The effects of correlated bias uncertainties are considered for both single and comparative tests, and the implications of these effects on experimental results and their uncertainties are investigated. In single tests, correlated bias uncertainties can cause a significant increase or a significant decrease in uncertainty in the result as compared to the uncertainty when there are no correlation effects. In comparative tests, the experimental result is the difference of two test results or the ratio of two test results. If the same test apparatus and instrumentation are used to obtain the two test results, the systematic uncertainty in the difference or the ratio can be significantly less than the systematic uncertainly in the individual test results. However, the effects of systematic uncertainties do not cancel out in comparative tests in which the result is the difference in the results of two tests, contrary to assertions made in some engineering standards.

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