Believe it or not, most errors on proficiency test submittals stem from a few causes, and could be avoided or corrected by a careful review of data before submission. Here are the most common errors and how to prevent them.
#1 Correlation of data:
Participants fail to correlate data before submitting it. This is the number one error found in PT submittals. To avoid this error, data should be reviewed carefully to determine whether it needs to be correlated. Correlation is most commonly needed for temperature, torque, force, humidity, and pressure measurements, but there may be others, depending on the measurement process.
What is meant by correlation of data?
When a nominal value on the data reporting sheet cannot be achieved exactly by the reference standard, the difference between the reference and nominal values needs to be taken into account when reporting results. It is essential to correct and account for any differences between actual applied values, reported values and nominal values.
For example, if the reference value applied to the unit under test (UUT) is 100.001 and the UUT reads 100.005, and the nominal value on the data reporting sheet is 100.000, the UUT reading needs to be adjusted by the difference between the reference and nominal value.
In this case, the reported value on the data reporting sheet would be 100.004.
#2 Reporting Units:
Participants report values or uncertainties in different units from what is on the Data Reporting Sheet (DRS). The units on the DRS are not always the same as what is read on the UUT (or the standard). Similarly, the units used to report uncertainty must match the units reported for the measurement. A review of data and units could prevent this type of error.
#3 Questionable Uncertainty Calculation:
Participants fail to include all contributors when determining the uncertainty for their measurement process. Uncertainties should include the resolution of the unit under test (UUT), and several other factors. The uncertainty of the standard(s) used in the measurement should be included in the calculation but should never be the only contribution to the uncertainty of the measurement process. Further, the uncertainty reported should not come from the lab’s scope of accreditation. It should be based on the actual measurement made. Underestimation of uncertainty will lead to unsatisfactory PT results. There are many tools and resources available to help labs develop reasonable uncertainty budgets.
#4 Transposition of Data:
Participants record data in a different order from what is on the Data Reporting Sheet. If this is not corrected before submitting, it will generate an unsatisfactory result for the PT. Again, a careful review of data should catch this error.
#5 Decimal Point Placement:
Participants enter data (or record data on a DRS) and inadvertently place the decimal point in the wrong spot, causing a blunder or gross error. The ISO 17025 standard requires a review of calibration reports prior to issuance. The same care should be taken prior to submitting PT results.
#6 Measuring the Correct Value:
Participants measure a different value from what is listed on the Data Reporting Sheet (DRS), causing a gross error. There may be two reasons why this occurs. A lab may not be capable of making a measurement at the stated value. In this case, the Measured Value on the data sheet should be left blank. The other reason may be the lab normally tests the UUT at values other than the ones on the DRS. Regardless of this, the only way the lab’s results can be compared to other labs is if all labs make measurements at the same values. Therefore, the lab needs to review the DRS prior to accepting the PT to make sure it can make measurements at the stated values. No exception can be made to this requirement.
The key takeaway from all of this is a careful review of the Data Reporting Sheet and data entry can prevent many errors when participating in a proficiency test. While NAPT staff review all data submittals and do their best to notify participants of suspected blunders, they are prohibited from correcting data entry errors, so please do your part to ensure a successful PT!