What must a sampling plan include as per ISO 17025 guidelines?

A sampling plan must include clearly defined criteria as per ISO 17025 guidelines. This requirement ensures that the sampling process is consistent, reliable, and valid across different tests and analyses. Clearly defined criteria help specify the parameters that will guide the sampling process, such as the population being sampled, the sample size, and the methods for sample selection. These criteria contribute to the integrity of the results by ensuring that the selected samples accurately represent the whole population and that the results can be reproduced under similar conditions.

In contrast to the other options, which do not align with the standards set by ISO 17025, clearly defined criteria are essential for maintaining the quality and reliability of testing and calibration processes. Uncontrolled testing methods lack the structure necessary for consistent results, while only relying on theoretical approaches does not account for practical realities of sampling. Overlapping methods may introduce ambiguity and inconsistency rather than providing clarity in the sampling process.