ASTM E29: Using Digits In Test Data

Description

ASTM E29 is a widely recognised standard that provides guidance for the correct use of significant figures when reporting and interpreting test data for determining conformity with specifications. This practice ensures clarity, consistency and accuracy in data reporting across different sectors.

Purpose and Scope

• Purpose: Establishes a systematic approach for the use of significant figures in test data, assisting users in determining whether a product or material meets the established requirements.
• Scope: This standard applies to all numerical values derived from test results, including those used in design, manufacturing and quality assurance processes.

Key Terms of ASTM E29

1. Significant Figures

• Significant figures are the digits in a number that convey its accuracy.
• The standard emphasises that the reported data should reflect the precision of the test method used to generate them.

2. Rounding Rules

ASTM E29 contains specific rules for rounding numbers:

• If the first digit to be omitted is less than 5, the preceding digit is retained.
• If the first digit to be omitted is greater than 5, the preceding digit is increased by one.
• If the first digit to be omitted is exactly 5, round to the next even number to avoid rounding errors.

For example:

• 2.344 rounded to two significant figures becomes 2.34.
• 2.355 rounded to two significant figures becomes 2.36 (because the 5 is followed by an odd digit, thereby increasing the preceding digit).

3. Test Data and Specification Limits

The standard provides guidance on the following points:

• Comparison of test data with specification limits, ensuring both are expressed with the same number of significant figures.
• Avoidance of excessively precise comparisons that could misrepresent the accuracy of the test results.

For example:

• If a test result is 99.7 and the specification limit is 100, the result should not be rejected if the test method’s precision does not allow for a more detailed comparison (e.g. 99.7001 vs. 100.0000).

4. Establishing Conformity

• Conformity decisions must take into account the precision of both the test method and the specification.
• ASTM E29 helps to avoid disputes by ensuring that specifications and test results are expressed using the same number of significant figures.

- Further information is available at Stanford Advanced Materials (SAM).

Practical Applications

Quality Assurance:

Manufacturers use ASTM E29 to ensure that test data reported by laboratories conform with the specified tolerances for materials or products.

Data Reporting:

It standardises the presentation of test results in technical reports and certifications, thereby preventing misinterpretations due to inconsistent rounding or unnecessary levels of precision.

Design and Engineering:

It ensures that design tolerances are realistic and measurable, based on the test methods utilised to verify them.

Regulatory Compliance:

It assists companies in demonstrating adherence to industry standards by providing uniform criteria for data evaluation.

Examples of Applications

Testing Material Strength:

A steel specimen is tested and the yield strength is measured at 400.7 MPa. If the specification requires a minimum of 400 MPa, the rounding rules ensure that the result aligns with the specification.

Dimensional Tolerances:

The dimension of a component is measured with a calliper, accurate to ±0.01 mm, as 25.48 mm. When compared with a specification of 25.5 mm, ASTM E29 ensures the correct interpretation of the result.

Frequently Asked Questions

Why is ASTM E29 important?

ASTM E29 ensures consistency and accuracy in the reporting of test data by providing clear guidelines for the use of significant figures and rounding rules. It helps to align test results with specification limits and thereby minimises disputes and misinterpretations in quality assurance and regulatory compliance.

How does ASTM E29 handle rounding when the digit is exactly 5?

When the first digit to be omitted is exactly 5, ASTM E29 specifies rounding to the next even number. For example:

• 2.355 rounds to 2.36 (because 5 is followed by an odd digit, thereby increasing the preceding digit).
• 2.345 rounds to 2.34 (because 5 is followed by an even digit, so the preceding digit remains unchanged).

This rule avoids rounding errors in large datasets.

What is the relationship between the accuracy of the test method and significant figures?

The accuracy of the test method determines the number of significant figures that should be reported in the test results. Reporting more digits than the test method permits may misrepresent the data's accuracy, while reporting fewer digits may result in a loss of meaningful information.

Does ASTM E29 apply to all types of tests and industries?

Yes, ASTM E29 applies universally to all sectors and testing procedures where numerical results are used to evaluate conformity with specifications. These include applications in manufacturing, construction, environmental testing and materials science.

How does ASTM E29 assist in conformity decisions?

ASTM E29 ensures that both test results and specification limits utilise the same number of significant figures, thereby eliminating discrepancies caused by differing levels of precision. The standard provides a fair and transparent method to determine whether a material or product meets the required specifications.