and Differences of the two devices The differences will be used to estimate bias standard paired t-test with n-1 degrees of freedom Performing a correlation analysis analysis on the sums and differences is used to estimate precision of the two devices follow student’s t distribution with n-2 degrees of freedom Provides two independent tests one for bias of the two measurement devices one for precision equivalency

Simultaneous Test for Precision and Bias
Multivariate approach instead of independent tests Differences are regressed on the Sums Difference = Intercept + Slope * Sums Model F-test uses the UNCORRECTED SUMS OF SQUARES to get the correct number for the df instead of the familiar corrected sums of squares the overall Type I error (Alpha) rate is exact

Simultaneous Test for Precision and Bias
If the Model F indicates significance, then tests for the Bias and Precision are just the individual F-tests the overall test is generally more powerful it can reject the equivalence assumption of the two devices even though each individual test does not

Simultaneous Test for Precision and Bias
The Precision test is exact The Bias test is exact only when the precision between the two devices is equal use the paired difference t-test otherwise more powerful Uses UMVU (uniform minimum variance unbiased) estimate of the variance

Type I Error Rate is exact Overall test could reject even though individual test do not power of test Statistical modeling usual array of diagnostics residuals

Regression approach Regression can still be used, but an adjustment to the model must be made In Simple Linear Regression, the results (Beta Hat) are achieved by minimizing the sum of squared residuals in the direction of the dependent variable The correction is to achieve Beta hat by minimizing the sum of squared residuals in the direction of -Lambda/Beta hat

Regression approach Lambda is a calculated value to adjust the slope calculations called the Precision Ratio ratio of the machines repeatability machine 1 / machine 2 or inverse Lambda is determined by performing the standard Gauge RxR studies or taking repeated values

Regression approach The approach still uses regression and Lambda value helps solve the asymmetry problem as lambda approaches infinity, implies X values approach 0, so Beta hat is Sxy/Sxx, which is where X is the independent variable as lambda approaches 0, implies Y values approach 0, so Beta hat is Syy/Sxy, which is where Y is the independent variable

Regression approach Approach handles precision but not bias
Uses Polar coordinates for confidence intervals Slope = TAN (Theta) Intercept = Tau/COS (Theta)

Example Device1 Device2 Sum Diff 5.00 4.73 9.73 0.27

Standard Regression Analysis
R-Square = 98.24% SQRT MSE = 0.31 Device 2 = * Device 1 95% Confidence Intervals Intercept (-1.07,0.52) Slope (1.07,1.31) Conclusion No Bias, but not similar

Simultaneous Test for Bias and Precision
Regress Diff on Sums (Uncorrected SS) Model F = ==> p-value = Therefore, devices are different relative to their bias and precision Individual Precision Test F = 17.5 ==> p-value Individual Bias Test (not exact) F = ==> p-value = Paired t-test has a T value = 6.98 and p-value less then

Correct Conclusion The two devices measure differently Strong Bias
Strong lack of precision (repeatability)

Potential Problems Methods do not account for a difference in Gain, or slope of devices Devices might measure equally well or poor at the low and high ends of the scale, but the relationship is not constant collect data at one end of the data range power of the test could be compromised

Multiple Measuring Devices
Grubbs and others propose technique for three measuring devices comparisons when one device is a “Standard” with three devices, get a more powerful test Multivariate methods lead to fuller choice of sub-hypothesis and can be used regardless of the number of measurement devices

Multiple Measuring Devices
One method involves performing a multivariate regression on q-1 measurement devices independent variable = mean of each part dependent variable = deviations from that mean Independent variable is averaged each part across all the measurement devices Dependent variable is calculated by the differences of each value from that mean

Multiple Measuring Devices
Generally have to fit a Full model and a Reduced model (intercepts only) Then compare the two models usually through some matrix manipulation Technique can be performed by most software packages that can perform MANOVA techniques

Authors Opinion As the title says, this is just my opinion and not based on any concrete proof such as simulation studies My preferred method of analysis would be the Multivariate approach using Blackwood and Bradley’s Regression with the Uncorrected Sums of Squares this procedure seems to have a more powerful test in finding differences eliminates the possibility of getting a negative variance, which Grubbs method could get

Authors Opinion With the Multivariate approach, there is a natural extension to testing more than two measuring devices Of course, there is no reason to try both the multivariate approach and Grubbs approach since they are easily computed using standard data analysis techniques

Final Comments These methods are not to replace Gage RxR studies, but to evaluate two devices against each other Each device should be tested for bias and repeatability and linearity as desired corrective action should be taken as needed

Final Comments The test for Bias is only a test for agreement between the two devices, not a bias against a standard both devices could be grossly off from the standard (but in the same direction and amount) If there is a claim that one device is “superior” to another (better precision), these methods could prove the validity of the claim and provide the precision estimates

References for Two Device Comparison
Grubbs, F.E. (1973). “Errors of Measurement, Precision, Accuracy and the Statistical Comparison of Measuring Instruments” , Technometrics Vol. 15 pp Bradley, E.L. and Blackwood, L.G (1991). “An Omnibus Test for Comparing Two Measuring Devices”, Journal of Quality Technology, Vol. 23 pp Tan, C.Y. and Iglewicz, B. (1999). “Measurement-Methods Comparisons and Linear Statistical Relationship”, Technometrics, Vol. 41 pp

References for Multiple Device Comparison
Christensen, R. and Blackwood, L.G (1993). “Tests for Precision and Accuracy of Multiple Measuring Devices”, Technometrics, Vol. 35 pp Bedrick, E.J. (2001). “An Efficient Scores Test for Comparing Several Measuring Devices”, Journal of Quality Technology, Vol. 33 pp