Why are the original results and check test results combined together?

The IWTO Specifications (test methods) and Regulations set very clear criteria to be used in determining whether or not an error has occurred. When a Check Test is completed the results are compared with the original results issued on the test Certificate. If the difference between the two exceeds a specified limit, then the original is discarded and a new Certificate issued using the check test result. Otherwise, the difference is assumed to be due simply to the random errors that always occur due to variation in samples, preparation, equipment and operators, and the average of the two results is calculated, as this provides a more accurate estimate of the parameter involved.