Contemplating Precision and Bias
Form and Style Can Help You Along
Precision and bias statements are an integral part of all ASTM International test methods. The Precision and Bias section in ASTM test methods was mentioned in the previous Rules&Regs column, and it was noted that a follow-up would more thoroughly examine A21, the Precision and Bias section of Form and Style. We’ll now look at what Form and Style has to say about precision and bias.
The first things you’ll notice when you begin to read A21 are references to ASTM standards that will aid you in the pursuit of precision and bias:
Form and Style states that precision is “the closeness of agreement between test results obtained under prescribed conditions.” A statement on precision provides potential test method users the means to assess how useful the test method would be for proposed applications.
As A21.2.2 notes: “Every test method shall contain a statement 1) regarding the precision of test results obtained in the same laboratory under specifically defined conditions of within-laboratory variability (repeatability conditions), and 2) regarding the precision of test results obtained in different laboratories (reproducibility conditions).” An example precision statement is provided.
A temporary statement must be included if the responsible committee decides that an interlaboratory study should be delayed. This statement addresses repeatability and notes that reproducibility is being determined.
Bias is defined in A21.3.1 as “a systematic error that contributes to the difference between the mean of a large number of test results and an accepted reference value.”
Subsections A21.4.1 through A21.4.2 provide general considerations to keep in mind when preparing a precision and bias statement. These include providing a brief descriptive summary of the interlaboratory study that will permit test method users to judge the reliability of data.
Finally, some test methods may involve exceptions to the directions given in A21, and instructions on handling these cases are given in A21.5 along with sample precision and bias statements to address these issues. Examples of exceptions include, but are not limited to:
While this column covers some of the basics, committee members are urged to study A21 further when considering the precision and bias statements for their test methods.