The ratio of the accuracy (or tolerance) of the unit under test to the accuracy of the reference standard used to calibrate it. TAR is the predecessor to TUR and does not account for measurement uncertainty.
The Test Accuracy Ratio (TAR) is an older metric that compares the accuracy specification of the instrument being calibrated to the accuracy specification of the reference standard. A TAR of 4:1 means the reference standard is four times more accurate than the unit under test. While simple to calculate, TAR has significant limitations because it only compares specifications and ignores the actual measurement uncertainty of the calibration process.
TAR was widely used before the modern uncertainty-based approach became standard practice. It considers only the ratio of the stated accuracies or tolerances of the two instruments, without accounting for environmental effects, repeatability, resolution, or other sources of uncertainty that affect the real-world calibration measurement. Because of these limitations, TAR has been largely superseded by TUR in modern calibration practice.
In calibration management, understanding the difference between TAR and TUR is important because some legacy procedures and specifications still reference TAR. When converting from TAR-based to TUR-based approaches, organizations often find that their actual TUR is significantly less favorable than the TAR suggested, because measurement uncertainty encompasses more error sources than instrument specifications alone. Modern standards like ISO 17025 and ANSI/NCSL Z540.3 require uncertainty-based (TUR) analysis rather than specification-based (TAR) comparisons.
In aerospace calibration labs, TAR calculations are critical when calibrating precision torque wrenches used for critical fasteners. For example, when calibrating a 100 ft-lb torque wrench with ±2% accuracy using a deadweight torque standard with ±0.5% accuracy, the TAR is 4:1 (2%/0.5%). This meets typical aerospace requirements but ignores measurement uncertainty contributions from environmental factors and operator technique. Medical device manufacturers face similar challenges when calibrating infusion pump flow meters. A flow meter requiring ±2% accuracy calibrated against a gravimetric flow standard with ±0.4% accuracy yields a TAR of 5:1. However, this approach fails to account for temperature variations, fluid viscosity effects, and timing uncertainties that could significantly impact the actual measurement uncertainty. Getting TAR wrong leads to serious audit findings - auditors frequently cite labs that rely solely on manufacturer specifications for TAR calculations without considering real measurement uncertainty. A defense contractor recently faced a major nonconformance when their pressure transducer calibrations showed adequate TAR ratios on paper, but actual measurement uncertainty analysis revealed the calibrations were inadequate for their 0.1% accuracy requirements, leading to suspect test data on critical components.
TAR is referenced in ANSI/NCSL Z540.3-2006 Section 6.2.1.1 as a legacy approach for establishing measurement traceability ratios, though the standard emphasizes that TUR (Test Uncertainty Ratio) is the preferred method. ISO/IEC 17025:2017 Section 6.5.2 requires consideration of measurement uncertainty rather than simple accuracy ratios, effectively superseding TAR-based approaches. AS9100D Section 7.1.5 mandates that measurement equipment accuracy be suitable for measurements being made, but doesn't specifically mention TAR, instead requiring uncertainty analysis per ISO/IEC Guide 98-3 (GUM). ISO 13485:2016 Section 7.6 requires measurement equipment to provide valid results but emphasizes measurement uncertainty over simple ratio calculations. ILAC P14:01/2013 Section 5.4.4 explicitly states that simple ratio methods like TAR are insufficient for modern calibration requirements. During audits, assessors specifically look for evidence that labs have moved beyond TAR to comprehensive uncertainty analysis, checking that calibration certificates include measurement uncertainty statements rather than relying on manufacturer specifications alone. Auditors frequently cite nonconformances when labs cannot demonstrate how environmental conditions, drift, and other uncertainty sources are considered beyond basic TAR calculations.
CalibrationOS addresses TAR through its Standards Management module, which automatically calculates TAR ratios when users input both UUT specifications and reference standard accuracies during calibration planning. The system captures manufacturer tolerance specifications, reference standard uncertainties, and environmental conditions to provide comprehensive ratio analysis. The Certificate Generator module produces calibration certificates that clearly distinguish between historical TAR calculations and modern TUR/uncertainty-based results, helping labs transition from legacy practices. During audit preparation, the Compliance Dashboard flags instruments where only TAR ratios were calculated, prompting metrologists to perform full uncertainty analysis. The system's Measurement Uncertainty Calculator integrates with TAR data to demonstrate how simple ratio approaches can underestimate actual measurement uncertainty. CalibrationOS generates audit-ready reports comparing TAR and TUR values, showing auditors that the lab understands both historical and current best practices for establishing measurement traceability ratios.
Test accuracy ratio (TAR) is the ratio of the unit-under-test accuracy to the reference standard accuracy. It is a simpler but less rigorous metric than TUR because it does not account for the full measurement uncertainty.
Modern calibration standards (ISO 17025, ANSI/NCSL Z540.3) require TUR, which accounts for full measurement uncertainty. TAR is a legacy metric that may understate the actual risk of false accept decisions.
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