Independent assurance is the verification by a third party (not directly responsible for quality control or acceptance) of the product and/or the reliability of test results obtained from quality control and acceptance testing. This independent assurance ensures that (1) acceptance test results are accurate and provide a fair and equitable basis for construction acceptance and (2) quality control testing is accurate and thus will properly indicate process quality.
Quality control and acceptance tests are often performed by a multitude of laboratories (agency, contractor and independently owned) within a particular county, region or state. Even if all tests are performed at the same central laboratory, they are at least conducted at different times. Thus, independent assurance insures that sampling variability (the variation in sample characteristics from sample-to-sample that is attributable to variations in sampling technique), testing variability (the lack of repeatability of test results – operators, equipment condition, calibration, and test procedure all contribute to testing variability) and the differences in these variabilities between laboratories and over time do not unduly influence testing measurements. For example, the same sample tested by different laboratories should give roughly the same result.