ASTM E177 Standard Practice for Use of the Terms Bias and Precision in ASTM Test Methods

Get Certified ASTM E177 Testing

ASTM E177 evaluation is suitable for ASTM test-methods developers, analysts, technicians, quality-control personnel, engineers, laboratories, manufacturers, and organizations that need to understand or document the statistical performance of quantitative test methods. The practice is for preparation and interpretation of precision and bias statements for ASTM test methods. By applying principles of ASTM E177, qualified analysts can judge the amount of variation within and between test conditions, determine relationships to established reference values, and effectively explain statistical characteristics. This assists users in deciding if a test method performs well enough for the intended use.

• Quantitative Test Methods: Used when a measured property is given numerical values. 
• Precision: Discusses the range of variation that would be anticipated if a test method were used under specified conditions.
• Bias: Assesses the association between the average value of a test and the known reference value. 
• Sources of Variability: Considers factors such as operator, apparatus, environment, sample, and time.
• Statistical Evaluation: Provides concepts for evaluating and expressing statistical characteristics of test methods.
• Precision Statements: Guides communicating expected variability within and between testing conditions.
• Bias Statements: Guides describing the relationship between test results and accepted reference values.
• Test-Method Reporting: Provides examples and guidance for incorporating precision and bias information into ASTM test methods.

What are the uses of ASTM E177?

ASTM E177 is a standard that sets a standard basis for talking about the statistical performance of quantitative test methods. It aids analysts, method developers, and technical experts in identifying variability due to the test method from systematic differences due to reference values. The practice also supports clearer interpretation of precision and bias statements when comparing test results. Common uses include:

• Test Method Development: Supporting the development of precision and bias statements for quantitative ASTM test methods.
• Method Evaluation: Assessing the expected variability and statistical performance of a test method.
• Quality Control: Helping quality professionals evaluate consistency in test results.
• Interlaboratory Evaluation: Supporting assessment of variability among different testing organizations or facilities.
• Method Comparison: Providing statistical information for comparing test-method performance.
• Result Interpretation: Helping analysts understand the significance of precision and bias information.
• Technical Reporting: Providing a consistent basis for communicating precision and bias characteristics.

Which Materials Can Be Evaluated According to ASTM E177?

ASTM E177 does not prescribe testing for one specific material or product. Instead, it applies to quantitative test methods used to evaluate materials, products, systems, or services when those methods generate numerical results. The material being evaluated depends on the individual ASTM test method to which the precision and bias analysis is applied. Examples may include:

• Metals and Alloys: Quantitative chemical, mechanical, dimensional, or physical property test methods.
• Plastics and Polymers: Numerical measurements of physical, thermal, mechanical, or chemical properties.
• Construction Materials: Quantitative measurements of cement, concrete, aggregates, coatings, and related materials.
• Chemicals: Numerical measurements of composition, purity, concentration, or physical properties.
• Petroleum Products: Quantitative measurements of chemical and physical characteristics.
• Environmental Samples: Numerical analytical measurements obtained using applicable ASTM methods.
• Other Products and Systems: Any material, product, system, or service evaluated through a suitable quantitative ASTM test method.

Why is ASTM E177 important?

ASTM E177 is important because test results can vary even when analysts follow the same test method. Understanding precision helps users determine the magnitude of expected variability, while understanding bias helps users evaluate the relationship between test results and accepted reference values. The practice provides a common statistical framework for communicating these characteristics. This improves the interpretation of ASTM test methods and helps engineers. analysts, manufacturers, and quality professionals make better decisions based on numerical test results.

ASTM E177 Testing Process and Data Collection

Technicians follow the specified test procedure, prepare representative specimens or samples, operate the required equipment, and record the resulting measurements under controlled conditions. Analysts then organize the available data for statistical evaluation. The analyst evaluates the sources of variability associated with the experimental realization of the test method, including operator, apparatus, environment, sample, and time. The analyst determines the appropriate statistical characteristics needed to describe precision and evaluates bias when an accepted reference value is available. Technicians should record sample identification, specimen information, test conditions, equipment information, measurement results, and other data required by the underlying test method. Analysts use these data to calculate or assess the applicable statistical measures and prepare appropriate precision and bias statements.

Common Challenges and Troubleshooting

A common challenge in ASTM E177 evaluations is uncontrolled variability in the underlying test method. Differences in operator technique, equipment performance, environmental conditions, sample preparation, or measurement procedures can increase observed variation. Analysts should identify and control these sources before interpreting precision data. Another challenge is confusing precision with bias. Precision concerns the variability of test results, whereas bias concerns the relationship between typical results and an accepted reference value. Analysts should also verify that the data are suitable for the statistical evaluation being performed.

• Precision results describe the variability associated with the test method under specified conditions. Depending on the applicable evaluation, the results may address repeatability, reproducibility, or other appropriate measures of precision.
• Bias results describe the relationship between typical test results produced under specified conditions and an accepted reference value, when an appropriate reference value is available. Bias may be expressed using the statistical approach applicable to the test method and data set.
• The technician records sample identification, specimen information, test conditions, measurement values, equipment information, and other relevant test details from the underlying ASTM method. The analyst documents the statistical data, precision estimates, bias assessment, sources of variability, and applicable calculations or interpretations.
• The report contains the test method identification, material or sample information, testing conditions, numerical test results, applicable precision information, bias information, statistical evaluation, and relevant limitations. ASTM E177 is a statistical practice and does not provide a single value or unit of measurement for use in all applications. 
• Analysts can produce reliable and meaningful precision and bias statements when test method execution is consistent, sampling is representative, testing is conducted under suitable conditions, and a suitable statistical analysis is applied, with equipment calibration and complete documentation.

Link to ASTM E177