A sample size of 40 patients will provide 80% or greater power for testing for equivalence between methods where the mean difference is .1 or less for up to a ten-fold increase in the estimates of the variability of the differences based on the ASTM precision measures and for a mean difference of .15 for up to a five-fold increase in these same measures. If the sample size is 40 then the power for the test of equivalence, where the difference is .15 and the standard deviation is a ten-fold increase, is 35%. Agreement testing: The Bland-Altman test will be used to test the agreement of the two methods. This test will permit a check for significance of bias between the two measures (the average difference between the two measures) as well as a check for significant trending over the range of the measurements. If the bias is significant then the results of the two differ by an overall offset in their measurements. If the trending (slope of the regression line) is significant then the difference between the two measurements changes as their magnitude changes which means the two methods are not in agreement.