For example, several tests conducted in close proximity in both time and place can result in screeners for later tests being in a state of heightened alert. For a variety of reasons, screening personnel can sometimes be cued beforehand that a test is coming. These cueing factors contribute to a possible upward bias in performance estimates. Although some of these factors can be identified, it is difficult to measure their impact. Sampling Bias Error - Field agents may conduct tests more frequently at locations that have known problems or in conditions that have records of previous failure.
Factors causing test failures are more likely to be present in such tests.
Verification and Validation Field agent test reports are manually reviewed for accuracy and completeness at the field and regional office levels. Headquarters-based employees also perform ad hoc testing and reviews to verify and validate testing protocols and data. Databases for machine data are manually examined to detect serious technical problems.