ASTM E2554 for Estimating and Monitoring the Uncertainty of Test Results
ASTM E2554 Standard Practice for Estimating and Monitoring the Uncertainty of Test Results of a Test Method in a Single Laboratory Using a Control Sample Program
10. Individual Tests
10.1 A reference material or a stable and homogenous material representative of the material normally tested can be used.
10.1.1 A minimum of about 25 sample test results should be completed with reasonable time intervals between readings. The standard deviation of this set of individual readings will serve as an estimate of uncertainty.
10.1.2 This estimate of standard deviation shall be used to provide "control limits" for the review of past results and ongoing monitoring of the method. Limits of overall average more or less 3 standard deviations would be appropriate.
10.1.3 A graph showing the sequence of readings with the limits added allows examination to determine if unusual conditions may be present and contributing to the overall variation.
10.2 Example 2 - Vanadium in Oil:
10.2.1 Vanadium is an impurity in oil. A critical level is approximately 300 ppm. Test Method D 5184 is followed to determine the level of impurities. A quantity of control material was prepared using a typical batch of oil with a level of impurities similar to those encountered in practice (vanadium at approximately 300 ppm). This overall batch was subdivided into multiple containers and individual samples were drawn daily. Each sample was processed and tested along with regular samples in a single laboratory following standard procedures.
10.2.2 Multiple operators, different days, and conditions as generally experienced were incorporated into the program.
10.2.3 Forty consecutive samples were taken and are listed in Table 3 (units of mg/kg). These are used to provide an estimate of the variation of the test.
10.2.4 The average and standard deviation of the 40 values are found as: average = 292.5 and standard deviation (sd) = 13.3.
10.2.5 Based on these values, limits that would include 99.75 % of all readings would be within more or less 3 sd of the average. These limits would be:
292.5 + 3*13.3 = 332.4
and
292.5 - 3*13.3 = 252.7
10.2.6 A graph of the values including the limits for these 40 samples is shown in Fig. 4. No points fall outside the limits.
10.2.7 The value of standard deviation then serves as an estimate for the uncertainty standard deviation.
10.2.8 This uncertainty standard deviation is considered the laboratory's uncertainty for vanadium determinations at about 300 ppm.
11. Ongoing Studies
11.1 Additional factors should be introduced and identified over time. These should include, when appropriate, different operators, recalibration of equipment, weather changes (temperature, humidity), and so forth.
11.2 It is expected that these additional factors will result in an increase in the magnitude of the uncertainty estimate.
11.3 The uncertainty shall be reviewed to determine if there are changes either as improvement or a worsening of the degree of variation within laboratories.
12. Keywords
12.1 control chart; control sample; intermediate precision; test result; uncertainty
